NTT Technical Review

June 2023 Vol. 21 No. 6
View from the TopKeigo Kajimura, Senior Executive Vice President, NTT Communications

Front-line ResearchersYutaka Miyamoto, NTT Fellow, NTT Network Innovation Laboratories

Rising ResearchersYasutoshi Ida, Distinguished Researcher, NTT Computer and Data Science Laboratories

Feature Articles: Toward Quantum Technology Innovation

Expectations and Prospects for Innovation in Quantum Technology



Optical Technologies for Optical Quantum Computing with Continuous Variables



Quantum Information Technology Based on Superconducting Quantum Circuits



Optical-lattice-clock-network Technology for Gravitational Potential SensingExtracting Quantum Power by Using Algorithms and Their Verification

Improving the Performance of Quantum Key Distribution



Toward a Quantum Internet

Regular Articles


Identification of Transcription Factors and the Regulatory Genes Involved in Triacylglycerol
Accumulation in a Unicellular Red Alga

Global Standardization Activities


Recent Activities of QoE-related Standardization in ITU-T SG12

Practical Field Information about Telecommunication Technologies


DeteriorationofTelecommunication

Equipment

andFacilities

inEnvironments—Case Studies of Corrosion in Guy Wires and Maintenance Holes
External Awards

Salt-damage

View from the Top

Human Resources Is the Foundationof Our Business and Empathy Is
the Driving Force That Creates
New Value
Keigo Kajimura
Senior Executive Vice President,
NTT Communications
Abstract

In 2022, the new DOCOMO Group was formed after a reorganization of
NTT DOCOMO, NTT Communications, and NTT COMWARE. Since
then, the Group has integrated the three companies’ business functions andclarified their respective roles to improve the value it provides to customers
and boost growth. Under its brand slogan “Changing worlds with you,” thenew DOCOMO Group aims to reform the structure of society/industry andcreate new lifestyles. We interviewed Keigo Kajimura, senior executive
vice president of NTT Communications, which is responsible for the enterprise business, about the initiatives he is focusing on and his beliefs as a top
executive.
Keywords: digital transformation, customer experience, human resource

Contributing to digital transformation insociety and industry through multiplication
rather than addition—It has been more than a year since the reorganization of NTT DOCOMO, NTT Communications, andNTT COMWARE, how do you feel looking back?
In accordance with the New DOCOMO Group
Medium-Term Strategy announced in 2021, management policies of NTT DOCOMO, NTT
Communications (NTT Com), and NTT COMWARE
were merged in January 2022, and the three companies were reorganized, including transferring certain
businesses among them, in July 2022. I was appointed senior executive vice president of NTT Com at the
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Vol. 21 No. 6 June 2023

time of the reorganization. As head of the Platform
Service Division, I am in charge of all services provided by NTT Com. I am also in charge of corporate
affairs, technology and innovation, internal systems,
information security, and promoting transformationin all areas, i.e., digital transformation (DX), customer experience (CX), green transformation, andemployee experience.
Through this reorganization, NTT Com can now
provide total solutions in a one-stop-shop manner toall customers nationwide by combining our corebusinesses, such as fixed networks, cloud computing,and datacenters, with NTT DOCOMO’s fifth-generation mobile communications system (5G) andInternet of Things (IoT) and NTT COMWARE’s
software-development capabilities.
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View from the Top

At the time of the reorganization, NTT Com offeredabout 100 services, to which NTT DOCOMO added
about 200, thus increasing the total number of services to about 300. I see this increase as multiplication rather than addition, and I was very excited thatthe possibilities would expand infinitely if we further
strengthened our sales capabilities.
During the past year, in addition to carrying out theStart Dash Program in which we focus on offering
newly added services to existing NTT Com customers, we have also endeavored to create new value by
combining technologies such as 5G, drones, robots,
IoT, and XR (cross/extended reality) and provide it toour customers according to their needs.
These efforts at the first stage of our transformation
have begun to bear fruit in various areas, and I feelthat customers are expecting to be offered an evenwider range of products from NTT Com. In 2023,
which marks the next stage of our transformation, we
aim to make even greater strides and provide concretevalue to our customers in a timely manner to meettheir expectations.

we jointly verified in an experiment that the AI (artificial intelligence) face-recognition mobile gate system developed by Kumahira Co. Ltd. combined with
5G and docomo MECTM improves the efficiency ofadmission-control operations at large-scale events.
The number of people passing through the gate perminute increased by 23% compared with that possible with the previous setup. We are also seeing theresults of experiments on automated driving andautonomous drone flight, and we would like to turnthese results into commercial products in the future.
In addition to addressing a growing demand formobile services from customers who are operating
their businesses globally, we also plan to strengthen
our lineup of fixed-mobile convergence services bycreating services for small- and medium-sized businesses. We will continue to contribute to the DX ofsociety and industry by providing advanced solutions
with the mobile and cloud-first approach.

—Your attitude toward meeting customer expectations is encouraging. Can you give us specific examples of your efforts?

—It’s exciting to think about the new value you canoffer and the impact you can have on society. So,
could you introduce NTT Com’s unique technology as
well as your service strategy?

For large enterprises, IoT solutions for remote management and optimization (energy, automobiles, etc.)
and solutions using 5G and docomo MECTM (Multiaccess Edge Computing) with low latency and highsecurity are beginning to show results. For example,
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It is important to create and sell services that areused by people

During my time as the general manager of the
Solution Services Department, I launched and completed various system integration (SI) projects tomeet individual customer requirements; however, SI2

View from the Top

tailored to individual customers inevitably requires a
lot of work, has long lead times, and does not scalewell. Accordingly, we have been working on company-wide innovation to address this issue by creating
repeatable models of solutions based on the value we
provide to our customers.
The service that supports these solution models is
the Smart Data Platform (SDPF), a platform that connects data and value to enable data-driven DX. Morethan 80 components, including network, cloud, storage, security, AI engine, and data-utilization functions, can be freely combined to meet the needs ofcustomer businesses. As I mentioned earlier, thereorganization has strengthened our capabilities
regarding mobile services, terminals, and software,so the added value provided by us thus far has
become much higher and wider by combining thosecapabilities with the SDPF.
Technologies that support this composable service
strategy include the VxF (Virtual Everything
Function) infrastructure, which virtualizes networkadded functions, and Qmonus, which is a DevOps(development and operations) platform for integrating components. We have contributed to the speedy
development and provision of various services bycustomers, including the Flexible Mobile Connect,
which was the first integrated service of the threecompanies. Qmonus has begun to be used by other
NTT Group companies and scheduled to be incorpo-

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rated into the multi-orchestrator part of the Cognitive
Foundation, which is one of the pillars of NTT’s
Innovative Optical and Wireless Network (IOWN).
Although we are only halfway, I believe that there isno other example of telecommunications carriers
working on this type of technology in-house.
We are currently using the SDPF in a wide range of
projects involving market-oriented service development, including for the expansion of zero-trust network services that use the strengths of telecommunications carriers. In the promotion of a smart worldthat addresses social issues, the SDPF is used as theplatform for data utilization in smart cities and forsupporting the Manabi Pocket, which has five million
user identifications in smart education.
—You provide services underpinned by solid technology, right?
I believe that it is important to create and sell services that are used by people and especially important
to enable people to use them safely and comfortably.
Since improving CX is crucial to provide services
that customers continue to choose, we are making
company-wide efforts to anticipate customer needs
and improve the value of CX throughout the entire
customer journey—from the proposal and introduction of services to post-introduction operations andresponse to problems that arise.
To this end, it is necessary to analyze the gap
between service expectations and the current situation on the basis of the voice of customer (VoC), andto implement a cycle of improvement from the customer’s perspective. For example, we provide a customer portal for customers who are using our services; unfortunately, the 2021 VoC survey showed very
low Net Promoter Scores (NPS; an indicator of recommendation) and satisfaction levels. In response tothese results, we established a cross-organizational
project structure and spent a year establishing “personas” (profiles of typical customers), organizing thecustomer journey for each persona, and redefining
the value provided by the portal to improve thosescores. Therefore, we were able to significantly
improve both NPS and satisfaction levels in the 2022
VoC survey. We thus intend to promote such initiatives across the organization at each touch point withour customers.

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If you don’t listen to others, you will stop
growing as a person—How do you view this era in which new technologies are being created one after another?
It may seem off the subject, but you’ve probably
heard of a science-fiction television series and movie
called “Star Trek,” right? I love science fiction, particularly Star Trek. It was one of the reasons I joinedNTT. I joined NTT in 1989 and was involved indesigning telephone services such as toll-free telephone numbers. At that time, there were already signs
that the era of exchanging data over networks wascoming, but I had no idea that it would develop so
rapidly.
Exactly as depicted in Star Trek, innovation has
advanced and a world that was unthinkable in thetelephone age has become a reality. In 2000, people
doubted the new era of space development, but it is
now a business trend. The future beyond that era isalso becoming a reality through the development of
IOWN.
Bringing about such innovations that transformsociety is the best part of my job, and to make that
happen, I believe that top management must lookahead to the world five or ten years from now andshow the company the way it should be and the direction it should take.
I think few people would dare to review a businesswith stable earnings or want to start something newthat spans the whole organization. However, in such
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a stable environment, innovation is unlikely to occur.
For that reason, we focus on taking a broad view ofthe business in general. Our ongoing efforts embody
this attitude.
I also feel the need to look at things from multiple
angles. People tend to stop listening to others as wemove up the corporate ladder or get older. Such a wayof being may stop people from growing, not only
those at the top management. Therefore, I try to communicate with our employees without fail.
—How did you come to realize the importance ofcommunication?
The trigger was so-called “failures” at work. I experienced several failures, and each time I experienced
a failure, many people helped me and I was able tofind a breakthrough by listening to them carefully.
I believe that the foundation of our business ishuman resources. I’d like to nurture human resources
by improving their skills in the ability to create services, assemble services, and empathize (activationof communication that transforms human connections into power), which is the driving force behind
new innovations.
An “encounter” between a company and its customers and between employees creates a connection,and the empathy that arises from this connection isthe driving force that creates new value. I want to create an “empathetic organization” in which people
cooperate autonomously and inspire one another sothat innovation can continue to take place.
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View from the Top

I also think we must hold the core of our company
firmly in place and not let its axis waver. However,today is not the era of a planned economy, and innovation is accelerating. We can no longer take things
with the sense of speed that we have in the past, so I
try to make sure that the core that cannot be compromised remains, and the rest can be changed according
to the situation.
—Please say a few words to our engineers, employees, and customers.
To our engineers. Hearing from professionals about
technologies that I don’t know much about is not only
educational but also fun and exciting. Innovation
occurs when people with various expertise andknowledge connect, so I think it is important to havetechnological exchanges between different industries
and competitors. Technology is truly a common language. I also expect you to become customer-oriented
engineers who connect technologies with the market.
To our employees. There is no end to our transformation, so let’s work together toward the next stage.
Finally, we cannot solve social issues alone. NTT
Com has established OPEN HUB Park at Otemachi

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Place in Tokyo as a place for demonstration and cocreation of various technologies, including IOWN.
We have also established the IOWN Promotion
Office to accelerate efforts to create software-based
networks, the application of the All-Photonics
Network to datacenter interconnection, and fielddemonstrations using the IOWN testbed. We would
like to work with our customers to co-create solutions
to social issues while refining various technologies
toward the future. We look forward to working withyou.
Interviewee profile
Career highlights
Keigo Kajimura joined NTT in 1989. In his
career at NTT Communications, he became general manager of the Systems Department in 2012,
member of the Board and general manager of the
Solution Services Department in 2017. He
became president and representative member ofthe Board of NTT Com Engineering in 2020. He
assumed his current position in June 2022.

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Front-line Researchers

Turning an Innovative Idea into
Common Sense and Making It Usefulto Society
Yutaka Miyamoto
NTT Fellow, NTT Network Innovation
Laboratories
Abstract

With the increase in video-data distribution, the development ofcloud technology, and spread of new information and communication services, such as 5th-generation mobile communications
and remote work, information and communication traffic has
been increasing and will continue to increase. Implementation of
the All-Photonics Network of IOWN (the Innovative Optical andWireless Network) to meet the demand for diverse services andexponential growth of data traffic will require a huge increase intransmission capacity as well as drastic reductions in power consumption and latency. NTT Fellow Yutaka Miyamoto at NTT Network Innovation Laboratories is pioneering new optical network technology to overcome the capacity crunch. We interviewed him about the
progress of his research and what is the best part of being a researcher.
Keywords: scalable optical transport network, optical-amplification repeater, space-division multiplexing

Developing fundamental technologies forhigh-capacity scalable optical transport networks—Could you give us an overview of the research you
are currently conducting?
To implement a scalable optical transport networkwith petabit per second (Pbit/s)-class link capacitythat can accommodate communication traffic thatwill increase due to the expansion of cloud services
and spread of smartphones, I’m conducting researchon four fundamental technologies: (i) large-scale
digital-signal-processing technology for optical communications; (ii) photonics-electronics convergence
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integrated circuit (IC) technology; (iii) ultra-lownoise optical-amplification technology for signal-tonoise-ratio improvement; and (iv) space-divisionmultiplexing (SDM) optical-transmission-systemtechnology.
NTT has been a world leader in research and development (R&D) of optical communication technology. Since the practical application of time-divisionmultiplexing systems in 1981, NTT has continued todrive three paradigm shifts concerning optical transmission systems: optical-amplification repeater systems, wavelength-division multiplexing (WDM)
systems, and digital-coherent systems. As a result of
these shifts, transmission capacity has increased6

Transmission speed per
wavelength (Tbit/s)

Front-line Researchers

2.4
Our result
2.0

NTT’s latest results
NTT’s previous results
Results from other institutes
2.02-Tbit/s
PCS-144QAM signal

1.6
1.2
0.8

10

100
1000
Transmission distance (km)

10000

PCS: probabilistic constellation shaping
QAM: quadrature amplitude modulation

Fig. 1. Extending transmission distance of multi-terabit-class optical signals by increasing modulation rate.

approximately one million times in the past 40 years.
Data traffic has continued to increase at a rate ofabout 1.4 times per year, and as 5th-generation
mobile communications (5G) and the Internet ofThings services begin to be rolled out widely and 6G
services are also in sight, communication traffic is
expected to continue to grow exponentially. In fact, itis predicted that long-distance transmission with
Pbit/s-class capacity will be necessary in the 2030s.
To meet such future communication demands, theInnovative Optical and Wireless Network (IOWN)
aims to economically provide higher capacity
through the All-Photonics Network (APN).
However, recent research has shown that the physical limit on transmission capacity (namely, the capacity crunch) of long-distance transmission using optical fibers currently in use becomes apparent around
100 terabits per second (Tbit/s). To overcome thetechnical challenges posed by the capacity crunch
and implement a Pbit/s-class optical communication
infrastructure that can accommodate more than 100
times the current data traffic in a low-power consumption and economical manner, my research colleagues and I are conducting R&D on scalable opticaltransport technologies. We are driving the fourthparadigm shift through technological innovation thatcombines the advancement of optical transmission
technology that we have been developing with new
optical transmission medium (optical fiber) technology.
—Since our last interview two years ago, you havecontinuously tackled the technical issues concerning
the capacity crunch.
In the previous interview, I talked about the resultsof our R&D at that time, including (i) SDM optical
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communication technology that can increase transmission capacity per link to more than 1 Pbit/s, which
was more than 125 times the capacity of practical
WDM systems using current optical fiber (100Gbit/s/wavelength), and (ii) the demonstration of
long-distance WDM transmission exceeding 1 Tbit/s
per wavelength, both of which were the world’s firstat the time.
These achievements were the result of timely collaboration with the transmission medium and device
research departments within NTT laboratories as well
as external research institutes. We have continued todevelop these technologies through close collaboration among our research departments, especiallythose among young researchers, and were able toproduce world-first and world-leading research
results in FY 2022.
For example, in 2022, we experimentally demonstrated the world’s first optical amplified transmission using digital-coherent optical signals exceeding
2.02 Tbit/s per wavelength over 240 km (Fig. 1).
To expand transmission capacity per wavelength byovercoming the speed limit of silicon complementary
metal oxide semiconductor circuits, we had to both
broaden bandwidth and increase output power of thedriver amplifier for driving the optical modulator aswell as achieve extremely accurate compensation fordifferences in tributary-signal-path lengths and variations in tributary-signal-path loss in the optical transmitter and receiver circuits. We addressed this issue
by integrating an ultra-broadband baseband-amplifier
IC module and digital-signal-processing technologythat enables ultra-high-precision compensation ofloss variation and distortion in the optical transmitterand receiver circuits, both of which were developed
by NTT, and achieved optical amplified transmission
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Front-line Researchers

Optical amplification gain (dB)Conventional EDFA optical amplification (C band: 4 THz)

Our result: 12 THz (S, C, L bands)
20
10
PPLN waveguide-based
module

0
−10
−20

89:
1500

;<=AB

( .*

1520
1540
1560
Optical signal wavelength (dB)

0?*0@
1580

1600

Fig. 2. B roadening the amplification bandwidth and expanding wavelength resources by applying optical-parametricamplification repeater system using PPLN waveguide modules.

over a distance of 240 km at 2.02 Tbit/s. This result
indicates the potential for further scalability of digital-coherent optical-transmission technology that canachieve both high capacity and long-distance transmission. A paper reporting this achievement was
accepted as a postdeadline paper—a paper presented
at the most-challenging session—at the European
Conference on Optical Communication (ECOC2022).
Conducting experiments in historic facilities at
NTT Yokosuka R&D Center—You are developing core technology for the IOWN
APN, right?
We also made significant progress in terms ofbroadband optical-amplification repeater technology
using NTT’s proprietary periodically poled lithiumniobate (PPLN) waveguide modules.
In close cooperation with the device research
department, we proposed an optical-amplification
repeater system that combines an optical parametricinline amplifier that handles polarization-divisionmultiplexed digital-coherent optical modulation anddemodulation signals, which are currently the mainstream. We conducted the world’s first ultra-wideband WDM optical amplified transmission experiment over 240 km by applying an optical parametricamplifier to multiplex optical signals at 1 Tbit/s per
wavelength within the amplification bandwidth of 12
THz (Fig. 2).
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While the WDM signal bandwidth of the widelyused optical amplifier (erbium-doped fiber amplifier(EDFA) in the C band) is approximately 4 THz, ournewly developed optical-parametric amplifier canamplify a bandwidth of 12 THz or greater, which isapproximately three times that of an EDFA, and cancover the wavelength regions (S, C, and L bands) inwhich optical fiber has low loss. It is thus possible toexpand wavelength resources by broadening bandwidth. This broadband optical-amplification repeater
technology, with its wide bandwidth and low distortion, is expected to provide further high-capacity
optical amplification enabling implementation of the
APN, which dynamically uses abundant wavelength
resources in accordance with the IOWN concept proposed by NTT. This accomplishment was also accepted as postdeadline papers at the Optical Fiber Communication Conference (OFC) in 2021 and 2022.
—You are also working closely with the transmission
medium research department on SDM optical communication technology.
In addition to broadening the bandwidth of conventional single-mode optical fibers (SMFs), we arepromoting R&D on optical fibers for SDM opticalcommunication technology with the aim of fundamentally overcoming the capacity crunch. Working
closely with the transmission medium research
department, we are developing prototypes of various
types of SDM optical fibers, including multicore
optical fibers (MCF) having multiple cores (which
8

Front-line Researchers

are pathways for light) in a single fiber and multimodeoptical fibers having multiple propagation states(spatial modes) in a single core, and are assessing thefeasibility of these optical fibers as new transmission
media for future optical communications. To maximize the performance of the above-mentioned newtransmission media, we are also studying high-capacity SDM optical communication systems.
Since SDM optical fiber with a standard cladding
diameter of 125 µm (which is the same diameter asthat of current SMF) is suited for mass production of
optical-fiber cables, we are investigating SDM optical communication technology that can increase thecapacity of such fiber more than ten times that of current SMF while maintaining a standard cladding
diameter. We have been focusing our research on
mode-multiplexed transmission technology that uses
and controls multiple spatial modes to overcome thelimitation on transmission distance imposed by interference (crosstalk) between different spatially multiplexed optical signals, which is an issue regarding
SDM optical communication systems. Mode-multiplexed transmission technology consists of the following three technologies: (i) mode-multiplexing
optical-fiber cabling technology, which can control
spatial modes; (ii) mode-multiplexed multiple-input
multiple-output (MIMO) digital-signal-processing
configuration technology, which can multiplex anddemultiplex multiple different optical signals at thesame wavelength in accordance with dynamic opticalcharacteristics attributed to the cable-installation
properties; and (iii) fundamental transport technology that unifies the spatial-mode-multiplexing opticalamplification repeater technology with (i) and (ii).
Regarding mode-multiplexed MIMO digital-signal-processing configuration technology, NTT successfully demonstrated in a proof-of-principle
experiment long-distance transmission over 6000 km
of mode-multiplexed optical communications using
six independent spatial modes. For this experiment,
we developed a MIMO signal-processing system andoptical-amplification repeater system that havestrong compensation characteristics against transmission loss and propagation-delay differences that
occur between different spatial modes. In March
2023, we also reported in our highly scored OFC
2023 paper the demonstration of long-distance modemultiplexed optical-amplified transmission with ten
spatial modes.
Research on the above three technologies has partly
been supported by National Institute of Information
and Communications Technology under the “R&D of
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basic technologies of spatial mode control type optical transmission for the Beyond 5G era” program(started in 2021). We are collaborating with four
domestic research institutes and NTT Access Network Service Systems Laboratories to establish an
SDM optical transmission system that will enable us
to build a high-capacity, long-distance backbone
optical network in the Beyond 5G era. Specifically,
we are investigating the following technologies: (i)design, installation, and connection technologies forcoupling-type MCF cable that has a standard cladding diameter and can control spatial modes; (ii)optical-amplification repeater technology that iscompatible with MCF cabling technologies (i); and(iii) a new low-complexity MIMO signal-processing
technology that can follow dynamic changes in thetransmission link. By investigating these three technologies, we aim to establish a basic technology for
spatial-mode-control optical transmission thatenables long-distance transmission of spatially multiplexed signals of ten or more.
Over the last few years, our research has advanced
to the stage of installing various types of SDM optical-fiber cables in underground facilities at the NTT
Yokosuka R&D Center and testing their transmission
characteristics under conditions similar to those in
the field. The NTT Yokosuka R&D Center, whichcelebrated its 50th anniversary in 2022, has an underground facility (conduit) built in the 1970s to verify
wired communication system technology. Opticalfiber cables developed at the dawn of optical-fibercommunication systems in the mid-1970s were laid,
and tests on the characteristics of those cables interms of practical use were conducted. We are taking
on the challenge of conducting new demonstration
experiments at this historical site, where our predecessors have been testing technology and putting it topractical use for more than 40 years.
Collaborating with others like the tale of the“three arrows” to accumulate achievements—Can you tell us what is key in your research activities?
Optical communication systems cannot be put into
practical use by using only a single technology. The
process of achieving target performance in a timely
manner by combining several of our technologies
with those of various companies and institutes asnecessary is important. Given the rapid pace ofchange in the world today, it is essential to promote
9Front-line Researchers

collaboration within other research institutes andother laboratories within NTT in a timely manner totake the lead in standardization activities and keepahead of global competition. We need to combine
multiple advanced technologies as in the tale of the“three arrows” (which conveys that one arrow can beeasily broken, but three bundled arrows cannot) toachieve results and build a track record.
I believe that one of my roles as an NTT Fellow is
to create opportunities for young researchers to test
their ideas and collaborate effectively with otherleading researchers. To fulfil that role, it is of course
important to have advanced technologies of our own,
but it is also important to share the goal to be reachedwith other researchers and research institutes thathave advanced technologies and build a relationshipof trust with them. I try to find partners who share ourgoals through joint experiments and at academic conferences and collaborate with them in a timely manner. I believe that if our ultimate goal is in line withour collaborative partners, we can get back to thebasics and move forward without wavering, even if
difficulties arise. If we are working with overseas
research institutes, we may face differences in cultures, business practices, and mentalities, so we mayneed to take subtle tactics; even so, when taking up achallenge concerning a new technology, I want tokeep a positive mindset and say, “It looks interesting
and challenging, so let’s give it a try.”
As I mentioned in the previous interview, I believe
it is important to be prepared on a daily basis so as not
to miss any critical “This is it!” moments. For thisreason, it is also important to identify the areas inwhich we produce technology that surpasses others
and has value. We also need to pay attention to whatvalue we can provide to our collaborative partners.
Since this perspective can be lost when focusing on
cutting-edge technologies only, we are working on
both R&D and practical applications in conjunction
with our colleagues in the practical application
department.
—What is important to be a good researcher?
I said before that an important perspective for system researchers is to “turn an innovative idea into
common sense and make it useful to society.” I wantto tell young researchers that everything starts froman innovative idea. For example, when we first pre-NTT Technical Review

Vol. 21 No. 6 June 2023

sented a research result at a conference, the reaction
was not so great; however, at the next conference,
many people presented something similar, creating atechnological trend.
I urge all researchers to pursue what you find interesting without fear of failure. However, it may bedifficult to determine how long you need to pursue it.
There were many cases in which a technology that
had remained obscure became widespread whencombined with another technology 20 years later. It isalso important to investigate things over a long timespan without being too concerned about the results infront of you and obtain rights to your ideas in preparation for the day that they are put to practical use.
As a final point, in the spring of 2021, on behalf of
all my colleagues, I was honored with receiving theMedal with Purple Ribbon for the development of ahigh-capacity optical transmission scheme using
coherent-multicarrier multilevel modulation. The
subject of this award is related to the R&D on longhaul high-capacity optical-fiber communication systems, with which I was involved from 1995 to 2010.
In 2007, the technology that won this award was put
to practical use for the first time in NTT Group’s
1.6-Tbit/s WDM optical communication system witha channel capacity of 40 Gbit/s per wavelength. Our
major contributions include the practical applicationand international standardization of (i) digital-signal
multiplexing/framing—called the optical transport
network (OTN)—equipped with strong error-correcting codes to flexibly accommodate data traffic in
WDM optical networks and (ii) multilevel differential phase shift keying for replacing the intensitymodulation direct-detection technology. The above
technologies we have developed facilitated the
spread of the Internet and global broadband services
via optical access networks and smartphones (4G)
and continue to support the communication infrastructure that is deeply involved in the transformationof people’s businesses and lifestyles. It is most gratifying to me that the contribution of the research fieldthat we have been pursuing, including those of ourpredecessors who have guided us and the many people that we have worked hard with to advance R&D
and practical applications, have been recognized.
Being able to engage in R&D that will produce
results that will be used around the world and contribute to society as a whole is the best part of being a
researcher.

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Front-line Researchers

Interviewee profile
Yutaka Miyamoto received a B.E. and M.E. inelectrical engineering from Waseda University,
Tokyo, in 1986 and 1988 and Dr.Eng. in electricalengineering from the University of Tokyo in
2016. He joined NTT Transmission Systems
Laboratories in 1988, where he engaged in R&D
on high-speed optical communications systems
including the 10-Gbit/s first terrestrial opticaltransmission system (FA-10G) using EDFA
inline repeaters. He was with NTT Electronics
Technology Corporation between 1995 and 1997,
where he engaged in the planning and productdevelopment of a high-speed optical module atthe data rate of 10 Gbit/s and beyond. Since 1997,
he has been with NTT Network Innovation Labs,
where he has researched and developed opticaltransport technologies based on 40/100/400-Gbit/schannel and beyond. He is currently investigating
and promoting a future scalable optical transport
network with Pbit/s-class capacity. He is a member of the Institute of Electrical and Electronics
Engineers (IEEE) and a fellow of the Institute ofElectronics, Information and Communication
Engineers (IEICE).

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Rising Researchers

Fast Sparse Modeling Technology
Opening Up the Future with
Ultra-high-dimensional Data
Yasutoshi Ida
Distinguished Researcher,
NTT Computer and Data Science
Laboratories
Abstract

As one pillar of NTT’s IOWN (Innovative Optical and Wireless
Network) vision, Digital Twin Computing aims to construct theworld in a digital space. To this end, it is essential that data be
obtained from people and things by sensors. However, advancesin sensing technologies have created a new issue in which anincrease in the number of data dimensions makes processing time
longer, which makes it difficult to analyze and use collected data
within a realistic period of time. We asked NTT Distinguished
Researcher Yasutoshi Ida to tell us about “fast sparse modelingtechnology” as a means of solving this problem.
Keywords: sparse modeling, deep learning, high-dimensional data

Creation of a new technique for overcoming
increased processing time ofultra-high-dimensional data
—Dr. Ida, what exactly is “sparse modeling” that you
are now researching?
Sparse modeling is a technology that applies sparsity to data usage in the sense that “the amount of
information needed is only a small part of all information obtained—most of the remaining informationis unnecessary.” Here, “sparse” has the meaning of“thin” or “scattered” based on the idea that “weshould not make more assumptions than necessary to
explain a certain matter” (Occam’s razor) put forth by
William of Ockham, a 14th-century English theoloNTT Technical Review

Vol. 21 No. 6 June 2023

gian.
To give an example of sparse modeling, let’s consider the case of predicting tomorrow’s temperaturein Tokyo from weather data consisting of temperature, wind direction, pressure, etc. throughout Japan.
In this case, the hypothesis that “data related to Tokyo
temperature prediction is only that from some of theneighboring prefectures” can be postulated taking
geographical relationships into account. This hypothesis is a basic precondition for using sparse modelingexpressed as “important data is only a small part ofthe entire amount of data—all other data is unnecessary.” Incorporating this prior knowledge called sparsity into our analysis enables us to specify those prefectures related to the prediction of tomorrow’s temperature in Tokyo.
12

Rising Researchers

Example of high-dimensional data
In a global observation system consisting of ships, buoys, airplanes, weather satellites, etc.,
the number of dimensions in weather data collected by a sensing network can reach from several
tens of thousands to several hundreds of millions or more.
Weather data

Number of dimensions
Region A

Region B

Region C

Region D

Region E

Region X

Region Y

Region Z

Region A

Region B

Region C

Region D

Region E

Region X

Region Y

Region Z

Region A

Region B

Region C

Region D

Region E

Region X

Region Y

Region Z

Region A

Region B

Region C

Region D

Region E

Region X

Region Y

Region Z

Region A

Region B

Region C

Region D

Region E

Region X

Region Y

Region Z

Number of samples

2000
2001

2021

Fig. 1. Number of dimensions and number of samples in weather data.

The background to the need for this technique liesin the increase in the number of data dimensions thatcan be obtained due to recent advances in sensing
technologies. In recent weather analysis, for example, the number of items of weather information(number of dimensions) that can be observed inregions throughout the world can reach several tensof thousands to several hundreds of millions withrespect to the number of years (number of samples)on the vertical axis (Fig. 1). However, machine learning in recent years has widely adopted the approachthat using a massive number of samples in training
improves accuracy, but handling data in which thenumber of samples is relatively smaller than the number of dimensions is difficult. The aim of sparse modeling is to therefore solve this problem at least in partby using sparsity to select only those dimensions
needed for analysis.
Model compression in deep learning is anotherexample of how sparse modeling can be used. In deeplearning, which can be called a fundamental technology of modern-day artificial intelligence (AI), accuracy has continuously improved by increasing thenumber of model parameters. However, a massive
number of parameters increase memory consumption
and processing time, so applying AI to edge-side
hardware having limited memory capacity, for example, can be difficult. Under these conditions, incorporating sparsity in deep learning in the sense that
“important parameters are only a small part of all
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parameters—all other parameters are unnecessary”
decreases the number of AI parameters and theamount of memory consumed.
—What are the strengths of “fast sparse modelingtechnology” compared with existing technology?
Fast sparse modeling that I’m researching isachieving speeds up to 35 times faster than existingsparse modeling technology, which requires muchprocessing time to compute a score expressing theimportance of each dimension. Fast sparse modeling,on the other hand, replaces this score with the score’s
upper and lower bounds that can be computed at highspeeds. In this way, fast sparse modeling has beensuccessful in greatly shortening processing time
while preventing degradation in accuracy. Thisapproach is influenced by the high-speed techniqueused in database research by NTT Distinguished
Researcher Yasuhiro Fujiwara, who was my research
mentor when I entered NTT. By combining sparsemodeling with databases, which are seemingly unrelated, I was able to achieve high speeds with an
original technique not found in the existing research
field of sparse modeling (Fig. 2).
Armed with this approach, I have my sights on theprocessing of ultra-high-dimensional data of several
hundreds of millions of dimensions or more of which
there are not many examples in existing sparse modeling technology. In 2019, the fast algorithm I just13

Rising Researchers

Fast sparse modeling—overall algorithm
Executing in the order 12 optimizes all parameters without omission and prevents degradationin accuracy. It also theoretically guarantees that no degradation in accuracy can occur.
1. Focus on and optimize only those groups
deemed important

2. Optimize all parameters while skipping
unnecessary score computation

Extract an important group by thescore’s lower boundIdentify an unimportant group by thescore’s upper boundFig. 2. Computational algorithm of fast sparse modeling.

mentioned increased processing speed by as much as
35 times, and the paper describing this achievement
was accepted at Neural Information Processing Systems (NeurIPS), a leading conference in this field.
This technology was subjected to tests under a varietyof environments over a period of about one year sothat it could be provided to NTT Group companies,
and it was found that the algorithm could increase
speeds in many analysis examples.
Yet, it was also found that high speeds could not be
attained for a very small number of use cases. On
conducting a detailed analysis as to why this occurred,
we found that there were cases in which the overhead(pre-computation) for computing the score’s upperbound in the fast algorithm was too large dependingon the properties of the target data. We are now developing an algorithm with the aim of reducing thisoverhead as far as possible, and if successful, we
expect a twofold increase in speed, which means that
we should be able to improve processing speed by
35×2, or in other words, by a maximum of 70 times.
In parallel with these activities, we are studying anapproach that combines sparse modeling with deeplearning that achieves high accuracy with large-scaledata. Basically speaking, deep learning requires alarge number of samples, so we are researching how
deep learning could be applied to use cases with asmall number of samples. Combining the knowledge
gained from this research with sparse modeling, weare studying technology that can achieve high accuracy even for high-dimensional data in which the
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number of samples is small. This is research with along-term view, and my aim here is to improve theaccuracy of sparse modeling by 10%.
—What kind of world can we expect to see throughfast sparse modeling?
Fast sparse modeling can be applied mainly to tasks
that perform analyses and predictions from ultrahigh-dimensional data. In the Industrial Internet ofThings targeting factories, for example, applying fastsparse modeling to the preprocessing of analysis for
identifying time slots affecting an increase/decreasein production can speed up the PDCA (Plan–Do–
Check–Act) cycle in data analysis and shorten thelead time to decision-making. This increase in speed
also allows for the handling of time-series dataspanning even longer periods thereby enabling highspeed analyses that have so far not been possible.
Similarly, in genome-wide association studies, fastsparse modeling can be applied to the preprocessingof analysis for identifying genetic factors, or single
nucleotide polymorphisms (SNPs), related to cancer
and other diseases. Increasing processing speed inthis way will enable the analysis of SNP data on aneven larger scale. Moreover, in a nuclear fusion reactor, fast sparse modeling can be applied to the preprocessing of analysis for identifying control operations
and sensors related to plasma disruption and maintenance. This capability should contribute to the explanation of phenomena related to the stable operation of
14

Rising Researchers

nuclear fusion reactors.
In addition, Digital Twin Computing in NTT’s
IOWN (Innovative Optical and Wireless Network)
vision aims to create digital twins using large
amounts of sensor data collected from sensors
attached to people and things. Amid this initiative
toward the future, sensing technologies are progressing and sensor data is becoming increasingly highdimensional, so I think that the opportunities forusing sparse modeling that is especially adept at highdimensional data analysis are increasing. Moreover,since the processing time devoted to sparse modelingwill be increasing as data becomes increasingly highdimensional, I think that establishing technology formaking sparse modeling even faster will become all
the more important.
—How do you think fast sparse modeling will evolve
going forward?
From here on, a key issue will be spatial complexity, that is, “To what extent can the amount of memory consumption be minimized?” The speed-up factor
of fast sparse modeling is top class even from a
global perspective, and in terms of shortening processing time, it’s reaching milestones. It must be kept
in mind, though, that memory consumption naturally
increases as data takes on more dimensions. For
example, when performing technical testing of fastsparse modeling to support the implementation ofdeveloped technologies in services, it may happen
that memory consumption becomes too large. As aresult, the number of dimensions at which processing
can be performed can hit a ceiling even if no problems are occurring in the speed-up factor. Of course,
bolstering the amount of memory through additional
facility investment can solve this problem, but
depending on the type of service being targeted, thisNTT Technical Review

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method may prevent a profit from being made thereby creating a new problem. To resolve these issues,
we are searching for a new method of fast sparsemodeling that can hold down memory consumption
while maintaining the speed-up factor.
Another direction that I can envision for fast sparsemodeling is a “quantum leap in speed.” As data
becomes increasingly high dimensional together withadvances in sensing technologies and limits toincreasing speed are reached whatever the algorithm,
I believe that we will have to consider new integrated
approaches to achieving higher speeds that include
distributed processing platforms and the use ofadvanced hardware. To tackle this research issue, collaborating with experts in other fields is essential—
we must carefully search out a solution taking a longterm perspective. At NTT Computer and Data Science Laboratories, the research and development ofadvanced computers is also underway, so we are now
investigating whether even faster sparse modeling
could be achieved by making good use of such computers.
Determined to create practical technology after
experiencing a “valley of death”—What do you think is an important attitude to adopt
in the work of research and development?
Based on what I learned from participating in a
venture company during my university days, I placemuch importance on research and development
focused on practical use. It was in 2010 during mythird year of undergraduate studies that I first encountered machine learning. At that time, I was researching “topic modeling” technology that can visualize
what kinds of topics occur in a document, and I felt
strongly that I would like to incorporate this technology into an actual service. As a result, I participatedin a venture company on the invitation from a friend.
Once there, however, I was confronted with a numberof problems not present at the research stage, such as“the mixing of data I had not expected with training
data,” “the model consequently behaving in unexpected ways,” “output results that could not be interpreted by humans,” and “a scale of data that was solarge overall that training could not be completed.”
As a result, we never got to the point of implementing
the technology in an actual service. The barrier
between research and services is sometimes called
the “valley of death.” This was my first experience
with this “valley of death” in which I was not able to15

Rising Researchers

implement machine learning into some kind of a service. From this experience, my idea of wanting tocreate machine-learning technology that could solve
problems in real-world settings took root. Today, I am
committed to “developing practical algorithms thatcan be applied to actual services” and am researchingsuch algorithms on a day-to-day basis while exchanging information with people at actual work sites.
Of course, there’s more to research than simply
adopting a practical approach. If a researcher is to
produce research results that are truly groundbreaking and original, I believe it’s also necessary to takeup long-term research that is somewhat removed
from practical considerations. A researcher who pursues both short-term and long-term research may findthat the knowledge gained from practical research
can also be useful in long-term research, and conversely, that some of the ideas being pondered inlong-term research can be useful in practical research.
In short, a researcher can take on real-world problems
in practical research and feed back the knowledge
gained there to long-term research, and vice versa,
can share ideas nurtured in long-term research withpractical research to solve problems in the real world.
Long-term research may also lead to technologies or
papers that can produce groundbreaking results.
These two types of research with different properties
can mutually interact with each other or create a feedback loop, which I think is an ideal form of research.
—Dr. Ida, could you leave us with a message forother researchers, students, and business partners?
Yes, of course. I feel that research and development
at NTT spans a wide range of fields and has a portfolio of unique research fields even on a global basis.
This is especially advantageous in problem solving.
For example, to speed up deep learning, severe targets can be imposed on the speed-up factor. NTT,
however, has a lineup of experts across many areasfrom algorithms to hardware and interconnects, so by
simply talking to a colleague sitting right next to you,it may be possible to discover an integrated solution
in no more than ten minutes. I believe that this abilityof coming up with an approach to a difficult problem
in a short period of time is something that only the
NTT environment can provide thanks to its original
portfolio and diverse lineup of research personnel.
In addition, I think that NTT Computer and Data
Science Laboratories that I belong to is an organization that researches a broad layer of technology
related to data science in terms of both the breath of
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fields and short-term/long-term perspectives. For
example, data analysis technology developed with afocus on real-world problems is researched on thepremise that it will be introduced into business relatively soon, which I think contributes to added value
in NTT services. On the other hand, I think that the
research and development of advanced computersand searching out applications for them play animportant role in the long-term development of data
science.
Finally, if working as a corporate researcher, surrounding conditions can suddenly change due, forexample, to an organizational restructuring. At thistime, if you significantly change your direction ofresearch in an attempt to adapt to changes in the environment, you will not be able to use the technicalabilities that you have so far developed and results
will not be forthcoming, all of which can lead tostress and a loss of confidence. In my experience,
however, if you have a universal, unshakeable corecharacter and refuse to compromise, I think that you
will be able to adapt to your environment more often
than not without sacrificing your good qualities. Thisway of thinking can be expressed as “gentle inappearance but tough in spirit,” or in other words,“my core research philosophy is uncompromising,but apart from that, I will adapt in a flexible manner
to my environment.” To students reading this who arelooking to become a corporate researcher, I would
like them to enjoy research while discovering andtreasuring one’s core character.

Interviewee profile
Yasutoshi Ida received his M.E. from Waseda
University in 2014 and entered NTT in the same
year. He received his Ph.D. in informatics from
Kyoto University in 2021. He has been a distinguished researcher at NTT Computer and Data
Science Laboratories since 2022. He is engaged
in the research of fast-and-accurate sparse modeling and supports the implementation of developed technologies in services. He administers aliaison meeting that brings together AI-related
engineers in the NTT Group for informationexchange (NTT Deep Learning Liaison Meeting).
His papers on developed technologies in this field
have been accepted at many leading conferences(NeurIPS/ICML/AAAI/IJCAI/AISTATS).

16

Feature Articles: Toward Quantum Technology
Innovation

Expectations and Prospects forInnovation in Quantum Technology
Tetsuomi Sogawa
Abstract

NTT laboratories have been engaged in basic research on various quantum-related technologies,
including quantum information, since the mid-1980s before research on quantum computers took off.
The Feature Articles in this issue present the latest global trends in quantum technology and NTT’s
research portfolio regarding quantum technology. In addition to quantum computing, quantum sensing
and quantum networks are also extensively discussed from both experimental and theoretical perspectives.
Keywords: quantum computer, quantum sensing, quantum internet

1. Trends in quantum technology
The first “quantum-computer boom” was triggeredby the development of Shor’s algorithm in 1994,
which drew attention to the potential threat to thesecurity of the information technology (IT) society inwhich public-key cryptography is widely used.
Numerous important advances, particularly in academia, have been made including the demonstrationof qubit operation in various physical systems and thedevelopment of theory of quantum error correction.
Around 2010, the technical difficulties in implementing those advances as a quantum computer became
widely recognized and excessive expectations began
to subside.
However, in 2011, the sudden announcement by
D-Wave Systems of their development of a dedicatedmachine for solving combinatorial optimization
problems (called a quantum annealer) by using acompletely different technology called quantum
annealing came as a surprise to all concerned. The
significant improvement in performance of superconducting qubits around 2014 prompted IT companies
such as Google, IBM, and Microsoft to enter intofull-scale research and development of quantum
computers and triggered a second quantum-computer
boom, which has since continued as venture capital
investments continue to increase.
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Vol. 21 No. 6 June 2023

Facing the added perspective of security, researchand development of quantum technologies, such as
quantum computers, quantum security, and quantum
sensing, is in the midst of fierce global competition.
Around 2015, Europe, the U.S., and China began tosignificantly expand government support under theirquantum technology strategies. Although a few years
behind them, Japan formulated the “Quantum Technology and Innovation Strategy” in 2020 [1] and the“Vision of Quantum Future Society” in 2022 [2],which aims to create new industries and business
opportunities and address social issues on the basis ofquantum technology. In response to this action, theQuantum STrategic industry Alliance for Revolution(Q-STAR) [3] was established in 2021 with the aimof making Japan a “quantum technology innovationoriented nation,” and Q-STAR is accelerating efforts
to implement quantum technology in society.
2. Fundamental characteristics of quantum
nature and application areas
As shown on the left side of Fig. 1, the world welive in is described by classical mechanics represented by Newtonian mechanics, i.e., a world that can be
actually observed and touched. In contrast, as shownon the right side, the quantum world represented by
the behavior of atoms and electrons is defined by
17

Feature Articles

Classical mechanics:
Newtonian mechanics (1687)ddt(mv) = F

In the macroscopic (real) world, objects,
particles, etc. exist with certainty ata certain position x.

Quantum mechanics (1900 −)i (r,t )
t

=

2

–

2m

+ V (r)

 (r,t ) In the microscopic world, this certainty
breaks down (uncertainty principle).

Fig. 1. Contrast between the real (classical) world and quantum world.

Duality

Quantum superposition

Property of particles
Two properties: particles andwaves

One quantum has multiple statesat the same time.

Property of waves
Quantum entanglement

Individual states of the quanta are undetermined;
only their relationships are fixed.
The quanta affect each other instantly, even when
they are at a distance from each other.

Achievement of large-capacity, high-precision, high-reliability, and low-power-consumption computing and communications
 Potential for innovation in various fields

Fig. 2. Three characteristics of quantum behavior.

mathematical expressions that represent waves withexistence probability under the laws of quantum
mechanics, therefore far from our common sense andexperience. There are various types of quanta, including extremely small ones, such as atoms and electrons, as well as light and relatively large superconducting quantum circuits; however, as shown in
Fig. 2, regardless of the type, they all share the same
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Vol. 21 No. 6 June 2023

characteristics of duality, quantum superposition,and quantum entanglement. Duality is the simultaneous possession of two properties: property of particles and property of waves. Quantum superposition
enables two values, “0” and “1,” to be held in a single
state, as illustrated in the figure with the upward
arrow (corresponding to “0”) and downward arrow(corresponding to “1”) side by side. Quantum
18Feature Articles(1) Quantum computing(2) Quantum communications
Quantum security
Used forcomputation as bits

Carried safely overgreat distances
Absolute
security

Ultra-highspeed

Quantum(3) Quantum sensing

Altered by the external
environment

Create an environment
in which quantum
behavior is manifested(4) Quantum materials
Quantum devices

Newphenomena

Ultra sensitivity

Fig. 3. Four application areas of quantum technology.

entanglement is a phenomenon in which the individual states of two quanta are undetermined, but therelationship between them is fixed in a way that even
if the quanta are far apart, they can instantly affecteach other when one is measured, even if they are at
the other ends of Earth or even at opposites sides of
the universe. In 1935, Einstein and other researchers
published a paper arguing that the spooky behavior ofquantum entanglement could not be sufficiently
explained; therefore, quantum mechanics is incomplete. After 80 years of debate among many theorists
and experimentalists, in 2015, it was finally concluded that quantum entanglement is real. Note thatthe 2022 Nobel Prize in Physics was awarded forsignificant work concerning this quantum entanglement controversy. The hope that harnessing thesethree quantum wonders will lead to technologies that
achieve high-capacity, high-precision, high-reliability, and low-power-consumption computing and communications is drawing attention to the potential for
innovation in a wide variety of fields.
The four main application areas of quantum technology—as shown in Fig. 3—are (i) quantum computing, which uses quanta as bits for high-speed
computation; (ii) quantum communications andquantum security, which guarantee safety by using
the property that quanta cannot be duplicated; (iii)
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Vol. 21 No. 6 June 2023

quantum sensing, which uses the quantum sensitivity
to the external environment to execute highly sensitive detection; and (iv) quantum materials, which
create environments in which quantum behavior ismanifested, and quantum devices, which use functions unique to quanta.
3. Recent progress in application ofquantum technology
3.1 Quantum computers
Quantum computers can be broadly classified as
two types: gate-based quantum computers, which canrun a variety of algorithms and be used as generalpurpose computers, and Ising machines, which are
dedicated solvers for solving combinatorial optimization problems. Several companies have announced
commercial gate-based quantum computers that are
based on superconducting and ion-trap systems, butthey can currently solve only small-scale problems.
Various other approaches, such as using neutral
atoms, photons, and semiconductor quantum dots,
are also being researched and developed. In contrast,
Ising machines have reached the practical applicationlevel of solving real problems of a certain scale. NTT
reported that its coherent Ising machine called
LASOLVTM can solve combinatorial optimization
19

Feature Articles

Error tolerant

NISQ
No error correction

With error correction

Ion-trap
quantum computer

Opticalquantum
computer

Difficulty of actualization

Quantum simulator/
Ising machine

Ion-trap
quantum simulator
Quantum annealer(D-Wave Systems)

Topologicalquantum
computer

Quantum computer
based on siliconquantum dots

Superconducting quantum computer
Gate-based
quantum computing

Opticalquantum computer(NISQ)

PhysicalIsing machine(NTT, LASOLVTM)

Target time

Digital Ising machinein 10 years

in 20 years

in 50 years

Fig. 4. Overview of quantum-computer development.

problems (maximum cut problems) of a 100,000node graph 1000 times faster than a digital machine[4]. Annealing machines to solving combinatorial
optimization problems using digital technology havealso been commercialized by several Japanese companies, which are thereby giving Japan a strong presence in this field.
The development of quantum computers is illustrated on the basis of NTT’s own indexes in Fig. 4.
The current gate-based quantum computer is called anoisy intermediate-scale quantum computer (NISQ),
namely, a quantum computer that cannot be scaled up
due to noise, and its applications are limited by its
lack of error-correction capability. To implement
quantum error correction, a large number of qubits(several dozen to ten thousand) are combined to forma single logical qubit, which requires large-scale integration and connection between qubit chips via aquantum network. Many technical challenges, suchas the enormous size of the refrigerator and the complexity of the control system, must also be overcome.
Under these circumstances, it is still unclear whichquantum bit will be the most favored approach.
Superconducting qubits are not suitable for integration of 1000 qubits or more owing to their large element size; accordingly, silicon qubits based on semiconductor processing technology are promising for
NTT Technical Review

Vol. 21 No. 6 June 2023

achieving higher integration. Optical quantum systems—which can place a large number of qubits on atime axis—are also being studied. Topological quantum computers, which are protected by the inherently
stable property of matter called topology, thus do not
require error correction, are also attracting attention.
On top of these developments in hardware, researchand development of quantum algorithms, which
exploit the high speed of quantum computers, is also
continuing apace.
3.2 Q
 uantum communications and quantum
security
Since the Ministry of Internal Affairs and Communications took the lead in launching the world’s firstquantum-cryptography testbed called the TOKYOQKD network in 2010, Japan has possessed worldclass technological capabilities in quantum-cryptography communications, and verification experiments
for genome information, electronic medical records,
financial transactions, etc. using the TOKYO QKD
network are underway. China has established a
2000-km-long quantum-cryptography network
stretching from Shanghai to Beijing, and is further
extending it by using satellites. Since quantum cryptography requires the use of extremely weak light,
namely, a single photon (the smallest unit of light),
20

Feature Articles

the transmission distance of quantum cryptography islimited due to transmission loss in optical fibers, andthat limit is thought to be about 100 km for actual use.
Post-quantum cryptography (PQC), a cipher thatcannot be solved even by a quantum computer, isunder development, mainly in the U.S., and hybrid
schemes combining quantum cryptography and modern cryptography such as PQC are also being developed. Quantum relay technology is an essential element of the quantum internet, and quantum memory
to retain the received quantum state is being actively
researched. All-photonic quantum repeaters, which
do not use quantum memory, are also being demonstrated.
3.3 Quantum sensing devices and materials
One type of quantum sensing is diamond nitrogenvacancy (NV) centers, in which quantum effects exist
even at room temperature, and are expected to be
used to detect magnetic fields and temperature withhigher sensitivity than possible with conventional
sensors. Applying quantum sensing to medicine anddrug discovery is also being investigated, and technologies for real-time observation of drug efficacy
using magnetic resonance imaging by combining
materials containing isotope-controlled elements andnuclear-spin hyperpolarization technology are rapidly emerging.
Other developments include quantum inertial sensors based on atom-wave interferometers, which
theoretically offer ten orders of magnitude higher
performance than fiber-optic gyroscopes, and ultrahigh-precision clocks (optical lattice clocks). Quantum devices, including single-photon and entangledphoton light sources, highly sensitive photodetectors,
single-electron devices, and spin Seebeck devices
and quantum materials, including diamond NV centers, quantum dots, topological materials, and atomic-layer materials, have attracted attention.
4. Future prospects for quantum technology
As mentioned above, the creation of a quantum
computer that can surpass current digital computersin terms of computational power is still a long way
off; even so, quantum cryptography and quantum
sensing are expected to be implemented in society inthe near future if the cost issues can be overcome. It
is envisioned that there will be a demand for quantum
connections between quantum computers andbetween quantum sensors and quantum computers,
which will require a network capable of transmitting
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quantum states, that is, a quantum internet. The current structure of the Internet, however, cannot handle
quantum states, so innovative infrastructures such as
the IOWN (Innovative Optical and Wireless Network) All-Photonics Network will become increasingly important. Quantum computing is also important from the perspective of energy consumption in
the IT industry because, in principle, it is energyneutral computing.
The Feature Articles in this issue discuss an opticalquantum computer [5], quantum-information technology with superconducting qubits [6], an opticallattice-clock network [7], fast algorithms for quantum computers [8], high-performance quantum-key
distribution [9], and the all-photonic quantum internet [10] as representative examples of NTT’s quantum technology.
References
[1]

[2]

[3]
[4]

[5]

[6]

[7]

[8]

[9]

Integrated Innovation Strategy Promotion Council, “Quantum Technology and Innovation Strategy (Final Report),” Jan. 2020 (in Japanese). https://www8.cao.go.jp/cstp/tougosenryaku/ryoushisenryaku.
pdf
Outline in English, https://www8.cao.go.jp/cstp/english/outline_
quantum.pdf
Integrated Innovation Strategy Promotion Council, “Vision of Quantum Future Society,” Apr. 2022 (in Japanese).
https://www8.cao.go.jp/cstp/ryoshigijutsu/ryoshimirai_220422.pdf
Outline in English, https://www8.cao.go.jp/cstp/english/outline_
vision.pdf
The Quantum STrategic industry Alliance for Revolution (Q-STAR),
https://qstar.jp/en
NTT press release, “100,000-spin coherent Ising machine—Highspeed solution search for large-scale combinatorial optimization
problems enabled with a large-scale optical computer—,” Sept. 2021.
https://group.ntt/en/newsrelease/2021/09/30/210930a.html
T. Hashimoto, T. Umeki, T. Kashiwazaki, and A. Inoue, “Optical
Technologies for Optical Quantum Computing with Continuous Variables,” NTT Technical Review, Vol. 21, No. 6, pp. 23–28, June 2023.
h ttp s ://n tt- r e v ie w. jp /a r c h iv e /n ttte c h n ic a l. p h p ? c ontents=
ntr202306fa2.html
S. Saito, K. Mizuno, T. Takenaka, H. Toida, and K. Kakuyanagi,“Quantum Information Technology Based on Superconducting Quantum Circuits,” NTT Technical Review, Vol. 21, No. 6, pp. 29–35, June2023.
h ttp s ://n tt- r e v ie w. jp /a r c h iv e /n ttte c h n ic a l. p h p ? c ontents=
ntr202306fa3.html
K. Oguri, T. Akatsuka, H. Imai, T. Hashimoto, and T. Sogawa,“Optical-lattice-clock-network Technology for Gravitational Potential Sensing,” NTT Technical Review, Vol. 21, No. 6, pp. 36–42, June2023.
h ttp s ://n tt- r e v ie w. jp /a r c h iv e /n ttte c h n ic a l. p h p ? c ontents=
ntr202306fa4.html
S. Tani, S. Akibue, and Y. Takeuchi, “Extracting Quantum Power byUsing Algorithms and Their Verification,” NTT Technical Review,Vol. 21, No. 6, pp. 43–47, June 2023.
h ttp s ://n tt- r e v ie w. jp /a r c h iv e /n ttte c h n ic a l. p h p ? c ontents=
ntr202306fa5.html
T. Ikuta and S. Akibue, “Improving the Performance of Quantum Key
Distribution,” NTT Technical Review, Vol. 21, No. 6, pp. 48–52, June2023.
h ttp s ://n tt- r e v ie w. jp /a r c h iv e /n ttte c h n ic a l. p h p ? c ontents=

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ntr202306fa6.html[10] K. Azuma, “Toward a Quantum Internet,” NTT Technical Review,Vol. 21, No. 6, pp. 53–57, June 2023.

h ttp s ://n tt- r e v ie w. jp /a r c h iv e /n ttte c h n ic a l. p h p ? c o ntents=
ntr202306fa7.html

Tetsuomi Sogawa

Senior Vice President, Basic and Advanced
Research Principal, NTT Science and Core Technology Laboratory Group.
He received a B.S., M.S., and Ph.D. in electrical engineering from the University of Tokyo in
1986, 1988, and 1991. In 1991, He joined NTT
Basic Research Laboratories (NTT-BRL). From1999 to 2000, he was a guest scientist at Paul
Drude Institute in Berlin, Germany, investigating
acoustic spin manipulation in semiconductor
quantum structures. From 2004 to 2006, heworked for the Council for Science and Technology Policy, Cabinet Office, Japan. He served as
the director of NTT-BRL from 2013 to 2019 andthe director of NTT Science and Core Technology Laboratory Group from 2018 to 2022. He has
been a visiting professor at the Institute of Industrial Science, the University of Tokyo, since2014. He is a fellow of the Japan Society ofApplied Physics.

NTT Technical Review

Vol. 21 No. 6 June 2023

22

Feature Articles: Toward Quantum Technology
Innovation

Optical Technologies for Optical
Quantum Computing with
Continuous Variables
Toshikazu Hashimoto, Takeshi Umeki,
Takahiro Kashiwazaki, and Asuka Inoue
Abstract

Quantum computers are increasingly seen as a potential solution for computing problems that are difficult to solve with conventional technologies. A photon-based quantum computer is a promising technology that enables the development of large-scale, universal quantum computing at high speeds at room
temperature with large-scale quantum entanglement thanks to photons’ inherent characteristics. This
article explores NTT’s efforts to develop optical quantum computers using optical fiber communication
technology.
Keywords: continuous-variable quantum state, quantum computer, optical component

1. Introduction
For sustainable development of society, computer
technology must also be developed sustainably toaddress increasingly complex and challenging issues
such as the economy, climate change, energy, andexploration of materials and living phenomena
beyond our current knowledge. The rapid growth ofsemiconductor technology, as exemplified by
Moore’s law, is reaching its limits. Consequently,
there are growing expectations for new computing
technologies that can solve problems beyond thecapabilities of conventional computers in a practical
timeframe. Quantum computers that leverage quantum properties to execute calculations are attracting
attention as a potential solution. Quantum algorithms
use physical states with quantum properties, such as
superconductivity, ion traps, cooled atoms, and electrons, as a medium for calculation. Using a quantumof light as a computational medium is emerging as aleading candidate of an enabler for quantum computers. It possesses three key features: (1) it operates atroom temperature *1, (2) can achieve large-scalequantum entanglement, and (3) operates at highNTT Technical Review

Vol. 21 No. 6 June 2023

speeds. While feature (1) is intrinsic to photons, features (2) and (3) require technology that uses thecharacteristics of photons. NTT is working toward
developing a photon computer by applying opticaldevice technology for optical fiber communications.
This article outlines NTT’s efforts toward opticalquantum computers.
2. Continuous-variable
optical-quantum-information processing andlarge-scale quantum-entanglement generation
In many quantum computers, physical states called
qubits are spatially arranged, and quantum information is processed using the superposition of thesestates or quantum correlations (quantum entanglements). The optical quantum computer introduced inthis article, however, processes quantum informationon the basis of the quantum state of the amplitude of*1 It operates at room temperature: By converting the photon energy
ε to a temperature T using Boltzmann’s constant (kB), T = ε/kB is
2.93 × 104 K. This temperature is high enough for the thermal
noise generated at room temperature, suggesting that the system
can effectively operate at room temperature.

23

Feature Articles

SIN

Amps. of COS and SIN are
displayed as points on thecomplex plane

Local
oscillator

COS

Input signal

COS
SIN

Superposition of SIN and COS
SIN
COS

COS

Oscillator(Laser)COS
SIN Modulator
+90°

SIN

+90°

SIN

Output signal

COS

Detector

SIN
DSP

COS

Optical fiber
Quadrature phase and amp.
detection

Quadrature phase and amp.
modulation(a) Optical fiber communications (Digital coherent transmission)
Distribution of the squeezed state on
the complex plane (COS, SIN)
Squeezed state superposition
SIN

Squeezed pulse

Entangled state of 4 squeezed pulses
Optical fiber

Quadrature phase and amp.
measurement

SIN
Quantum
light sources(Squeezers)

1-bit
delay

COS Beam splitter

COS
Quantum state generator forstate transfer
Quantum
Optical switch
operations/modulator

Multi-bit
delay

Error
corrections

Control unit
e.g. FPGA for
operations(b) Continuous-variable optical quantum computer
DSP: digital signal processor
FPGA: field-programmable gate array

Fig. 1. Configuration of optical fiber communications and continuous-variable optical quantum computer.

light. (The amplitude of light is represented by a continuous complex number. When it is quantized, it
becomes what is called a continuous-variable quantum state.) It takes advantage of the fact that wave
superposition, interference, and so on are phenomenathat correspond directly to quantum operations. Optical quantum computing makes arranging pulse-likephoton states in the time domain possible. It thus
enables an optical quantum computer to executelarge-scale operations without restrictions due to thespatial arrangement of quantum qubits used withother physical quantum computers. Large-scaleentanglement states to execute large-scale operations
can also be generated relatively easily using simple
devices such as beam splitters, thanks to the wave
nature of photons. Continuous-variable optical quantum computers use superposition of optical quadrature phase amplitudes, similar to that used in digitalcoherent transmission technology for optical fiber
communications (Fig. 1(a)). The digital coherent
transmission superposes modulated quadrature comNTT Technical Review

Vol. 21 No. 6 June 2023

ponents (cosine and sine components) of laser light(coherent light) for transmitting. Each quadrature isacquired by interfering with the light of a local oscillator reconstructing the original signal by using digital signal processing for receiving. However, simply
adding the cosine and sine components does notproduce a superposition of quantum states used incontinuous-variable optical quantum computers. To
obtain quantum effects, we need to use a quantum
state of light called squeezed light, which has intentionally distorted distribution. Figure 1(b) shows anintentionally biased distribution of squeezed light asa dotted ellipse, superposing two quantum states
spread in the cosine and sine directions. A continuous-variable optical quantum computer calculates byexecuting quantum operations on squeezed light.
Pulses of squeezed light (the spheres in Fig. 1(b))
emitted from a squeezed light source (quantum lightsource) propagate through an optical fiber and areseparated by a beam splitter that functions as a half
mirror to achieve a state in which different squeezed
24Feature Articles

Table 1. C orrespondence and differences between optical fiber communications and continuous-variable optical quantum
computer.
Optical fiber communications

Continuous-variable optical quantum computer

Operations

Digital coherent transmission(compensates for large dispersion)

Measurement-based quantum computing(computation with large quantum entanglement)

Representation space forsignals and information

Coherent light (carrier wave)
and quadrature phase and amp.

Squeezed lightand quadrature phase and amp.

Generation of carrier waves

Laser light source (coherent light source)

Quantum light source (squeezed light source)

Beam splitter

Beam splitter

Signal generationand quantum operations

Phase shifter

Phase shifter

Modulator

Displacement operator (modulator)
Squeezing operator
Third-order nonlinear operator

Signal demodulation

Quadrature phase and amplitude detection

Quadrature phase and amplitude measurement
Red text indicates differences.

light pulses are superposed simultaneously. Thesestates are entangled quantum-mechanically, and hugeentangled states can be created by shifting lightpulses in the time domain and repeatedly mixing
them with beam splitters [1]. This massively entangled state enables measurement-based quantum calculations, which are known to correspond to gatetype quantum calculations. Measurement-based
quantum computing*2 is a very interesting computational method. However, we focus on the technology
required for continuous-variable optical quantum
computers and the correspondence of optical fiber
communication technology as an optical technology
for continuous-variable optical quantum computers.
3. Application of optical fiber communication
technology to a continuous-variable opticalquantum computer
Table 1 shows the similarities and differences
between optical fiber communications and continuous-variable optical quantum computers. To conduct
arbitrary continuous-variable quantum calculations, asqueezer, demultiplexer (beam splitter), phase shifter,
displacement operation (optical modulator), andthird-order nonlinear gate are all necessary [2, 3]. The
beam splitter, phase shifter, and optical modulator are
elements used in classical optics and components of
optical fiber communications. Continuous-variable
optical quantum computers also use quadratureamplitude measurements with the same configuration
as that used for quadrature amplitude detection in
NTT Technical Review

Vol. 21 No. 6 June 2023

optical fiber transmissions. The squeezers and thirdorder nonlinear gates require technologies not directly used in optical fiber communications. As a quantum light source corresponding to a laser light sourcein optical fiber communications, a squeezer is an
essential element of an optical quantum computer.
However, a squeezer with good characteristics applicable to an optical quantum computer had not been
implemented because it requires large optical nonlinearity to generate squeezed light.
To achieve squeezer characteristics suitable for
optical quantum computing, NTT has implemented awaveguide-type periodically poled lithium niobate(PPLN), which has been researched and developed
for optical fiber communications. For optical fiber
communications, phase-sensitive amplification, astate of an optical parametric amplifier, has been considered a preferred technology that does not increase
noise. NTT has been developing waveguide-type
PPLN technology to provide high-performance optical parametric amplifiers. PPLN is an optical device
that enhances nonlinear conversion efficiency byperiodically inverting the polarization of lithium niobate, a nonlinear optical crystal. NTT further*2 Measurement-based quantum computing: A method of quantum
computation that achieves operations equivalent to quantum operations by repeatedly changing the projection direction of a projection-valued measure for subsequent pulses on the basis of themeasurement results. A projection-valued measure obtains a specific state component as output by changing the incident state orreference state in a quadrature phase amplitude measurement. A
typical example is a Bell measurement that determines the components of a quantum entangled state called a “Bell state.”

25Intensity [dBm/nm]

0−10Intensity [dBm/nm]

Feature Articles

Multi-pulse delay(Optical fiber)

Shot noise level
Squeezed level

Quantum light source(PPLN squeezer)

−20
−30

Beam splitter

Waveguide-type PPLN

−40
−50

6.0 THz

50 cm−601490 1510 1530 1550 1570 1590
Wavelength [nm](a) Bandwidth of quantum light source(b) Quantum-light-source
module1-pulse delay(Optical fiber)(c) Components of a large-scale quantumentanglement generator

Fig. 2. Q uantum-light-source module and fiber-based continuous-variable optical quantum computer (components of alarge-scale quantum-entanglement generator).

processed the optical waveguide structure on the
PPLN and demonstrated that it is applicable as anamplifier for long-distance transmission, which had
been difficult, by confining the light propagating in
the optical waveguide in a narrow area and increasing
the nonlinearity [4]. We have confirmed that thishigh-performance waveguide PPLN technology has
advantages not only in phase-sensitive amplification
but also in quantum light sources, or squeezers, by
fabricating quantum light sources. Figure 2 shows alarge-scale quantum-entanglement generator for acontinuous-variable optical quantum computer consisting of a quantum-light-source module and fibertype devices. The quantum-light-source module uses
a waveguide-type PPLN to improve nonlinear conversion efficiency, eliminating the need to reciprocate
inside the resonator to obtain the optical path length
required for conversion, which causes frequency
dependence, and expands the bandwidth to 6 THz. It
also achieves a squeezing level of 6 dB, an index of
the asymmetry of compression in the distribution of
the squeezing light. It was the highest squeezing level
achieved by a non-resonator type of squeezer [5].
This wide bandwidth can accommodate many quantum pulses and various quantum states corresponding
to waveforms in optical fiber communications. We
have demonstrated that the module can generate
pulses of arbitrary quantum states in time slots [6].
It has been too difficult to achieve third-order nonlinear gates because they require larger optical element nonlinearities than those needed for squeezers.
However, an alternative method of generating thirdNTT Technical Review

Vol. 21 No. 6 June 2023

order nonlinearity using quantum teleportation has
been proposed [7] and is becoming feasible as technology advances.
4. Summary and future prospects
We have shown that we can provide the components of a continuous-variable optical quantum computer by using optical fiber communication technology. As well as optical communication components
acting as components of optical quantum computers,
further developments in component technology for
optical fiber communications will enable the implementation of important optical quantum computercomponents such as squeezers and third-order nonlinear gates. For a continuous-variable optical quantum computer to operate as a system, in addition tothe optical devices, it is necessary to develop theelectronic devices that control optical components
and the software that operates them. Measurementbased quantum calculations executed by a continuous-variable optical quantum computer require thehigh-speed operation of electronic circuits and control systems because the measurement results mustreflect instantaneously to update the projection-valued measure system and quantum states. The middleware will also become essential to reflect quantum
algorithms in actual devices. By addressing thesechallenges, NTT aims to develop a continuous-variable optical quantum computer using optical components by 2030 and a chip for an optical quantum
computer by 2050, as shown in Fig. 3. A challenge is26

Feature Articles

Chip-scale integrated
opticsFiber optics
Spatial optics

2020
Implementation with lab
equipment

2030
Optical quantum
computer in a rack2050
All-optical quantum
computer on chipFig. 3. Roadmap for continuous-variable optical quantum computers.

improving the characteristics of current optical devices. For example, the squeezing level of the squeezer
as a quantum light source for fault-tolerant operations, which use GKP codes to error-correct largescale quantum entangled states, must be several dB
higher than the current level of 6 dB [8]. The losses
introduced by the constituent optical components
also remain a significant noise source that disturbs
the quantum state, thus must be reduced. These arechallenges that are difficult to overcome even withthe high-performance device technology cultivated in
optical fiber communications. Through our efforts toachieve a continuous-variable optical quantum computer, we aim to further advance optical technology
and bring about explosive developments in quantum
computing and optical fiber communications.

References
[1]

[2]
[3]
[4]

[5]

Acknowledgments
This research was conducted in collaboration with
the Furusawa Laboratory, The University of Tokyo.
Part of this research was also supported by Japan Science and Technology Agency (JST), Moonshot R&D
program JPMJMS2064.

[6]

[7]

[8]

NTT Technical Review

Vol. 21 No. 6 June 2023

W. Asavanant, Y. Shiozawa, S. Yokoyama, B. Charoensombutamon,
H. Emura, R. N. Alexander, S. Takeda, J. Yoshikawa, N. C. Menicucci, H. Yonezawa, and A. Furusawa, “Generation of Time-domainmultiplexed Two-dimensional Cluster State,” Science, Vol. 366, No.
6463, pp. 373–376, Oct. 2019. https://doi.org/10.1126/science.
aay2645
L. Seth and S. L. Braunstein, “Quantum Computation over Continuous Variables,” Phys. Rev. Lett., Vol. 82, No. 8, 1784, Feb. 1999.
https://doi.org/10.1103/PhysRevLett.82.1784
S. L. Braunstein and P. V. Loock, “Quantum Information with Continuous Variables,” Rev. Mod. Phys., Vol. 77, No. 2, 513, 2005.
https://doi.org/10.1103/RevModPhys.77.513
T. Kobayashi, S. Shimizu, M. Nakamura, T. Umeki, T. Kazama, R.
Kasahara, F. Hamaoka, M. Nagatani, H. Yamazaki, H. Nosaka, and Y.
Miyamoto, “Wide-band Inline-amplified WDM Transmission Using
PPLN-based Optical Parametric Amplifier,” J. Light. Technol., Vol.
39, No. 3, pp. 787–794, Feb. 2021. https://doi.org/10.1109/
JLT.2020.3039192
T. Kashiwazaki, T. Yamashima, N. Takanashi, A. Inoue, T. Umeki,
and A. Furusawa, “Fabrication of Low-loss Quasi-single-mode PPLN
Waveguide and Its Application to a Modularized Broadband Highlevel Squeezer,” Appl. Phys. Lett., Vol. 119, 251104, 2021. https://doi.
org/10.1063/5.0063118
K. Takase, A. Kawasaki, B. K. Jeong, T. Kashiwazaki, T. Kazama, K.
Enbutsu, K. Watanabe, T. Umeki, S. Miki, H. Terai, M. Yabuno, F.
China, W. Asavanant, M. Endo, J. Yoshikawa, and A. Furusawa,“Quantum Arbitrary Waveform Generator,” Sci. Adv., Vol. 8, No. 43,
2022. https://doi.org/10.1126/sciadv.add4019
K. Miyata, H. Ogawa, P. Marek, R. Filip, H. Yonezawa, J. Yoshikawa,
and A. Furusawa, “Implementation of a Quantum Cubic Gate by an
Adaptive non-Gaussian Measurement,” Phys. Rev. A, Vol. 93, No. 2,022301, 2016. https://doi.org/10.1103/PhysRevA.93.022301
K. Fukui, “High-threshold Fault-tolerant Quantum Computation with
the GKP Qubit and Realistically Noisy Devices,” arXiv:1906.09767v1,
June 2019.

27

Feature Articles

Toshikazu Hashimoto

Senior Distinguished Researcher, Group
Leader, Optoelectronics Integration ResearchGroup, NTT Device Technology Laboratories.
He received a B.S. and M.S. in physics from
Hokkaido University in 1991 and 1993 and Ph.D.
in engineering from Kyushu University in 2022.
He joined NTT Photonics Laboratories in 1993
and has been researching hybrid integration ofsemiconductor lasers and photodiodes on silicabased planar lightwave circuits and conducting
theoretical research and primary experiments on
the wavefront-matching method. He is a memberof the Institute of Electronics, Information andCommunication Engineers (IEICE), the PhysicalSociety of Japan, and the Japan Society ofApplied Physics (JSAP).

Takeshi Umeki

Distinguished Researcher, HeterogeneousMaterials and Devices Research Group, NTT
Device Technology Laboratories.
He received a B.S. in physics from Gakushuin
University, Tokyo, in 2002 and M.S. in physics
and Ph.D. in nonlinear optics from the Universityof Tokyo, in 2004 and 2014. He joined NTT
Photonics Laboratories in 2004 and has been
involved in research on nonlinear optical devices
based on PPLN waveguides. He was a visiting
researcher at the National Institute of Standards
and Technology (NIST), Boulder, CO, USA in
2020. He is a member of JSAP, IEICE, and theInstitute of Electrical and Electronics Engineers(IEEE) Photonics Society.

NTT Technical Review

Vol. 21 No. 6 June 2023

Takahiro Kashiwazaki

Researcher, Heterogeneous Materials andDevices Research Group, NTT Device Technology Laboratories.
He received a B.E. and M.E. in materials science from Keio University, Kanagawa, in 2013
and 2015. In 2015, he joined NTT Device Technology Laboratories, where he has been engaged
in research on nonlinear optical devices for optical communication and quantum computation.
He is a member of IEICE and JSAP.

Asuka Inoue

Researcher, Heterogeneous Materials andDevices Research Group, NTT Device Technology Laboratories.
She received a B.E., M.E., and Ph.D. in electrical and electronic engineering from Kobe University, Hyogo, in 2015, 2016, and 2019. In 2019,she joined NTT Device Technology Laboratories, where she has been engaged in research on
waveguide devices for optical quantum computation. She is a member of JSAP.

28

Feature Articles: Toward Quantum Technology
Innovation

Quantum Information Technology
Based on Superconducting Quantum
Circuits
Shiro Saito, Kosuke Mizuno, Takaaki Takenaka,
Hiraku Toida, and Kosuke Kakuyanagi
Abstract

The research on superconducting quantum circuits has expanded greatly, from applications of magnetic field sensors using superconducting quantum interference devices to the development of quantum
computers based on superconducting quantum bits (qubits). Superconducting quantum circuits havebeen progressing rapidly, and as examples of applying these circuits to quantum sensing, this article
introduces electron spin resonance spectrometers having high sensitivity and high spatial resolution andan ultrasmall high-sensitivity thermometer operating at extremely low temperatures. As an application
to quantum computing, this article also introduces a bosonic qubit that executes quantum error correction
using the many degrees of freedom within a superconducting resonator.
Keywords: superconducting quantum circuits, quantum sensing, quantum computers

1. Superconducting quantum circuits
Superconducting quantum circuits*1 are being studied extensively from basic to application research bytaking advantage of the degrees of freedom in theircircuit design. We first introduce an example of basic
research. The origin of research into superconducting
quantum circuits lies in the fundamental question: To
what extent can quantum mechanics that holds in the
microscopic world be applied to macroscopic systems? To answer this question, we conducted anexperiment using a superconducting flux qubit*3 anddemonstrated that a qubit loop can achieve quantum
superposition in which a trillion electrons per second
are flowing in a clockwise current state and counterclockwise current state [1]. This result indicated thatquantum mechanics holds in macroscopic artificial
structures on a micrometer order as well as in microsystems consisting of electrons and atoms. Next,
through collaborative research with the National
Institute of Information and Communications Technology (NICT), we devised a circuit design that can
NTT Technical Review

Vol. 21 No. 6 June 2023

achieve strong coupling between an artificial atomand light, which cannot be achieved in ordinary atom/
light coupling. This achievement has opened the door
to the new field of cavity quantum electrodynamics*4.
We used a superconducting artificial atom (superconducting flux qubit) and microwave photons toachieve a deep-strong coupling regime, in which thecoupling energy of both is greater than the energy of*1 Superconducting quantum circuits: Superconducting circuits that
include circuit elements described by quantum mechanics. Circuit elements making up superconducting quantum circuits include Josephson junctions*2, inductors, and capacitors.
*2 Josephson junctions: A structure in which an extremely thin insulator is sandwiched between two superconductors. Since a nonlinear superconducting current flows in relation to the superconductor phase, Josephson junctions can be used as nonlinear devices in superconducting quantum circuits. In a superconducting
qubit, aluminum and aluminum oxide are widely used as the Josephson junction’s superconductor and insulator, respectively.
*3 Superconducting flux qubit: A superconducting circuit consistingof a superconducting loop including multiple Josephson junctions. Applying an optimal magnetic-field bias to this circuit generates two states—a clockwise current state and counterclockwise
current state—that can be treated as a quantum two-level system.

29

Feature Articles

each, and measured a previously unknown physicalphenomenon [2].
From the above studies, superconducting quantum
circuits are breaking new ground in basic physics
research, but there is also much activity in application
research. The most well-known example of suchapplications is the superconducting quantum computer being developed by Google, IBM, University of
Science and Technology of China, RIKEN, and otherresearch institutions. The superconducting quantum
circuits used for this application are qubits called
transmons, which can be used as the basic elementsof a quantum computer since their transition frequency remains nearly unchanged under an external
magnetic field and their coherence time*5 is long.
However, the transition frequency of a superconducting flux qubit changes significantly under an external
magnetic field, so its application to magnetic fieldsensors is expected. Progress is also being made at
D-Wave Systems, National Institute of Advanced
Industrial Science and Technology (AIST), and elsewhere in applying superconducting flux qubits to aquantum annealer*6 by using the tunability of thetransition frequency.
In this article, we introduce local electron spin
detection and a hybrid thermometer based on flux
qubits as examples of quantum sensors. We also
describe the prospects for a new type of quantum bit(bosonic qubit) that uses a transmon as an ancillaqubit.
2. Magnetic field sensors
A superconducting flux qubit can detect changes in
the magnetic field penetrating a loop with high sensitivity by measuring the transition frequency between
quantum levels. As described below, such a magnetic
field sensor is sensitive enough to detect a magnetic
field created by a small number of electron spins.
Since the loop size of a flux qubit is about several
micrometers, it also enables magnetic-field measurements with high spatial resolution. Taking the above
features into account, NTT began research on highsensitivity, high-spatial-resolution electron spin resonance (ESR) spectrometers.
Since ESR can be used to obtain information onelectrons that are not paired with each other (unpaired
electrons) in a substance, it has become an indispensable analytical technique in such fields as materialscience, solid state physics, drug discovery, andmedical care. A typical ESR spectrometer operates byplacing a several-milliliter spin sample in a cavity
NTT Technical Review

Vol. 21 No. 6 June 2023

Fig. 1. Local ESR spectrometer.

and measuring the microwave response at the cavity’s resonance frequency. Since the interaction
between the cavity and one electron spin is weak, it isnecessary with this technique to include about 1013
electron spins in the sample to measure an ESR signal. The ESR signal is averaged over the entire spin
sample, so local information cannot be obtained.
In contrast, a local ESR spectrometer using superconducting quantum circuits consists of a sensor,
such as a flux qubit or superconducting quantum
interference device (SQUID), and a readout circuit to
extract the information from the sensor (Fig. 1). On
irradiating the electron spin sample with microwaves,the magnetic field created by the spins changes andthe sensor detects that change. We apply magnetic
field B⊥ on the microtesla (µT) order for sensor control and magnetic field B|| on the millitesla (mT) orderfor electron spin polarization. In this spectrometer,
the irradiation of microwaves is executed from awideband microwave line for electron spin detection
without a resonator, so there is no limitation to thefrequency range in which ESR spectra can be measured. In other words, this spectrometer makes it possible to measure ESR spectra in a broad parameterregion while sweeping the device with frequencies
and magnetic field B|| to therefore obtain even more
information from the measurement target [3].
*4 Cavity quantum electrodynamics: Quantum theory that describes
the interaction between an atom and light confined within a cavity. It is also called circuit quantum electrodynamics when describing an interacting system between an artificial atom configured from a superconducting quantum circuit and microwaves ina superconducting resonator.
*5 Coherence time: The length of time that a qubit can retain its
quantum information.
*6 Quantum annealer: Equipment that uses the properties of quantum mechanics to solve optimization problems that are difficult
to solve with current computers as the scale of the problem increases.

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Feature Articles

Sensitivity (spins/Hz1/2)

106

SQ+SW
SQ+JBA
FQ+SW
FQ+JBA
RES

105

104

103

102

101
100

101

102

103

104

105

Sensing volume (fL)
FQ: flux qubit
JBA: Josephson bifurcation amplifier
RES: resonator
SQ: SQUID
SW: SQUID switching readout circuit

Fig. 2. Figure of merit of local ESR spectrometers.

Next, we introduce our efforts to date in improving
the figure of merit of local ESR spectrometers. At the
beginning of this research, we used SQUID as a magnetic field sensor and a SQUID switching readout
circuit to extract the sensor information. With thisconfiguration, we succeeded in detecting electron
spins originating in erbium atoms within an opticalcrystal (Y2SiO5) at a sensitivity of 106 spins/√Hz [4].
We then replaced SQUID as the magnetic field sensor
with a flux qubit and changed the readout circuit froma SQUID switching readout circuit to a switchingreadout circuit using a Josephson bifurcation amplifier*7. We achieved a sensitivity of 20 spins/√Hz,
resulting in an improvement of approximately fiveorders of magnitude [5]. By making the loop size ofthe magnetic field sensor smaller, we also succeeded
in decreasing the detection volume from 150 to 6 fL.
These results along with the figure of merit of a sensor using a two-dimensional superconducting resonator from other research groups are shown in Fig. 2.
3. Thermometers
We have succeeded in developing an extremely
small thermometer (Fig. 3) and making precise measurements of temperature at very low temperaturesby using an electron spin ensemble within a nanodiamond as a thermometer and a flux qubit as an electron
NTT Technical Review

Vol. 21 No. 6 June 2023

spin detector [6].
A compact temperature sensor has a small heat
capacity, and the quantity of heat flowing into thethermometer from the measured system is small. This
enables fast response speeds and measurements thatsuppress the back action to the measured system.
Such a temperature sensor can also improve the spatial resolution of temperature measurements, whichsuggests new applications as in measuring the spatial
distribution of temperature in a very small region.
However, developing a compact temperature sensor
and achieving high sensitivity at the same time is difficult. The reason for this is as follows. Temperaturesensing involves measuring the change in temperature of the amount of physical material making up thethermometer, but the amount of material decreases
when making the thermometer smaller, which lowers
measurement sensitivity. To solve this problem, we
achieved quantum sensing on the basis of a hybrid
system that couples a flux qubit and paramagnetic
electron spins within a nanodiamond (Fig. 3).
Various types of defects exist within a nanodiamond including nitrogen-vacancy (NV) centers*8 andP1 centers*9 that confine an electron. This electron
has spin, and applying a magnetic field from the outside creates a Zeeman split in which energy changes
in accordance with the spin state. The energy level ofan electron spin system is determined by the magnetic field as well as by hyperfine interaction with
nuclear spin and the energy of magnetic anisotropy
that appears when there is spin greater than or equal
to one. Under thermal equilibrium, the occupancy ofeach spin state is determined by temperature andenergy, so a nanodiamond gives rise to magnetization
dependent on temperature. This change in magnetization can be measured as a change in the magnetic flux
*7 Josephson bifurcation amplifier: A resonator that includes a Josephson junction exhibits a bistable state due to the nonlinearityof the junction. A device that uses this state to amplify an extremely small signal is called a Josephson bifurcation amplifier.
When reading a flux qubit, it makes use of the fact that the final
state of the resonator differs depending on the state of the qubit.
In contrast to a readout circuit using SQUID switching, a readout
circuit using a Josephson bifurcation amplifier features no generation of voltage by the resonator’s final state, negating the needfor cooling time to suppress heat buildup. This makes it possible
to make repeated measurements in a short period, thereby improving signal accuracy by averaging.
*8 NV centers: A compound defect in which a nitrogen atom replaces a carbon atom in diamond and lies adjacent to a carbon vacancy is called an NV center. It is a color center that produces acolor close to pink in diamond.
*9 P1 centers: A defect in which a nitrogen atom replaces a carbon
atom in diamond is called a P1 center. It is a color center thatgives diamonds their yellowish tint.

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Feature Articles

Temperaturechange ofnanodiamonds

Magnetization
change ofnanodiamonds

Flux change
through the qubit

Transition-frequency
shift of the qubit

Fig. 3. Hybrid thermometer.

Fig. 4. Figure of merit of thermometers.

penetrating the flux qubit (Fig. 3). On converting themagnetic-field sensitivity of the flux qubit to temperature sensitivity, we confirmed a high-sensitivity
figure of merit at 1.3 μK/√Hz at the base temperatureof 9.1 mK in a dilution refrigerator.
On comparing the figure of merit for various thermometers (Fig. 4), the above results enable the development of a compact and high-sensitivity thermometer. Commonly used resistance thermometers must
be calibrated for all temperature regions, but thetemperature change associated with the measured
magnetization can be theoretically predicted, whichmeans that measurements across all temperature
regions can be conducted by calibrating at only one
temperature point. There is also a type of sensor using
a single quantum dot that can serve as a compact
thermometer, but our thermometer has the advantage
of enabling the measurement of lattice temperature,
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Vol. 21 No. 6 June 2023

not the electron temperature, as with the quantum dot
thermometer. While the temperatures that can bemeasured with our thermometer are extremely low, it
is expected to contribute to measuring extremely low
temperatures at the region where accurate measurements have been difficult due to slow response
caused by thermometer self-heating and large heat
capacity.
4. Bosonic qubits
Research and development of superconducting
quantum computers is progressing steadily, as
reflected by the 2019 announcement from Googlethat it had proven “quantum supremacy” with a53-qubit processor that outperformed a conventional
supercomputer [7] and the 2022 announcement fromIBM of a 433-qubit processor. However, a massive
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Feature Articles

Table 1. D esigned and measured parameters of a bosonicqubit.
Designed

Measured

Storage cavity (S)

(MHz)

4419

4425

Ancilla qubit (A)

5368

5531

Readout resonator (R)

7534

7400

Anharmonicity of A

129

152.3

Coupling between S and A

2.06

4.2

Coupling between A and R

0.34

0.32

Fig. 5. Components of a bosonic qubit.

(a)

0

3
2
1

Excitation amplitude (arb. unit)

Energy

(b)

4

3

2

1

0

Ancilla relative frequency (MHz)Fig. 6. (a) Photon number state in the storage cavity and (b) spectrum of the ancilla qubit while the storage cavity is excited.

number of qubits will be necessary to achieve a practical and fault-tolerant universal quantum computer.
This is because redundant qubits are needed for quantum error correction since qubits are vulnerable toexternal noise and errors occur frequently. For example, a trial calculation indicated that 20,000,000
qubits would be needed to factor a 2048-bit number[8]. In current designs, about 1000 qubits can bearranged on a chip, and any greater number would
require new technologies such as for inter-chip wiring. There would also be a need for breakthroughs in
high-frequency wiring to control such a massive
number of qubits, refrigeration technology, etc.
The bosonic qubit is a technology that is attracting
attention as a means of satisfying the need for a massive number of qubits. This technology encodes
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quantum information into an infinite number ofenergy levels within a superconducting cavity anduses that redundancy to execute quantum error correction. It uses a transmon as an ancilla qubit to prepare complex quantum states in the cavity. A resonator is needed to read out the transmon state. The
components of this bosonic qubit are shown in Fig. 5.
We carefully designed the frequencies of each component (storage cavity, ancilla qubit, readout resonator) and the coupling strength between components(Table 1) and successfully identified photon number
states*10 in the storage cavity (Fig. 6). This is the firststep toward accessing each photon number state andpreparing complex quantum states.

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5. Outlook
This article introduced local ESR spectrometers, anultrasmall thermometer, and bosonic qubit as examples of applying advanced superconducting quantum
circuits to quantum information technology.
Our objective in developing local ESR spectrometers is to enhance sensitivity to the level of detecting
a single electron spin using a long-lived flux qubit [9]and a readout circuit applying quantum electrodynamics. We also plan to expand our measurementtargets with this spectrometer beyond solid-stateelectron spin samples to biological electron spin
samples such as neurons and cell lines.
Our objective in researching bosonic qubits is to
encode quantum information in a superconducting
cavity using the lowest order binomial code*11, abosonic quantum-error-correcting code. We will thenseek to achieve a long-lived bosonic qubit beyond its
cavity lifetime by quantum error correction, in otherwords, to reach the break-even point.

[3]

[4]

[5]

[6]

[7]

References
[1]

[2]

K. Kakuyanagi, Y. Matsuzaki, H. Toida, H. Yamaguchi, S. Saito, andW. J. Munro, “Demonstration of Realism Violation on a Macroscopic
Scale,” NTT Technical Review, Vol. 15, No. 7, July 2017.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201707fa4.html
S. Saito, K. Kakuyanagi, S. Ashhab, F. Yoshihara, T. Fuse, and K.
Semba, “Discovery of a Stable Molecular State Consisting of Photons
and an Artificial Atom,” NTT Technical Review, Vol. 15, No. 5, May
2017.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201705ra1.html

[8]
[9]

H. Toida, Y. Matsuzaki, K. Kakuyanagi, X. Zhu, W. J. Munro, H.
Yamaguchi, and S. Saito, “Electron Paramagnetic Resonance Spectroscopy Using a Single Artificial Atom,” Commun. Phys., Vol. 2, 33,
Mar. 2019. https://doi.org/10.1038/s42005-019-0133-9
H. Toida, Y. Matsuzaki, K. Kakuyanagi, X. Zhu, W. J. Munro, K.
Nemoto, H. Yamaguchi, and S. Saito, “Electron Paramagnetic Resonance Spectroscopy Using a Direct Current-SQUID Magnetometer
Directly Coupled to an Electron Spin Ensemble,” Appl. Phys. Lett.,Vol. 108, 052601, Feb. 2016. https://doi.org/10.1063/1.4940978
R. P. Budoyo, K. Kakuyanagi, H. Toida, Y. Matsuzaki, and S. Saito,“Electron Spin Resonance with up to 20 Spin Sensitivity Measured
Using a Superconducting Flux Qubit,” Appl. Phys. Lett., Vol. 116,
194001, May 2020. https://doi.org/10.1063/1.5144722
K. Kakuyanagi, H. Toida, L. V. Abdurakhimov, and S. Saito, “Submicrometer-scale Temperature Sensing Using Quantum Coherence of aSuperconducting Qubit,” New J. Phys., Vol. 25, 013036, Feb. 2023.
https://dx.doi.org/10.1088/1367-2630/acb379
F. Arute, K. Arya, R. Babbush, D. Bacon, J. C. Bardin, R. Barends, R.
Biswas, S. Boixo, F. G. S. L. Brandao, D. A. Buell, B. Burkett, Y.
Chen, Z. Chen, B. Chiaro, R. Collins, W. Courtney, A. Dunsworth, E.
Farhi, B. Foxen, A. Fowler, C. Gidney, M. Giustina, R. Graff, K.
Guerin, S. Habegger, M. P. Harrigan, M. J. Hartmann, A. Ho, M.
Hoffmann, T. Huang, T. S. Humble, S. V. Isakov, E. Jeffrey, Z. Jiang,
D. Kafri, K. Kechedzhi, J. Kelly, P. V. Klimov, S. Knysh, A. Korotkov,
F. Kostritsa, D. Landhuis, M. Lindmark, E. Lucero, D. Lyakh, S.
Mandrà, J. R. McClean, M. McEwen, A. Megrant, X. Mi, K. Michielsen, M. Mohseni, J. Mutus, O. Naaman, M. Neeley, C. Neill, M.
Yuezhen Niu, E. Ostby, A. Petukhov, J. C. Platt, C. Quintana, E. G.
Rieffel, P. Roushan, N. C. Rubin, D. Sank, K. J. Satzinger, V. Smelyanskiy, K. J. Sung, M. D. Trevithick, A. Vainsencher, B. Villalonga, T.
White, Z. J. Yao, P. Yeh, A. Zalcman, H. Neven, and J. M. Martinis,“Quantum Supremacy Using a Programmable Superconducting Processor,” Nature, Vol. 574, pp. 505–510, Oct. 2019. https://doi.
org/10.1038/s41586-019-1666-5
C. Gidney and M. Ekerå, “How to Factor 2048 Bit RSA Integers in 8
Hours Using 20 Million Noisy Qubits,” Quantum, Vol. 5, p. 433, Apr.
2021. https://doi.org/10.22331/q-2021-04-15-433
L. V. Abdurakhimov, I. Mahboob, H. Toida, K. Kakuyanagi, and S.
Saito, “A Long-lived Tunable Qubit for Bosonic Quantum Computing,” NTT Technical Review, Vol. 19, No. 5, May 2021. https://doi.
org/10.53829/ntr202105fa3*10 Photon number states: The state in which n microwave photons
exist within a cavity is called a photon number state and denoted
as |n〉. On exciting a storage cavity with microwaves, the superposition state of photon number states called a coherent state appears. Therefore, a single photon number state cannot be prepared. The nonlinearity of an ancilla qubit is necessary to prepare
photon number states.
*11 Lowest order binomial code: One type of quantum-error-correcting code for a bosonic qubit. It is a relatively simple code inwhich the logical words are defined as |0L〉 = (|0〉 + |4〉) ⁄ √2̄, and|1L〉 = |2〉 using photon number states in a cavity. If using an ancilla qubit, photon number parity can be measured while maintaining the quantum states in the cavity. A measurement result of
odd parity can be taken to mean that a photon loss has occurred,so adding a photon back to the cavity makes quantum error correction possible.

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34

Feature Articles

Shiro Saito

Senior Distinguished Researcher and Group
Leader of Superconducting Quantum Circuits
Research Group, NTT Basic Research Laboratories.
He received a B.E., M.E., and Dr. Eng. inapplied physics from the University of Tokyo in
1995, 1997, and 2000. He joined NTT Basic
Research Laboratories in 2000. Since then he has
been engaged in quantum information processing
using superconducting circuits. He was appointed as Distinguished Scientist of NTT in 2012 andSenior Distinguished Researcher in 2021. He
was a guest researcher at Delft University ofTechnology from 2005 to 2006. He was a guest
associate professor at Tokyo University of Science from 2012 to 2020 and promoted to a guest
professor in 2020. He is a member of the PhysicalSociety of Japan (JPS) and the Japan Society ofApplied Physics (JSAP).

Kosuke Mizuno

Research Associate, Superconducting Quantum Circuits Research Group, NTT Basic
Research Laboratories.
He received a B.E., M.E., and Dr. Eng. in electrical and electronic engineering from Tokyo
Institute of Technology in 2016, 2018, and 2021.
He joined NTT Basic Research Laboratories in
2021. Since then he has been engaged in thedevelopment of superconducting bosonic qubits.
He is a member of JPS and JSAP.

Hiraku Toida

Senior Research Scientist, Superconducting
Quantum Circuits Research Group, NTT Basic
Research Laboratories.
He received a B.A., M.A., and Ph.D. in arts andsciences from the University of Tokyo in 2008,2010, and 2013. He joined NTT Basic ResearchLaboratories in 2013, where he studied coherent
coupling between an erbium-doped crystal andsuperconducting flux qubit. From April to June
2014, he was with NTT Microsystem Integration
Laboratories, where he studied numerical simulation of plasmonic waveguides. He is currently
studying highly sensitive spin sensing using
superconducting quantum circuits. He is a member of JPS and JSAP.

Kosuke Kakuyanagi

Senior Research Scientist, Supervisor, Superconducting Quantum Circuits Research Group,
NTT Basic Research Laboratories.
He received a B.S., M.S., and Ph.D. in sciencefrom Hokkaido University in 2000, 2002, and2005. He joined NTT Basic Research Laboratories in 2005 and has been studying superconducting qubits. He is currently engaged in an experimental study of physics on superconducting
quantum circuits. He is a member of JPS andJSAP.

Takaaki Takenaka

Researcher, Superconducting Quantum Circuits Research Group, NTT Basic ResearchLaboratories.
He received a B.E. in applied physics, andM.E. and Ph.D. in materials science from the
University of Tokyo in 2015, 2017, and 2020. He
joined NTT Basic Research Laboratories in
2020. He is currently engaged in an experimental
study on bosonic code with superconducting
circuits and high-Q superconducting cavity to
improve the lifetime of both cavity mode andqubits. He is a member of JPS and American
Physical Society.

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35

Feature Articles: Toward Quantum Technology
Innovation

Optical-lattice-clock-network
Technology for Gravitational
Potential Sensing
Katsuya Oguri, Tomoya Akatsuka, Hiromitsu Imai,
Toshikazu Hashimoto, and Tetsuomi Sogawa
Abstract

Optical lattice clocks have an ultrahigh frequency accuracy that exceeds that of cesium atomic clocks
by a few orders of magnitude, enabling quantum sensing of gravitational potential equivalent to an altitude difference of only 1 cm on the Earth’s surface. An optical-lattice-clock network, in which multiple
optical lattice clocks are interconnected by optical fibers, is expected to become a new infrastructure,such as for a real-time sensing network of crustal movements with 1-cm-level accuracy. In this article,
we introduce the elemental technologies comprising an optical-lattice-clock network. We also report on
the construction of the ultrahigh-precision optical frequency transmission fiber link in the Tokyo metropolitan area and the experiment to evaluate its transmission stability.
Keywords: optical lattice clock, ultrastable optical frequency distribution, planar-lightwave-circuitbased optical repeater

1. What is an optical lattice clock?
An optical lattice clock refers to the world’s mostaccurate atomic clock*1, which uses the electronictransition in the optical wavelength region in the neutral atoms captured in a light cage (optical lattice) asa standard for clock ticking (frequency). It was proposed by Professor Hidetoshi Katori of the University of Tokyo (UTokyo) in 2001 [1]. Its time accuracy
has reached the level of “1 second difference in 30
billion years.” When one considers that generalquartz clocks are accurate to “1 second off in a day,”one realizes the tremendous accuracy of optical lattice clocks. The accuracy of a clock is generally
evaluated by the ratio of frequency uncertainty (Δf) toa clock frequency (f), and the accuracy of a current
optical lattice clock is Δf / f ~ 1 × 10−18. Our common
time is based on the cesium atomic clock, and 1 second in the world is defined as the time which an electron in cesium atom oscillates 9,192,631,770 times.
The typical accuracy of cesium atomic clocks today
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is at the level of 1 × 10−15 – 1 × 10−16, or “1 second
difference in 30 million years.” Optical lattice clocks
are already 2–3 orders of magnitude more accurate,
so they are being studied worldwide as potential candidates for the next-generation definition of the second. The mechanism of an optical lattice clock isshown in Fig. 1. The heart of an optical lattice clock
is an atom cooled to a very low temperature (–1 mK),trapped in an optical lattice*2 made at a special wavelength called a magic wavelength. According to*1 Atomic clock: A standard frequency generator that uses the resonant absorption frequency of an atom (the property of absorbing
and radiating electromagnetic waves of a fixed frequency or its
frequency) as a frequency reference. We refer to frequency as aclock in the sense that it is equivalent to a time reference rather
than being the inverse of time. The definition of seconds in the SI(International System of Units) unit system also uses an atomic
clock based on this principle.
*2 Optical lattice: A periodic electronic potential is formed by creating a standing wave by the light of an opposing laser. We can arrange laser-cooled atoms in a lattice with this periodic light-induced potential.

36

Feature Articles(a) Capture atoms with an optical lattice. A beam of atoms isejected into a vacuum at high temperatures (about 640 K) to be
slowed and cooled with a laser. Magnetic fields are also used togather about 100,000 atoms in one place then trap each atom inan optical lattice, arranging about 10,000 atoms well apart fromeach other. The photo shows an example of a Sr atom.

(b) Clock-frequency reading. The frequency of the laser
source is stabilized by matching it exactly to the resonant
frequency (clock frequency) of the atoms caught in the
optical lattice. The figure is drawn as a two-dimensional
optical lattice for clarity.

Fig. 1. How an optical lattice clock works.

quantum mechanics, the electrons in an atom absorb
electromagnetic waves only at a specific frequency
called the resonance frequency (electronic transition). An optical lattice clock enables us to read theresonance frequency by making the frequency of alaser oscillator copy (stabilize) to exactly match thisresonant frequency. For example, in the case of strontium (Sr), the most commonly used atom, the resonant frequency is 429 228 004 229 873.0 Hz, roughly
equivalent to 698 nm in wavelength. This resonant
frequency is extremely accurate and can become thefrequency standard when we assume no interaction
with the various environments surrounding the atom
(temperature, density, electric, magnetic fields, etc.).
However, the resonant frequency is easily affected,
and environmental fluctuations cause it to fluctuatefrom the value of the frequency that would be expected if it were not affected by the environment at all.
The optical lattice is designed to minimize the effectsof the environment by creating electronic potential on
the basis of the interference of light and catching each
atom cooled to extremely low temperatures. The
most outstanding feature of optical lattice clocks isthe ability to accurately read the resonant frequency
of atoms in a short period by simultaneously measuring many atoms with reduced environmental fluctuations.
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2. Expectations for relativistic geodesy by
networking optical lattice clocks
According to Einstein’s general theory of relativity,
when one compares two clocks placed at different
heights, the higher clock ticks faster (with higher
frequencies) due to the effect of Earth’s gravitational
potential. Using this principle, a network that enables
altitude-difference measurement using relativistic
effects (relativistic geodesy) is attracting attention as
an application of new ultrahigh-precision clocks.
This network will become possible by installing
many optical lattice clocks in multiple remote locations nationwide and connecting them via opticalfibers and measuring their frequency differences
remotely (Fig. 2). In fact, the relativistic effect on thisclock is well known in conventional atomic clocks.
For example, an atomic clock mounted on a satellite
orbiting at an altitude of 20,000 km has a frequency
about 10–10 higher than the Earth’s surface because
the Earth’s gravitational potential is larger than thatof the Earth’s surface. Optical lattice clocks withmuch improved accuracy compared with conventional atomic clocks have made it possible to detect
relativistic effects arising from slight differences ingravitational potential (differences in height) at thesurface of the Earth. Relativistic effects that appear
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According to Einstein’s theory of general relativity, height shift of an atomic clock of 1 cm causes fractionalfrequency shift of 1×10–18. Optical lattice clocks connected by an optical-fiber network will provide a spacetime information infrastructure based on gravitational-potential measurement.

Fig. 2. Relativistic geodesy with a nationwide optical-lattice-clock network.

only on the enormous spatial scale of space, such asouter space, can now be recognized as everyday
effects via optical lattice clocks. This is a revolutionized aspect of such clocks. An optical lattice clockwith a frequency accuracy of 1 × 10−18, which is currently the most accurate in the world, has a detectablegravitational potential equivalent to an elevation difference of approximately 1 cm [2]. If the elevation ofeach area is constantly monitored with an accuracy of
1 cm using an optical-lattice-clock network, we can
expect to expand the role of optical-fiber networks
into a new infrastructure, such as a type of benchmark
based on gravitational-potential measurements(quantum benchmark) and long-term monitoring ofcrustal movements. Not only will it be possible todetect altitude-difference measurements with anaccuracy of 1 cm, which is difficult with the current
geodetic accuracy of the Global Navigation Satellite
System (GNSS), but it may also be applicable tolong-term monitoring of the movement of massive
underground materials, such as magma in volcanoes,
which must greatly affect the gravitational potential.
It goes without saying that optical-lattice-clock networks play a role in precise frequency-referencedelivery, such as what the current GNSS offers, as
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well as applications such as quantum sensing. If wethink back on the fact that the frequencies provided
by optical-lattice-clock networks are in the opticaldomain, they can also be an extremely accurate optical-wavelength reference. If the backbone is an infrastructure capable of synchronizing timing and wavelengths with ultrahigh precision, it is expected to be aplatform that encourages development of new optical-communication architectures as well as contributes to current wavelength-division-multiplexing
communications.
3. Ultrahigh-precision
optical-frequency-transmission device (repeater)
Optical-lattice-clock networks require multiple
optical lattice clocks to be connected by optical fibers
and their frequency differences measured, but opticalfibers are noisy media for transmitting such ultraprecise optical-frequency standards. Optical fiber isalways affected by various noise, such as stretchingof the fiber due to daily temperature changes andvibrations derived from the surrounding environment, which deteriorates the accuracy of the transmitted optical frequency. Amplifier devices, such as fiber
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Feature Articles

Fig. 3. Optical frequency transmission based on LRS with planar lightwave circuit (PLC).

amplifiers, which are usually used in an optical communication system, are also difficult to use because
they can degrade frequency accuracy. An ultrahighprecision optical-frequency transmission and relay
device (repeater) compensates for the noise causedby fiber transmission, maintains accuracy, recovers
the propagation loss associated with fiber transmission, and transmits and relays the signal to remotelocations (Fig. 3). The repeater combines the fibernoise-compensation function and relay function intoa single device, which transmits the fiber-noisecompensated optical frequency to the next section.
By the repetitive (cascade) connection of fiber-noisecompensation, the optical frequency can be transmitted to a remote location with as little degradation in
accuracy as possible. The transmission wavelength of
the ultrahigh-precision optical frequency was 1397
nm, which is exactly 2 times the wavelength of 698
nm and corresponds to the resonance frequency of a
Sr optical lattice clock. Because of the sub-harmonic
relationship, it is possible to convert the optical-frequency reference of a Sr optical lattice clock into awavelength band capable of fiber transmission with a
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simple configuration using a single wavelength-conversion device [3].
The most important component in the repeater is an
optical interferometer, which achieves two mainfunctions of the repeater. One is fiber-noise cancellation, which compensates for the fiber noise by interfering with the source light and the same fiber being
sent back in the opposite direction from the receiverafter fiber transmission, detecting the frequency
noise coming from the transmitted fiber, and adding
the noise in antiphase to that of the source light. The
other function is frequency-coherent relaying, inwhich the frequency accuracy is copied to the transmitting laser by interfering with the ultrahigh-precision optical frequency one wants to copy with thetransmitting laser, detecting the frequency difference
between the two, and controlling the frequency of thetransmitting laser by feedback. Optical interferometers used in conventional repeaters consist of spatial
optics and fiber couplers, but there is a problem inthat the optical interferometers cannot eliminate
interferometer noise. Therefore, the use of a differential detection Mach-Zehnder interferometer based on
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Feature Articles

a quartz-based planar lightwave circuit (PLC)*3
developed by NTT Device Technology Laboratories
has resulted in a smaller repeater and improved stability and detection sensitivity. By building an interference circuit in the optical chip with a precisely
designed optical-path length, it is resistant to environmental fluctuations, such as temperature, and has
succeeded in reducing noise from the optical interferometer. Differential detection of optical-interference
signals is also possible by using the optical interferometer’s differential output of light to improve detection sensitivity.
4. Construction of ultrahigh-precision
optical-frequency-transmission fiber link inmetropolitan area
Toward the implementation of this optical-latticeclock network, NTT Basic Research Laboratories has
been conducting field demonstrations since 2015 incollaboration with the Katori Laboratory in UTokyo,
RIKEN, and NTT EAST. Optical lattice clocks withthe world’s highest accuracy level can currently
achieve a frequency stability of 1 × 10−18 with anaveraging time (data integration) of more than 10,000
seconds. Therefore, to transmit an optical latticeclock without degrading its stability, optical-fibertransmission must reach 18 digits of frequency stability in less than 10,000 seconds of measurement time.
The scalability of the fiber distance is also an important factor if we assume that we will expand such an
optical-lattice-clock network on a nationwide scale.
Using the repeater developed by NTT, we constructed an ultrahigh-precision optical-frequency-transmission fiber link for a demonstration experiment
involving connecting multiple relay stations (telecommunications offices) from RIKEN’s Wako location (in Saitama Prefecture) to the NTT Atsugi R&D
Center (in Kanagawa Prefecture) via the UTokyo’s
Hongo campus (in Tokyo) (Fig. 4). This is a demonstration of an optical-lattice-clock network within anarea corresponding to one region consisting of several prefectures in Japan. The relay station was
equipped with a compact remote-controllable repeater system that can be set in a 19-inch-rack unit. The
transmission accuracy was evaluated using a 240-kmlong loop network in which an ultranarrow linewidthlaser was transmitted from Wako to Atsugi via therepeater at UTokyo then transmitted back to UTokyothrough the other optical fiber installed on the same
path. At UTokyo, we measured the frequency stability*4 by detecting the interference between the optical
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frequencies transmitted to Atsugi and those returnedby the loop network. By measuring the frequency difference between the transmitted light and the light
returning through the fiber network, we can assess
the noise (how much it degrades frequency stability)that originates from the transmitted fiber. We demonstrated transmission with frequency stability andaccuracy of 3 × 10−16 for 1-second averaging timeand 1 × 10−18 for 2600-second averaging time [4].
This frequency-transmission stability is an achievement that will lead to future application of relativistic
geodesy with 1-cm accuracy, which does not deteriorate the accuracy of the world’s most accurate opticallattice clock. Previously, UTokyo and RIKEN carriedout relativistic geodetic experiments between Hongo
and Wako by comparing the frequencies of 2 opticallattice clocks with 30 km of non-relaying fiber transmission. They demonstrated the principle of remote
altitude difference measurement with accuracy ofseveral centimeters [5]. Since the transmission distance without relaying is roughly limited up to 100
km due to the fiber-propagation loss, the cascaderelaying method through repeaters we demonstrated
will enable us to extend to a regional level of several
hundred kilometers or national level of several thousand kilometers with ultrahigh precision.
5. Summary and future outlook
We introduced an ultrahigh-precision transmission
technology of an optical frequency standard, whichenables optical-fiber transmission over 200 km without deteriorating the accuracy of the world’s highestperformance optical lattice clock. We plan to conduct
frequency-comparison experiments on optical latticeclocks installed at Wako and Atsugi using the fiber
link we constructed. This will enable the demonstration of relativistic geodesy to detect height differences with several-centimeter-accuracy between
remote locations on the 200-km scale. On theassumption of a nationwide optical-lattice-clock*3 Quartz-based PLC: This optical waveguide technology, which
has been put into practical use by NTT, enables optical waveguides to be manufactured in a process similar to large-scale integrated circuits and integration of various interferometers. PLCs
are superior for mass production because of their ability to automate manufacturing and are highly effective in reducing costs
during mass production. They are also characterized by low loss
and high reliability because they can form waveguides with thesame glass materials as optical fibers.
*4 Frequency stability: A measure of frequency precision and defined as how constant the frequency is for a specified time interval.

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Feature Articles(a) 240-km-long link stability is estimated by measuring a beatsignal at UTokyo.

(b) Frequency instabilities measured with UTokyo-NTT fiber link

Fig. 4. Optical frequency transmission experiment in Tokyo area.

network, we plan to develop repeaters capable ofstable operation with more relays and extend thisultrahigh-precision optical-frequency-standard fibertransmission technology to the 1000-km scale.
Acknowledgments
This research was conducted in collaboration with
Prof. Hidetoshi Katori and Prof. Ichiro Ushijima (theUniversity of Tokyo), Dr. Masao Takamoto (RIKEN),
Prof. Noriaki Ohmae (Fukuoka University at present), Dr. Takashi Go (NTT Electronics at present),
Prof. Atsushi Ishizawa (Nihon University at present),
and Prof. Hideki Gotoh (Hiroshima University atpresent). This research was partially supported by
JSPS Grant-in-Aid for Specially Promoted Research(JP 16 H06284) and JST-Mirai Program Grant Number JPMJMI18A, Japan.

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References
[1]
[2]

[3]

[4]

[5]

H. Katori, “Optical Lattice Clocks and Quantum Metrology,” Nat.
Photonics, Vol. 5, pp. 203–210, 2011. https://doi.org/10.1038/
nphoton.2011.45
W. F. McGrew, X. Zhang, R. J. Fasano, S. A. Schäffer, K. Beloy, D.
Nicolodi, R. C. Brown, N. Hinkley, G. Milani, M. Schioppo, T. H.
Yoon, and A. D. Ludlow, “Atomic Clock Performance Enabling
Geodesy below the Centimetre Level,” Nature, Vol. 564, pp. 87–90,
2018. https://doi.org/10.1038/s41586-018-0738-2
T. Akatsuka, H. Ono, K. Hayashida, K. Araki, M. Takamoto, T.
Takano, and H. Katori, “30-km-long Optical Fiber Link at 1397 nm
for Frequency Comparison between Distant Strontium Optical Lattice
Clocks,” Jpn. J. Appl. Phys., Vol. 53, 032801-1–5, 2014. https://doi.
org/10.7567/JJAP.53.032801
T. Akatsuka, T, Goh, H. Imai, K. Oguri, A. Ishizawa, I. Ushijima, N.
Ohmae, M. Takamoto, H. Katori, T. Hahimoto, H. Gotoh, and T.
Sogawa, “Optical Frequency Distribution Using Laser Repeater Stations with Planar Lightwave Circuits,” Opt. Express, Vol. 28, No. 7,
pp. 9186–9197, 2020. https://doi.org/10.1364/OE.383526
T. Takano, M. Takamoto, I. Ushijima, N. Ohmae, T. Akatsuka, A.
Yamaguchi, Y. Kuroishi, H. Munekane, B. Miyahara, and H. Katori,“Geopotential Measurements with Synchronously Linked Optical
Lattice Clocks,” Nat. Photonics, Vol. 10, pp. 662–666, 2016. https://doi.org/10.1038/nphoton.2016.159

41

Feature Articles

Katsuya Oguri

Executive Senior Research Scientist, Executive Manager, Quantum Optical Physics
Research Group, Advanced Applied PhysicalScience Laboratory, Research Planning Section,
NTT Basic Research Laboratories.
He received a B.S., M.S., and Ph.D. from the
University of Tokyo in 1996, 1998, and 2005. In
1998, he joined NTT Basic Research Laboratories. Since 1998, he has been engaged in the
study of high-power femtosecond-laser-based
x-ray sources and their application. He is currently studying extreme laser physics and its
application to PHz technology covering ultrashort pulse laser and attosecond science, ultrastable laser and optical clock technology, andoptical-frequency comb. He has been a guest
researcher at RIKEN since 2015 and an associate
professor at University of Tsukuba Graduate
School Cooperative Graduate School Systemsince 2020. He received the 26th Japan Societyof Applied Physics Japanese Journal of Applied
Physics (JJAP) Best Original Paper Award in2004 and the 18th Japan Society of Applied
Physics JJAP Young Scientist Presentation
Award in 2005. He received the 32nd Laser Society of Japan Best Original Paper Award in 2008.
He received the Best Poster Presenter Award at
International Symposium on Ultrafast Intense
Laser Science XVI in 2017. He is a member of
the Japan Society of Applied Physics (JSAP), thePhysical Society of Japan (JPS), the Laser Society of Japan (LSJ), the Japan Intense Light Field
Science Society (JILS), and Optica. He is a committee member of the Ultrafast Optoelectronics
Technical Group in the Institute of Electronics,
Information and Communication Engineers(IEICE).

Tomoya Akatsuka

Senior Research Scientist, Quantum Optical
Physics Research Group, Advanced Applied
Physical Science Laboratory, NTT Basic
Research Laboratories.
He received a B.E., M.E., and Ph.D. in applied
physics from the University of Tokyo in 2002,
2004, and 2008. He studied quantum electronics,
atomic physics, and laser optics at the Universityof Tokyo, Tokyo University of Science, and
RIKEN. He joined NTT Basic Research Laboratories in 2016 and studied the optical-latticeclock network. He was active in developing an
optical-fiber link that connects three laboratories
at RIKEN, the University of Tokyo, and NTT
with a frequency instability of 10–18. He is currently studying next-generation optical-frequency-transfer systems. He is a member of JPS andthe Institute of Electrical Engineers of Japan.

NTT Technical Review

Vol. 21 No. 6 June 2023

Hiromitsu Imai

Senior Research Scientist, Quantum Optical
Physics Research Group, Advanced Applied
Physical Science Laboratory, NTT Basic
Research Laboratories.
He received a B.E., M.E., and Ph.D. in physics
from Tokyo University of Science in 2006, 2008,
and 2011. He joined NTT Basic Research Laboratories in 2011 and studied atom optics including laser cooling and atom spectroscopy. He iscurrently studying the optical-lattice-clock network towards high-precision clocks. He has beena visiting scientist at RIKEN since 2015. He is amember of JPS.

Toshikazu Hashimoto

Senior Distinguished Researcher, Group
Leader, Optoelectronics Integration ResearchGroup, NTT Device Technology Laboratories.
He received a B.S. and M.S. in physics from
Hokkaido University in 1991 and 1993 and Ph.D.
in engineering from Kyushu University in 2022.
He joined NTT Photonics Laboratories in 1993
and has been researching hybrid integration ofsemiconductor lasers and photodiodes on silicabased planar lightwave circuits and conducting
theoretical research and primary experiments on
the wavefront-matching method. He is a memberof IEICE, JPS, and JSAP.

Tetsuomi Sogawa

Senior Vice President, Basic and Advanced
Research Principal, NTT Science and Core Technology Laboratory Group.
He received a B.S., M.S., and Ph.D. in electrical engineering from the University of Tokyo in
1986, 1988, and 1991. In 1991, He joined NTT
Basic Research Laboratories (NTT-BRL). From1999 to 2000, he was a guest scientist at Paul
Drude Institute in Berlin, Germany, investigating
acoustic spin manipulation in semiconductor
quantum structures. From 2004 to 2006, heworked for the Council for Science and Technology Policy, Cabinet Office, Japan. He served as
the director of NTT-BRL from 2013 to 2019 andthe director of NTT Science and Core Technology Laboratory Group from 2018 to 2022. He has
been a visiting professor at the Institute of Industrial Science, the University of Tokyo, since2014. He is a fellow of JSAP.

42

Feature Articles: Toward Quantum Technology
Innovation

Extracting Quantum Power by Using
Algorithms and Their Verification
Seiichiro Tani, Seiseki Akibue, and Yuki Takeuchi
Abstract

Quantum computers are expected to increase computational speed compared with current and futurecomputers that work in accordance with the conventional computational principle and to revolutionize
information processing. Such an increase in speed is achieved using quantum algorithms that extract
computational power from quantum-computer hardware. This article briefly introduces our recent
results on quantum algorithms, quantum-circuit optimization to support their efficient implementation,and quantum-circuit verification necessary for their reliable execution.
Keywords: quantum computer, quantum algorithm, quantum information processing

1. Quantum algorithm as core technology
Just as with current computer systems, the performance of quantum-computer systems will heavily
depend on what software (i.e., algorithms) we use on
quantum-computer hardware. In this sense, quantum
algorithms can be considered a core technology forincreasing quantum speed. Many studies developed
sophisticated quantum-algorithmic techniques andproposed a variety of applications for them withquantum advantage. However, there are still issues
regarding quantum algorithms that we need to better
understand. To achieve an effective increase in quantum speed, it is also essential to implement quantum
algorithms as compact quantum circuits and ensure
the reliability of circuit execution.
2. Quantum algorithms for classical problems
Most problems we encounter in daily life are unrelated to the concept of quantum mechanics, i.e., classical problems. If quantum computers can solve classical problems much faster than any computer basedon the conventional computational principle, i.e., anyclassical computer, they will significantly impact ourlives and society.
Previous theoretical studies have revealed many
problems that quantum computers can solve much
NTT Technical Review

Vol. 21 No. 6 June 2023

faster [1]. In particular, the collision problem has
been central in developing various quantum-algorithmic techniques, such as quantum walk, which
increase quantum speed in solving practically important classical problems such as matrix multiplication.
We take the collision problem as an example to consider quantum-algorithmic techniques from a different point of view, that is, from the viewpoint of
information-communication security.
Quantum computers can be used not only for
improving our lives but for malicious attacks such as
cracking ciphers. It is thus essential to assess the
security of ciphers against strong attacks involving
quantum computers (i.e., quantum attacks). For suchan assessment, one needs to know how much ability
attackers have by devising quantum-attack methods,
which are simply quantum algorithms. We devised
the fastest quantum-attack method against randomhash functions, a core technology in cryptography, on
the basis of the knowledge of quantum algorithms
that we have accumulated [2].
A hash function takes long data as input and outputs
short data. The hash functions used in ciphers are
unique in that the input data are hard to infer from theoutput data. Such hash functions have many applications that require falsification prevention, such as
electronic signatures and public-key cryptography.
Let us explain an attack against hash functions
43

Feature Articles

2-collision
ƒ(i ) = ƒ( j )jƒiA 2-collision of a hash function ƒ is a pair (i, j ) of two distinct elements mapped via ƒ into
the same value, i.e., a pair (i, j ) such that ƒ(i ) = ƒ( j ). Similarly, an ℓ-collision is an ℓ-tupple
of ℓ distinct elements mapped via ƒ into the same value.

Fig. 1. Collision of hash function.



1. Sample a subset I [N ] and compute its image ƒ(I ), where [N ] {1, …, N }.
2. Quantum search the preimage of ƒ(I ) for a set J [N ] \ I and compute its image ƒ(J ).
3. Quantum search the preimage of ƒ(J ) for an element k [N ] \ (I J ).
4. Output 3-collision (i, j, k).

[N ]

[N ]

i

I

ƒ(I ): Image of 1-collisions
ƒ(J ): Image of 2-collisionsJƒ(k ): Image of a 3-collisionjkƒFig. 2. Quantum algorithm for finding a 3-collision.

using the concept of collisions. We say that multiple
elements are a collision of a hash function f if they aremapped via f to the same value. More concretely, we
say that ℓ elements mapped via f into the same value
are an ℓ-collision. We also say that ℓ is the multiplicity of the collision. In Fig. 1, since two elements i, j
are mapped into the same value by f, the pair (i, j) is
a collision.
One can falsify electronic documents even withmessage authentication codes if they find a collisionof a hash function. It is thus required to assess thecomputational hardness of finding a collision (i.e.,
how long it takes to find a collision) before using ahash function as part of a cryptographic system. ThisNTT Technical Review

Vol. 21 No. 6 June 2023

assessment requires concrete algorithms for finding acollision. We designed a quantum algorithm thatquickly finds an ℓ-collision for any given ℓ (Fig. 2).
The computation time of this algorithm is optimal in
the sense that it achieves the theoretical limit, whichmakes it possible to rigorously assess the security ofcryptographic systems that include hash functions.
Figure 3 compares our algorithm with the previously best algorithm [3] in terms of the number of
evaluations of a hash function to find an ℓ-collisionof the function, approximating the time taken to
execute our algorithm. For every ℓ ≥ 3, our algorithm
is superior to the previous one. When ℓ = 2, bothalgorithms have the same number of evaluations
44

Feature Articlesℓ (multiplicity)

2

Previously bestalgorithm [3]N3

Our algorithm

1

1

N3

3

4

N9

3

N7

4

13

N 27

7

N 15

5

40

N 81

15

N 31

ℓ

…

1

…

N 2(

…

N 2(

1

1−

1
3ℓ−1

)

1−

1
2ℓ−1

)

Fig. 3. C omparison of our algorithm with the previously best algorithm provided in [3] in the number of evaluations of a hashfunction, approximating the time taken to find an ℓ-collision for every ℓ, where N is the image size of the hash function.

since the previous algorithm achieves the theoreticallimit. As a numerical example, let us consider when
N is 2000 bits, in which case our algorithm is a billion
times faster than the previous one.
To execute a quantum algorithm efficiently, wemust consider optimizing the quantum circuit that
implements the algorithm. In the next section, we
introduce some of our results on quantum-circuit
optimization.
3. Quantum-circuit optimization
We can represent the dynamics of any quantum
system, such as photons and electrons, as a unitarymatrix. Thus, a ‘command’ sent by a quantum algorithm to a quantum computer is also essentially represented by a unitary matrix. For example, Shor’salgorithm, which efficiently solves the factoring
problem, uses the unitary matrix called the quantum
Fourier transformation as an essential command.
However, we cannot accurately implement such unitary matrices as the corresponding dynamics sincequantum systems are fragile against noise. We thus
implement a desired unitary matrix by sequentially
implementing unitary matrices selected from a finite
set (called an elementary gate set), the elements ofwhich are implementable with negligible error.
Hence, it holds that (a) the number of elementarygates = (b) the number of commands × (c) the numberof elementary gates used to implement the unitarymatrix representing a single command. Since the runtime of a quantum algorithm can be estimated by (a),many studies have been devoted to reducing it. There
are two types of such studies: for reducing (b) byexploiting the structure of the algorithm and for
reducing (c) without using the structure. It mightseem that reducing (c) is unnecessary if we design
algorithms by regarding each command as an eleNTT Technical Review

Vol. 21 No. 6 June 2023

mentary gate. However, it is necessary to design
algorithms independently of the quantum system we
use to implement the algorithms since the elementarygate set heavily depends on the system.
To reduce (c), we search for a sequence of elementary gates that approximately implements a target
unitary matrix (this procedure is called synthesis)
since it is impossible to accurately implement an arbitrary unitary matrix by using a sequence of elementary gates chosen from a finite set. We can implement
an arbitrary unitary matrix within an arbitrarily small
approximation error by using a sufficiently longsequence of appropriate elementary gates. Therefore,
traditional synthesis approaches, called deterministic
synthesis, are used to find the shortest single sequenceamong all sequences that approximate a target unitarymatrix within the desired approximation error. A new
synthesis approach, called probabilistic synthesis,
has been demonstrated to improve the approximationby probabilistically implementing the target unitarymatrix. This also indicates that probabilistic synthesis
can achieve the desired approximation error with ashorter sequence than the deterministic one (seeFig. 4). However, the optimality of current probabilistic-synthesis algorithms was unknown.
We have shown the fundamental limitations on thesmallest approximation error achievable with probabilistic synthesis [4]. We have also designed a probabilistic-synthesis algorithm that is efficiently executable and outperforms current probabilistic-synthesis
algorithms with respect to approximation error.
Numerical simulation showed that our algorithm canreduce (c) by about 50% compared with the best
deterministic-synthesis algorithm. The mathematical
tools we developed for analyzing optimal probabilistic synthesis are expected to be beneficial to optimizing various types of quantum-classical hybrid information processing.
45

Feature Articles

Target unitary matrix

Sequence of elementary gates

Deterministic
synthesis

X

Y

U

Z
X

H

H

Probability
distribution Sequence of elementary gates
p1
Target unitary matrix

Y

X

Z

Y

Probabilistic
synthesis

U

p2

p3

Z
Y

(Top) Deterministic synthesis is used to find the shortest single sequence among all sequencesthat achieve a target unitary matrix within the desired approximation error. (Bottom) We canshorten the sequence without increasing the approximation error by probabilistic synthesis.

Fig. 4. Reduction of elementary gates by probabilistic synthesis.

To reliably execute quantum algorithms, it is necessary to verify the operation achieved on a real device
by a sequence of elementary gates.
4. Verification methods for quantum computing
Quantum computers are susceptible to errors
caused by noise. Two typical techniques for mitigating such errors are quantum error correction andmitigation and verification of quantum computation.
These techniques complement each other. As thename suggests, quantum error correction and mitigation can correct and mitigate the errors, respectively,
but they require information such as error models anderror probabilities. Verification of quantum computation, however, is applicable to any error, which may
be completely unknown, but cannot correct errors
and can only determine whether they exist. Verification of quantum computation is a helpful techniquefor handling errors because only correct (i.e., errorless) computational results can be selected by evaluating the presence or absence of errors.
In cloud quantum computing that enables us toaccess a remote quantum computer, it should be difficult to characterize errors. Therefore, verification ofquantum computation works well for this application.
Verification of quantum computation has recently
NTT Technical Review

Vol. 21 No. 6 June 2023

been applied to mitigate errors. The rest of this section describes our recent results on verifying quantum computation.
Although several verification methods have been
proposed, almost all are tailored for fault-tolerant
universal quantum computers. However, the current
or near-term quantum computers called noisy intermediate-scale quantum (NISQ) computers have noerror-correction capability. That is why we tried toclose this gap by proposing a verification method for
NISQ computers [5]. To verify the outputs of NISQ
computers, a simple and trivial verification method
requires another quantum computer with the same
number of qubits. We solved this problem by using
the idea of dividing quantum-verification circuits into
two small quantum circuits (see Fig. 5) and succeeded in efficiently verifying the outputs of NISQ
computers with small-scale quantum devices.
5. Outlook
Research on software (i.e., algorithms), as well as
hardware for quantum computers, is essential forhigh-speed quantum computing. With our theoreticalexpertise, we will explore how to design algorithms
that quickly solve fundamental problems on quantum
computers and develop theoretical techniques, such46

Feature Articles

Error

0

Small-scalequantum device 1

1

Classical post-processing
on outcomes ofquantum devices 1 and 2
Error isdetected.

010Small-scalequantum device 2

1
NISQ computer

Output quantum stateof NISQ computer

Our verification method

Fig. 5. Our verification method for NISQ computers.

as those presented in this article, to extract the maximum computational power from quantum-computer
hardware.
References[1][2]

A. Montanaro, “Quantum Algorithms: An Overview,” npj Quantum
Inf., Vol. 2, 15023, 2016. https://doi.org/10.1038/npjqi.2015.23
A. Hosoyamada, Y. Sasaki, S. Tani, and K. Xagawa, “Quantum Algorithm for the Multicollision Problem,” Theor. Comput. Sci., Vol. 842,

Seiichiro Tani

Distinguished Scientist, Computing Theory
Research Group, Media Information Laboratory,
NTT Communication Science Laboratories.
He received a B.E. in information science from
Kyoto University in 1993 and M.E. and Ph.D. incomputer science from the University of Tokyoin 1995 and 2006. He joined NTT LSI Laboratories in 1995 and moved to NTT Network Innovation Laboratories in 1998. Since 2003, he has
been studying quantum computing theory at NTT
Communication Science Laboratories. He is also
an associate member of the Science Council ofJapan and visiting professor at the Quantum
Computing Unit, International Research Frontiers Initiative, Tokyo Institute of Technology. He
was a member of ERATO/SORST Quantum
Computing and Information Project, Japan Science and Technology Agency (JST) from 2004 to2009 and visiting researcher at the Institute forQuantum Computing (IQC), the University ofWaterloo, Canada, from 2010 to 2011. Hereceived the Institute of Electronics, Information
and Communication Engineers (IEICE) Achievement Award and IEICE Information and Systems
Society Best Paper Award. He also received
Maejima Hisoka Award. He is a member of Association for Computing Machinery (ACM), theInstitute of Electrical and Electronics Engineers(IEEE), IEICE, and Information Processing
Society of Japan (IPSJ).

NTT Technical Review

Vol. 21 No. 6 June 2023

[3]

[4]
[5]

pp. 100–117, 2020. https://doi.org/10.1016/j.tcs.2020.07.039
A. Hosoyamada, Y. Sasaki, and K. Xagawa, “Quantum Multicollision-finding Algorithm,” Proc. of the 23rd International Conference
on the Theory and Application of Cryptology and Information Security (ASIACRYPT 2017), Part II, pp. 179–210, 2017.
S. Akibue, G. Kato, and S. Tani, “Probabilistic Unitary Synthesis with
Optimal Accuracy,” arXiv:2301.06307, 2023. https://doi.
org/10.48550/arXiv.2301.06307
Y. Takeuchi, Y. Takahashi, T. Morimae, and S. Tani, “Divide-andconquer Verification Method for Noisy Intermediate-scale Quantum
Computation,” Quantum, Vol. 6, 758, 2022. https://doi.
org/10.22331/q-2022-07-07-758

Seiseki Akibue

Researcher, Computing Theory ResearchGroup, Media Information Laboratory, NTT
Communication Science Laboratories.
He received a B.E., M.E., and Ph.D. in physics
from the University of Tokyo in 2011, 2013, and2016. He joined NTT Communication Science
Laboratories in 2016 and has been studying theoretical topics in distributed quantum computationand classical-quantum hybrid computation. He
received Bourses du Gouvernement Français in
2013.

Yuki Takeuchi

Associate Distinguished Researcher, Computing Theory Research Group, Media Information
Laboratory, NTT Communication Science Laboratories.
He received a Ph.D. in science from Osaka
University in 2018. He joined NTT Communication Science Laboratories as a research associate
the same year and was a researcher from 2019 to2023. Since April 2023, he has been in his current
position and engaged in the theoretical investigation of quantum information and is especiallyinterested in the verifiability of quantum computing. He received IPSJ Computer Science
Research Award for Young Scientists and Young
Scientist Award of the Physical Society of Japan.
He is a member of the Physical Society of Japanand IPSJ.

47

Feature Articles: Toward Quantum Technology
Innovation

Improving the Performance of
Quantum Key Distribution
Takuya Ikuta and Seiseki Akibue
Abstract

Encryption is an essential technology for secure communications. Quantum key distribution (QKD)
can enable ultimately secure cryptographic communications by using quantum mechanics. Toward
secure networks using QKD, NTT has been conducting various studies from theoretical security analysis
to experimental control of optical quantum states. In this article, we introduce our recent activities on
QKD using multi-valued information (high-dimensional QKD) and a scalable measurement device for
improving its error robustness.
Keywords: quantum key distribution, high dimension, error robustness

1. Basics of quantum key distribution andresearch in NTT
It is necessary to encrypt information for securecommunications via the Internet. For example, malicious eavesdropper can read information about acredit card if we send it without encryption. Publickey cryptography, such as Rivest–Shamir–Adleman(RSA), is used for this purpose. Its security is basedon a problem that is difficult to solve using a modern
digital computer, so called computational security.
However, a quantum computer having error correction capability can break RSA efficiently. There isanother encryption called one-time pad, which is acommon-key cryptography. This cryptography uses
the fact that a third party does not have enough information to break the encryption. Such an informationtheoretic security cannot be broken by any type ofcomputer. Despite its strong encryption, it has a serious disadvantage in that it is difficult to create a situation in which the third party does not have enoughinformation. We use a bit represented by 0 and 1 in acommunication system. To make one-time pad ultimately secure, only the legitimate users for the communications need to share random numbers*1 (secretkey), the length of which is equal to the bit length of
the information they really want to send. If we cansend the secret key securely, one-time pad looks useNTT Technical Review

Vol. 21 No. 6 June 2023

less because, in a rough consideration, we can directly send the information using such a secure method
used for sharing the secret key instead of one-time
pad.
Quantum key distribution (QKD) is a techniquethat solves this problem. QKD uses the fact that an
attempt to clone a quantum state changes the originalstate (Fig. 1). By monitoring such a change in thestate, we can estimate how much information was
leaked during communications. We cannot directly
send a massage we want to encrypt because we cannot prevent the message from being leaked. However,
if we send random numbers having no meaningful
information, we can generate a secret key by erasing
the leaked information from the shared random numbers after the communication. By using this secret
key for one-time pad, we can enable unbreakablesecret communications.
While we often hear about QKD on the news, it has
been studied since its invention in 1984 [1]. NTT hascontinued its long-term research on QKD [2], such as
security analysis, experimental demonstration, andthe proposal of our unique QKD protocol called differential phase shift QKD. Let us introduce our recent
activities on high-dimensional QKD and a technology*1 Random number: A random value that is impossible to predict
beforehand.

48

Feature Articles

Monitor and erase
the leaked information

Send
random numbers
PhotonState change due to copy
Eavesdropper

Fig. 1. Schematic diagram of QKD.

Enlarge dimensions(multi-valued)

Time-bin qubit
bit 0
t0

High-dimensional time-bin state
bit 00

t0

t1

t2

t3

t0

t1

t2

t3

bit 01

bit 1
t1

A photon in time slot t0t0t1

A photon in time slot t1

A photon in time slot t0
bit 10

t0

t1

t2

t3

A photon in time slot t2

A photon in time slot t1
t0

t1

t2

t3

bit 11

A photon in time slot t3

Used for conventional QKD

Fig. 2. Schematic diagram of time-bin state.

for improving its error robustness.
2. High-dimensional QKD and scalable
measurement device
High-dimensional QKD uses a high-dimensional
quantum state to enhance the secret-key rate, whichcorresponds to the communication speed of QKD. A
conventional QKD system uses a quantum bit (qubit)
representing 0 or 1. In our research group, we use atime-bin qubit, 0 or 1 of which are represented by two
temporal positions of a photon*2 (Fig. 2 left). If we
can encode multiple values, such as 0, 1, 2, …,
instead of a bit value, we can increase the amount of
information per photon (Fig. 2 right). A high-dimensional state is such a state representing multiple values and its concept is similar to pulse amplitude
modulation (PAM)*3 and quadrature amplitude modulation (QAM)*4 used in modern optical communications. A very high-speed secret-key generation of
26.2 Mbit/s has been reported by using four-dimensional time-bin states represented by four temporal
positions [3].
As we explained in the previous section, it is imporNTT Technical Review

Vol. 21 No. 6 June 2023

tant to estimate how much information of random
numbers was leaked for generating a secure secretkey. For this purpose, QKD uses superposed states. A
superposed state is a state in which we cannot essentially determine 0 or 1 for a qubit and 0, 1, 2, … for ahigh-dimensional state (Fig. 3). We can estimate theamount of leaked information by using superposed
states that satisfy a special relation called mutually
unbiased. In a d-dimensional system, we can use at
most (d + 1) measurements, which satisfy such a special relation. An experiment using two types of measurements to estimate the amount of leaked information achieved 26.2-Mbit/s secret key generation [3].
By using (d + 1) measurements, we can more precisely evaluate the change in the quantum state during communications. Therefore, we can more precisely estimate the amount of leaked information,*2 Photon: The minimum unit of an optical energy (an elementary
particle), which can be observed when we drastically decrease
the optical intensity.
*3 PAM: A technique for representing multiple values using an amplitude of light or radio wave for high-speed communications.
*4 OAM: A technique for representing multiple values using both
amplitude and phase of light or radio wave for high-speed communications.

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Feature Articles

Superposed state
Bit 0 and 1,
simultaneously
t0Superposition of
00, 01, 10, 11
t0

t1

t1

t2

t3

For
high-dimensional state

For qubit

Fig. 3. Schematic diagram of superposed state.

Delay line
Phase modulator

MZI(1 ns)

Delay line

MZI
(500 ps)Single-photon
detectors

Fig. 4. Measurement device for four-dimensional time-bin states.

3. Extension of the security proof
In the previous section, we explained that (d + 1)
measurements can be used for a robust high-dimenNTT Technical Review

Vol. 21 No. 6 June 2023

5.0
Symbol error rate (%)

which results in an improvement in the secret-key
rate. In other words, multiple measurements make ahigh-dimensional QKD system more robust against
errors.
NTT developed and demonstrated an implementation of a scalable measurement device of (d + 1)
measurements for high-dimensional time-bin states[4]. A measurement for time-bin states uses delay
Mach-Zehnder interferometers (MZIs)*5 and singlephoton detectors. In an implementation of a previous
measurement device, (d − 1) MZIs and (d + 1) singlephoton detectors were required even for two measurements [3]. By using the above scalable measurement device, all the (d + 1) measurements for d = 2N
can be implemented using N MZIs and three singlephoton detectors independent of d (Fig. 4). We conducted experiments involving five measurements forfour-dimensional time-bin states. The experimental
results indicated error rates lower than the thresholdrequired to generate a secret key (Fig. 5). Hence, we
expect that this device can be used for a high-dimensional QKD system that is more robust against errors.

4.5%

4.0

4.0%

4.3%

3.0
2.3%

2.0
1.0
0.0

0.6%
0

1
2
3
Type of measurements

4Fig. 5. Experimental results of error rate.

sional QKD system. However, a rigorous security ofthis QKD protocol was proven only if d is a prime
number (2, 3, 5, …). Therefore, we cannot directly
apply that security proof for a four-dimensional system with the above scalable measurement device. To
circumvent this problem, we also extended the previous*5 Delay MZI: An optical interferometer in which a light is split at
the input, and combined after a temporal delay is introduced for
split light. It is widely used for, for example, measuring the timebin state and differential-phase detection in modern optical communications.

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Feature Articles

security proof [5] for prime power dimensions (e.g.,
d = 2, 4, 8, and 3, 9, 27). In this security proof, we use
operators that describe operations on a quantum state.
Corresponding to the operators used in the previous
security proof, there is a generalized operator using
the Galois field*6, which is used in, e.g., coding theory [6]. Because we can use the Galois field as long as
d is a prime power, we could extend the previous
security proof by using these generalized operators
for prime-power dimensions. Therefore, we can use
the scalable measurement device for a four-dimensional QKD system to ensure rigorous security.
4. Toward a practical QKD system
We introduced high-dimensional QKD and a scalable measurement device to enhance the error robustness as recent activities in NTT to improve the performance of QKD. We conducted a proof-of-principle
experiment on generations and measurements ofhigh-dimensional quantum states for such a QKD
system. To implement a practical QKD system, we
also need to consider other problems, for example, afinite key analysis which is a detailed treatment ofstatistical errors due to a finite number of measurement results. It is also important to explore otherapplications of the scalable measurement device

because the mutually unbiased relation can be foundin quantum communications and information processing other than QKD. Although we introduced
only the approach of using a high-dimensional statein this article, NTT will continue to conduct researchon other approaches toward practical quantum information technologies.
References
[1]

[2]
[3]

[4]

[5]
[6]

C. H. Bennett and G. Brassard, “Quantum Cryptography: Public Key
Distribution and Coin Tossing,” Proc. of IEEE International Conference on Computers Systems and Signal Processing, pp. 175–179,
Bangalore, India, 1984.
“Feature Articles: Quantum Cryptography,” NTT Technical Review,Vol. 9, No. 9, 2011. https://ntt-review.jp/archive/2011/201109.html
N. T. Islam, C. C. W. Lim, C. Cahall, J. Kim, and D. J. Gauthier,“Provably Secure and High-rate Quantum Key Distribution with
Time-bin Qudits,” Sci. Adv., Vol. 3, No. 11, e1701491, 2017. https://doi.org/10.1126/sciadv.1701491
T. Ikuta, S. Akibue, Y. Yonezu, T. Honjo, H. Takesue, and K. Inoue,“Scalable Implementation of (d + 1) Mutually Unbiased Bases ford-dimensional Quantum Key Distribution,” Phys. Rev. Res., Vol. 4,
No. 4, L042007, 2022. https://doi.org/10.1103/PhysRevResearch.
4.L042007
L. Sheridan and V. Scarani, “Security Proof for Quantum Key Distribution Using Qudit Systems,” Phys. Rev. A, Vol. 82, No. 3,030301(R), 2010. https://doi.org/10.1103/PhysRevA.82.030301
T. Durt, B.-G. Englert, I. Bengtsson, and K. Życzkowski, “On Mutually Unbiased Bases,” Intl. J. Quantum Inf., Vol. 8, No. 4, pp. 535–
640, 2010. https://doi.org/10.1142/S0219749910006502*6 Galois field: A set of finite number of elements associated withappropriately defined four arithmetic operations (+, –, ×, ÷). It isalso known as finite field.

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51

Feature Articles

Takuya Ikuta

Researcher, Quantum State Control ResearchGroup, Quantum Science and Technology Laboratory, NTT Basic Research Laboratories.
He received a B.E., M.E., and Ph.D. in engineering from Osaka University in 2014, 2016,
and 2023. He joined NTT Basic Research Laboratories in 2016 and has studied quantum opticalcommunications. He is also engaged in research
on an optical computing system using opticalparametric oscillators. He received the Young
Scientist Presentation Award from the Japan
Society of Applied Physics (JSAP) in 2017. He isa member of JSAP.

NTT Technical Review

Vol. 21 No. 6 June 2023

Seiseki Akibue

Researcher, Computing Theory ResearchGroup, Media Information Laboratory, NTT
Communication Science Laboratories.
He received a B.E., M.E., and Ph.D. in physics
from the University of Tokyo in 2011, 2013, and2016. He joined NTT Communication Science
Laboratories in 2016 and has been studying theoretical topics in distributed quantum computationand classical-quantum hybrid computation. He
received Bourses du Gouvernement Français in
2013.

52

Feature Articles: Toward Quantum Technology
Innovation

Toward a Quantum Internet
Koji Azuma
Abstract

A quantum internet holds promise for accomplishing quantum metrology and quantum computer networks as well as quantum communication among arbitrary clients all over the globe. Its actualization is
an important long-term scientific and technological goal. In this article, I explain what a quantum internet is and what is needed for its actualization as well as recent relevant progress in the field of quantum
information.
Keywords: quantum internet, quantum repeaters, all-photonic approach

1. Introduction
In modern physics, quantum mechanics gives themost precise description of nature in the range ofphenomena from the level of elementary particles toour universe. Quantum mechanics just predicts theprobability of an event occurring, in contrast to thedeterminism given by classical mechanics, and it is abroader paradigm than classical mechanical views.
Even if there is a theory with the view of determinism, quantizing it is still one of most important
attempts in physics. At the end of the last century, thepossibility of applying the quantum mechanical pointof view to information processing began to be
explored; as a result, quantum information processing was established. Quantum information processing accomplishes tasks that are intractable by conventional means and the concept includes the paradigm of conventional information processing [1].
For instance, a quantum computer can factor large
integers efficiently. Thus, if a quantum computer ishanded to an eavesdropper, most widely used publickey cryptosystems, such as RSA (Rivest–Shamir–
Adleman), can be cracked. Quantum key distribution(QKD), however, presents information-theoreticallysecure communication even if eavesdroppers can use
arbitrary attacks enabled by quantum mechanics(including eavesdropping by using an arbitrarily
large quantum computer). As long as quantum
mechanics is correct in the description of nature, such
quantum information processing allowed by the lawsof quantum mechanics must be the ultimate form of
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achievable information processing.
What is the ultimate form of quantum information
processing? Given that the current internet could be
regarded as the largest information-processing network on Earth, its quantum version, called a quantum
internet [2], would be the ultimate form of information-processing networks. In this article, I explain
what a quantum internet is and what is needed for its
actualization as well as recent relevant progress in thefield of quantum information (see review paper [3]for detail).
2. What is a quantum internet?
A quantum internet is a quantum-information-processing network (Fig. 1) in which quantum-information-processing nodes (such as quantum computersand quantum memories) are connected by quantum
communication channels (such as optical fibers andfree space). It enables arbitrary clients around theglobe to achieve various quantum-information-processing tasks beyond those served by the current
internet [4]. For example, it enables arbitrary clientsin the network to use QKD, which could be the basisof a referendum, top-level meeting, financial deal,
exchange of genetic/biological information and soon, thanks to its feature of high security. A quantum
internet would also make it possible to transfer
unknown quantum states faithfully to a distant placeat the speed of light. This is the basis of distributed
quantum computation, cloud quantum computing,and networking of quantum computers. A quantum
53

Feature Articles

Quantum communication channel(such as an optical fiber and free space)
C10

C11

Quantum-information-processing node(such as a quantum computer and memory)
B

C2

C15

C1

C4

C3
A

C17

C5
C7

C12

C13

C14

C6
C16

C8

C18

C9

Fig. 1. Schematic of a quantum network.

internet would also be used for synchronizing atomic
clocks with unprecedented stability and accuracy in asecure manner. It would also enable us to make baselines of telescope arrays unprecedentedly longer,
contributing to progress of astronomy.
3. Constructing a quantum internet
How can we construct a quantum internet? The roleof a quantum internet is to distribute quantum entanglement to clients efficiently. Quantum entanglement
can be demonstrated only with quantum systems suchas atoms and photons, and it is a peculiar correlationthat cannot be explained in the framework of classicalmechanics. This correlation was used by Einstein,
Podolsky, and Rosen to point out that quantum
mechanics includes a prediction contradicting the
local realism (which ought to hold in classicalmechanics), that is, to pose a question on the validityof quantum mechanics [5]. Ironically, the existence ofquantum entanglement has been confirmed experimentally. In 2002, Aspect, Clauser, and Zeilinger,
who had conducted these experiments, were awarded
the Nobel Prize in Physics [6]. Quantum entanglement is now identified as a universal resource not
only for quantum communication but also for quantum computation. Therefore, by distributing entanglement as such a universal resource efficiently, aquantum internet will serve clients with various functions.
Quantum networks for QKD have been developed
worldwide, as exemplified by the SECOQC networkin Europe [7], the Tokyo QKD network in Japan [8],and the 2000-km Shanghai-Beijing network in China[9]. However, all the nodes in these networks, includNTT Technical Review

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ing repeater nodes, are not quantum-informationprocessing nodes but classical information-processing nodes that can process only classical signals at
best even if they receive quantum signals. Therefore,
we cannot distribute quantum entanglement to arbitrary clients in those networks. Even if we specialize
the use of the networks only for QKD, we cannot
distribute a secret key unless we can fully trust all thenodes in the networks. In this regard, such existing
networks are categorized as trusted-node networks,
different from a quantum internet, and they cannot be
used to actualize a quantum internet.
If all clients are connected completely with quantum communication channels, that is, if all the clientscan communicate with each other by using point-topoint quantum communication schemes, the network
can work as a secure QKD network, which could be
used to actualize a quantum internet. However, it is
not realistic in terms of cost and efficiency if we actualize a quantum internet by expanding such a pointto-point quantum communication network worldwide. For example, if we use a standard optical fiber
as the quantum communication channel, its transmittance decreases exponentially with its length. In particular, the probability with which a single-photon
level of light emitted by a sender to convey quantum
information is detected by a receiver is about 10% fora distance of 50 km, about 1% for 100 km, and about
0.1% for 150 km, decreasing as it is multiplied by 0.1
every 50 km. Therefore, if a sender and receiver
execute point-to-point quantum communication witha 1000-km optical fiber, even with the use of a GHz
clock system, the expectation time needed to establish an entanglement pair between them is on theorder of hundreds of years, which is not realistic.
54

Feature Articles(1) Entanglement generation (EG)Photons for EG

Repeater

SenderMatter quantum memory

Repeater

Repeater

1 Transmissionof photons Receiver

2 EG process

Quantum bit(2) Entanglement swapping

3 Bell measurement(3) Completion

Quantum entanglement

Fig. 2. Memory-based quantum repeaters.

Hence, it is impossible to actualize a quantum internet only by combining point-to-point quantum communication schemes. To do so, we need to constructa network in which quantum-information-processing
nodes, called quantum repeaters, are installed
between clients [10, 11].
4. What is a quantum repeater?
Even in conventional communication, if a senderand receiver are far apart, their communication isestablished not with point-to-point means but via
repeater nodes set between them. In conventional
communication, the role of classical repeaters located on the repeater nodes is to amplify the received
weakened signal then send the amplified signal to thenext repeater node (or the receiver if the repeater isneighboring the receiver). However, this method
does not work for quantum communication because
the quantum no-cloning principle forbids copying
and amplifying quantum signals [12]. Thus, quantum
repeaters do not rely on signal amplification, although
they are located on the repeater nodes between asender and receiver similarly to classical repeaters.
The quantum repeater protocol based on the use ofquantum repeaters is composed of two processes;
entanglement generation and entanglement swapping. In a memory-based approach [10] (Fig. 2),
quantum repeaters, equipped with matter quantum
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memories and quantum interfaces, are connected toeach other by optical fibers. By exchanging photons
between adjacent repeater nodes, we attempt to generate quantum entanglement between them and store
the entanglement in quantum memories in the quantum repeaters once the attempt succeeds. By repeating this entanglement-generation process, if all theadjacent repeater nodes are connected with entanglement, we then move on to entanglement swapping. In
this process, by applying a measurement—called theBell measurement—to halves of entangled pairs, theentanglement pairs are transformed into an entangled
pair that directly connects the sender and receiver.
Therefore, in the memory-based approach, we canpresent entanglement between the sender and receiver by conducting entanglement generation, followed
by entanglement swapping.
An all-photonic approach [11] is based on a timereversed version of the memory-based repeater protocol, where we first carry out operations associatedwith entanglement swapping, followed by entanglement generation (Fig. 3). In this approach, all the
repeater nodes first prepare photons in an entangledstate, called a graph state, to execute entanglement
swapping. The halves of the graph states are sent toneighboring repeater nodes. On receiving the halves,
the receiving nodes apply entanglement generation to
the received pulses and, depending on this result,
execute measurements on remaining photons. With a55

Feature Articles

SenderRepeater

Quantum bit

Encoded photons

Photons for EG(1) Entanglement swapping

Repeater

Repeater

Receiver1 Preparation of photons in a graph state(2) EG

3 EG process

2 Transmission of photons(3) Completion

Quantum entanglement

Fig. 3. All-photonic quantum repeaters.

high probability, an entangled pair is then presented
to the sender and receiver. Quantum repeaters in thisall-photonic approach do not necessitate matter quantum memories and quantum interfaces and the protocol works only with optical devices (such as singlephoton sources, linear optical elements, active feedforward controllers, and photon detectors), in contrast
to the memory-based approach. Therefore, its repetition rate does not depend on the communication distance and is determined only by the clock speed of
optical devices. Therefore, the all-photonic approach
has high affinity with the current all-optical trend inconventional communication, and the served communication speed will be faster.
5. Discussion
Thanks to recent theoretical progress in the understanding of quantum networks (see review article[13]), the quantum capacity—the number of transmittable quantum bits per optical mode—and theprivate capacity—the number of securely transmittable bits per optical mode—of an optical fiber with atransmittance η are identified as −log2(1 − η). This
describes the fundamental limit of the point-to-point
quantum communication based on the use of an optical fiber, showing the impossibility of long-distancequantum communication as well as a quantum internet, without quantum repeaters (on considering η =NTT Technical Review

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e−l/latt for the length l of the fiber and a constant latt).
The quantum/private capacity for two-party communication over any optical-fiber network with arbitrary
topology was derived and achieved by aggregating
quantum repeater protocols. This indicates that quantum repeaters play essential roles in establishing aquantum internet that achieves or approaches such
ultimate performance.
There is a technological and conceptual gap
between currently available point-to-point QKD
technology and quantum repeater technology necessary for a quantum internet. Exploring and implementing schemes to bridge this gap is now being
investigated in QKD [14]. For instance, the adaptive
measurement-device-independent QKD protocol andtwin-field QKD protocol have been found to be candidates for bridging such a gap. Proof-of-principle
experiments on quantum repeaters, with the all-photonic approach as well as the memory-based approach,have been conducted. For the memory-based
approach, experiments of the working principle ofentanglement swapping have been conducted withthe use of quantum memories, such as silicon-vacancy color centers integrated inside a diamond nanophotonic cavity at Harvard University and Massachusetts Institute of Technology in the US [15], ensembles of Rubidium atoms at Tsinghua University in
China [16], and nitrogen-vacancy centers in diamondat Delft University of Technology in the Netherlands
56

Feature Articles[17]. For the all-photonic approach, experimental
demonstration of the principle of time-reversed
entanglement swapping has been carried out using an
entangled graph state, called a Greenberger-HorneZeilinger state, generated through the parametricdown conversion process, at Osaka University, NTT,
the University of Toyama, and the University ofToronto [18], and at the University of Science andTechnology of China [19, 20].
References
[1]
[2]
[3]

[4]
[5]

[6]
[7]

[8]

[9]

M. A. Nielsen and I. L. Chuang, “Quantum Computation and Quantum Information,” Cambridge University Press, 2000. https://doi.
org/10.1017/CBO9780511976667
H. J. Kimble, “The Quantum Internet,” Nature, Vol. 453, pp. 1023–
1030, 2008. https://doi.org/10.1038/nature07127
K. Azuma, S. Economou, D. Elkouss, P. Hilaire, L. Jiang, H.-K. Lo,
and I. Tzitrin, “Quantum Repeaters: From Quantum Networks to the
Quantum Internet,” arXiv:2212.10820, 2022 (to appear in Rev. Mod.
Phys.). https://doi.org/10.48550/arXiv.2212.10820
S. Wehner, D. Elkouss, and R. Hanson, “Quantum Internet: A Vision
for the Road Ahead,” Science, Vol. 362, No. 6412, 2018. https://doi.
org/10.1126/science.aam9288
A. Einstein, B. Podolsky, and N. Rosen, “Can Quantum-mechanical
Description of Physical Reality Be Considered Complete?”, Phys.
Rev., Vol. 47, pp. 777–780, 1935. https://doi.org/10.1103/
PhysRev.47.777
Press release issued by the Royal Swedish Academy of Sciences, Oct.
4, 2022. https://www.nobelprize.org/prizes/physics/2022/pressrelease/
M. Peev, C. Pacher, R. Alléaume, C. Barreiro, J. Bouda, W. Boxleitner, T. Debuisschert, E. Diamanti, M. Dianati, and J. F. Dynes, “TheSECOQC Quantum Key Distribution Network in Vienna,” New J.
Phys., Vol. 11, 075001, 2009. https://doi.org/10.1088/13672630/11/7/075001
M. Sasaki, M. Fujiwara, H. Ishizuka, W. Klaus, K. Wakui, M. Takeoka, S. Miki, T. Yamashita, Z. Wang, A. Tanaka, K. Yoshino, Y. Nambu,
S. Takahashi, A. Tajima, A. Tomita, T. Domeki, T. Hasegawa, Y.
Sakai, H. Kobayashi, T. Asai, K. Shimizu, T. Tokura, T. Tsurumaru,
M. Matsui, T. Honjo, K. Tamaki, H. Takesue, Y. Tokura, J. F. Dynes,
A. R. Dixon, A. W. Sharpe, Z. L. Yuan, A. J. Shields, S. Uchikoga, M.
Legré, S. Robyr, P. Trinkler, L. Monat, J.-B. Page, G. Ribordy, A.
Poppe, A. Allacher, O. Maurhart, T. Länger, M. Peev, and A. Zeilinger,“Field Test of Quantum Key Distribution in the Tokyo QKD Network,” Opt. Express, Vol. 19, No. 11, pp. 10387–10409, 2011. https://doi.org/10.1364/OE.19.010387
Y.-A. Chen, Q. Zhang, T.-Y. Chen, W.-Q. Cai, S.-K. Liao, J. Zhang, K.

Chen, J. Yin, J.-G. Ren, Z. Chen, S.-L. Han, Q. Yu, K. Liang, F. Zhou,
X. Yuan, M.-S. Zhao, T.-Y. Wang, X. Jiang, L. Zhang, W.-Y. Liu, Y.
Li, Q. Shen, Y. Cao, C.-Y. Lu, R. Shu, J.-Y. Wang, L. Li, N.-L. Liu, F.
Xu, X.-B. Wang, C.-Z. Peng, and J.-W. Pan, “An Integrated Space-toground Quantum Communication Network over 4,600 Kilometres,”Nature, Vol. 589, pp. 214–219, 2021. https://doi.org/10.1038/s41586020-03093-8[10] H.-J. Briegel, W. Dür, J. I. Cirac, and P. Zoller, “Quantum Repeaters:
The Role of Imperfect Local Operations in Quantum Communication,” Phys. Rev. Lett., Vol. 81, pp. 5932–5935, 1998. https://doi.
org/10.1103/PhysRevLett.81.5932[11] K. Azuma, K. Tamaki, and H.-K. Lo, “All-photonic Quantum Repeaters,” Nat. Commun., Vol. 6, 6787, 2015. https://doi.org/10.1038/
ncomms7787[12] W. K. Wootters and W. H. Zurek, “A Single Quantum Cannot Be
Cloned,” Nature, Vol. 299, pp. 802–803, 1982. https://doi.
org/10.1038/299802a0[13] K. Azuma, S. Bäuml, T. Coopmans, D. Elkouss, and B. Li, “Tools for
Quantum Network Design,” AVS Quantum Sci., Vol. 3, No. 1,
014101, 2021. https://doi.org/10.1116/5.0024062[14] M. Curty, K. Azuma, and H.-K. Lo, “A Quantum Leap in Security,”
Phys. Today, Vol. 74, No. 3, pp. 36–41, 2021. https://doi.org/10.1063/
PT.3.4699[15] M. K. Bhaskar, R. Riedinger, B. Machielse, D. S. Levonian, C. T.
Nguyen, E. N. Knall, H. Park, D. Englund, M. Lončar, D. D.
Sukachev, and M. D. Lukin, “Experimental Demonstration of Memory-enhanced Quantum Communication,” Nature, Vol. 580, pp. 60–64,
2020. https://doi.org/10.1038/s41586-020-2103-5[16] Y.-F. Pu, S. Zhang, Y.-K. Wu, N. Jiang, W. Chang, C. Li, and L.-M.
Duan, “Experimental Demonstration of Memory-enhanced Scaling
for Entanglement Connection of Quantum Repeater Segments,” Nat.
Photon., Vol. 15, pp. 374–378, 2021. https://doi.org/10.1038/s41566021-00764-4[17] S. L. N. Hermans, M. Pompili, H. K. C. Beukers, S. Baier, J. Borregaard, and R. Hanson, “Qubit Teleportation between Non-neighbouring Nodes in a Quantum Network,” Nature, Vol. 605, pp. 663–
668, 2022. https://doi.org/10.1038/s41586-022-04697-y[18] Y. Hasegawa, R. Ikuta, N. Matsuda, K. Tamaki, H.-K. Lo, T. Yamamoto, K. Azuma, and N. Imoto, “Experimental Time-reversed Adaptive Bell Measurement towards All-photonic Quantum Repeaters,”Nat. Commun., Vol. 10, 378, 2019. https://doi.org/10.1038/s41467018-08099-5[19] Z.-D. Li, R. Zhang, X.-F. Yin, L.-Z. Liu, Y. Hu, Y.-Q. Fang, Y.-Y. Fei,
X. Jiang, J. Zhang, L. Li, N.-L. Liu, F. Xu, Y.-A. Chen, and J.-W. Pan,
“Experimental Quantum Repeater without Quantum Memory,” Nat.
Photon., Vol. 13, pp. 644–648, 2019. https://doi.org/10.1038/s41566019-0468-5[20] R. Zhang, L.-Z. Liu, Z.-D. Li, Y.-Y. Fei, X.-F. Yin, L. Li, N.-L. Liu, Y.
Mao, Y.-A. Chen, and J.-W. Pan, “Loss-tolerant All-photonic Quantum Repeater with Generalized Shor Code,” Optica, Vol. 9, No. 2, pp.
152–158, 2022. https://doi.org/10.1364/OPTICA.439170

Koji Azuma

Distinguished Researcher, Theoretical Quantum Physics Research Group, NTT Basic
Research Laboratories.
He received a B.E., M.E., and Ph.D. in physics
from Osaka University, the University of Tokyo,
and Osaka University in 2005, 2007, and 2010.
He joined NTT Basic Research Laboratories in2010. He has been a guest associate professor of
Graduate School of Engineering Science, Osaka
University, since 2019. He is a member of the
Physical Society of Japan.

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Regular Articles

Identification of Transcription Factors
and the Regulatory Genes Involved in
Triacylglycerol Accumulation in a
Unicellular Red Alga
Sousuke Imamura
Abstract

Triacylglycerols (TAGs) generated by microalgae are a raw material for liquid biofuel production, so
increasing the amount of TAGs generated will contribute to reducing the environmental impact of, forexample, greenhouse gas emissions. Since transcription factors (TFs) regulate the expression of a groupof genes with related functions, it is thought that TAG accumulation can be enhanced by identifying TFs
involved in TAG accumulation and enhancing their functionality. In this study, my research colleagues
and I used transcriptomic and phosphoproteomic data—obtained under conditions of TAG accumulation
in a unicellular red alga, Cyanidioschyzon merolae, to identify 14 TFs that may regulate TAG
accumulation. To verify the function of these TFs, we constructed functionally enhanced strains
overexpressing each TF and analyzed changes in TAG accumulation. The analysis results indicate thatthe amount of TAGs regarding the four overexpressing strains increased 2.2 to 3.8 times compared withthat regarding the control strain, so we can consider that those four TFs are involved in TAG accumulation.
Transcriptome analysis of each of the four TF overexpression strains showed that among the group ofgenes related to TAG synthesis, only a gene-encoding endoplasmic reticulum-localized lysophosphatidic
acid acyltransferase 1 (LPAT1) significantly enhanced. In strains that overexpressed LPAT1 andenhanced its functionality, TAG accumulation increased 3.3 times compared with that in the control
strain. These results (i) explain the mechanism by which four types of TFs regulate the TAG amount in
C. merolae by altering the expression of a group of target genes, including LPAT1, and (ii) indicate thatenhancing TAG accumulation by strengthening functionality TF is useful.
Keywords: a lgal biofuel, lysophosphatidic acid acyltransferase, red alga, transcription factor,
triacylglycerol

1. Introduction
Microalgae store excess energy in lipid droplets(LDs), which are mainly composed of neutral lipids,in their cytoplasm. Neutral lipids contained in LDs
are mainly triacylglycerols (TAGs), which can beused as a raw material for producing biofuel. Since
the carbon that constitutes TAGs is derived from carbon dioxide (CO2) fixed by photosynthesis, biofuel
production using microalgae is expected to contribute
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to reducing environmental load such as mitigating
global warming [1–3]. However, a large-scale system
for industrial biofuel production using microalgae
has not been established. The main reason is due tothe lack in the understanding of the underlying
molecular mechanisms that control TAG accumulation in microalgae.
Our research group has aimed to explain the regulatory mechanism of TAG accumulation by using amodel unicellular red alga Cyanidioschyzon merolae
58Regular Articles

Overexpression of one target gene

Overexpression of one transcription factor

Gene A

Transcription
factor gene “X”

Protein A

Transcription factor “X”

Gene B
Gene C

Effect of protein A

Gene D

Protein B

Protein C

Protein D

Effects of many proteins regulated bytranscription factor “X”

Fig. 1. Comparison of effects of functional modification of a target gene and a transcription factor.

[4]. It has been observed that exposure of C. merolae
to nitrogen-deficient (-N) conditions enhances TAG
accumulation [5], which has been observed for manyother microalgae. The target of rapamycin (TOR) has
also been shown to govern the accumulation of TAGs
under -N conditions [5]. TOR is a highly conserved
protein kinase among eukaryotes and plays a central
role in cell growth and stress response by sensing
environmental conditions [6]. The kinase activity of
TOR is specifically inhibited by rapamycin, an inhibitor of TOR [7]. In our previous studies, when
rapamycin-sensitive C. merolae strains F12 or SF12[8, 9] were grown under normal culture conditions inwhich TAGs do not accumulate, the addition of
rapamycin to the culture medium resulted in accumulation of almost the same amount of TAGs as
observed under -N conditions [5, 9]. TAG accumulation after inhibition of TOR activity has also been
observed in other algae [10–12], in other words, themechanism that regulates TAG accumulation by TOR
has been shown to be common to all algae [13].
Several studies have reported on the use of over
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expression of a single gene involved in synthesis of
TAGs as a method for enhancing TAG accumulation[14, 15] (Fig. 1). Compared with such methods,
simultaneously enhancing the expression of multiple
genes involved in TAG synthesis could further
enhance TAG accumulation. One way to achieve thissimultaneous multiple-gene enhancement is toenhance the function of the transcription factors(TFs) that control gene expression. This is possible
because each TF regulates a group of genes withrelated functions [16] (Fig. 1).
The aims of this study were (i) identify TFs
involved in TAG accumulation in C. merolae, (ii)enhance TAG accumulation by strengthening thefunctions of the TFs, and (iii) identify genes that are
regulated by the identified TFs.
2. Identification of candidate TFs involved in
TAG accumulation
Using previously reported microarray data [5, 10]
and phosphoproteome data [17] obtained under -N- and59

Regular Articles(a)Upregulated transcript after -N
CML101C, CMO347C,
CMS371C, CMT134C,
CMJ282C, CME102C,
CMT433C, CMA095C,
CMO257C, CMG129C

CMT067C

CML246C,
CMR124C,
CMT597C

CMK212C,
CML282C,
CMM055C,
CMB028C,
CMR472C

CML277C,
CMR165C

Decrease inphosphorylation status
after TOR-inactivation

Upregulated transcript
after TOR-inactivation(b)
Gene number

Predicted enzyme

CMB028C
CMK212C
CML101C
CML282C
CML246C
CML277C
CMM055C
CMO347C
CMR124C
CMR165C
CMR472C
CMS371C
CMT067C
CMT597C

Bromodomain-containing protein
Bromodomain-containing protein
MYB-related protein
Basic leucine zipper domain TF
MYB-related protein
Heat shock TF
Protein containing DNA-binding domain SAP
MYB-related protein
MYB-related protein
RNA polymerase sigma factor
PHD-type zinc finger protein
MYB-related protein
E2F TF
Heat shock TF

Fig. 2. Identification of candidate TFs involved in TAG accumulation. (Figure modified from Takahashi et al. 2021)

TOR-activity-inhibiting conditions (under which
TAG accumulation in C. merolae is enhanced), we
identified genes encoding TF with an increase in transcripts and TFs with varying degrees of phosphorylation. The TFs identified for each condition are shownin the Venn diagram in Fig. 2(a). The notation of each
TF corresponds to the number in the C. merolae database (http://czon.jp).
For the ten TFs with expression enhanced only
under -N conditions, the number of TF candidates
was narrowed due to their large number. In green
algae, ROC40 (an MYB-type TF) has been reported
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to be involved in TAG accumulation under -N conditions [18]. Accordingly, among the ten candidate TFs,
the ROC40 homolog CML101C, CMO347C, and
CMS371C were analyzed. A total of 14 types of TFs(as listed in Fig. 2(b)) were therefore analyzed. Notethat the four TFs, CML246C, CMR124C, CMT597C,
and CMT067C, showed enhancement under two conditions.

60

TFc

CMM055Cox

CMB028Cox

CMO347Cox

CMK212Cox

CMR124Cox

CML101Cox

CMR165Cox

CML282Cox

CMR472Cox

CML246Cox

CMS371Cox

CML277Cox

CMT067Cox

BODIPY

Chlorophyll

Bright field

BODIPY

Chlorophyll

Bright field

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Fig. 3. Accumulation of lipid droplets by overexpression of each TF. (Figure modified from Takahashi et al. 2021)

3. Enhancement of LD formation and TAG
accumulation by strengthening the functionof TFsIf the 14 candidate TFs play critical roles in TAG
accumulation in C. merolae, overexpression of each
TF would potentially lead to an accumulation of cytoplasmic LDs that contain TAGs. To examine this possibility, we constructed overexpression strains foreach candidate TF. It should be noted that the overexpression strain for CMT597C has not yet been successfully constructed. The reason is unknown, but it
is conceivable that enhanced expression of the
CMT597C protein may lead to cell death. Accordingly, CMT597C was excluded from subsequent
analysis, leaving a total of 13 TFs included in theanalysis (Fig. 3). The name of each overexpression
strain is shown with “ox” after the gene number. For
example, for CMB028C, the name of its overexpression strain is CMB028Cox.
Each of the generated overexpression strains wasgrown under normal culture conditions under which
TAGs do not accumulate as the wild type, and thepresence or absence of LD formation was determined. LDs were stained with BODIPY (4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene), i.e., a fluorescent reagent for neutral lipids, and
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observed using a fluorescence microscope. The
observation results indicate clear LD formation in the
CMK212Cox, CML101Cox, CML277Cox, andCMO347Cox strains (Fig. 3, white arrowheads).
Chlorophyll indicates the autofluorescence of chloroplasts, namely, the localization of chloroplasts, andscale bars indicate 2 μm. TAG accumulation in eachstrain was then measured using gas chromatography
under the same conditions as when LD formation wasobserved. The results indicate that TAG accumulation in the CMK212Cox, CML101Cox, CML277Cox, and CMO347Cox strains significantly
enhanced—2.2, 3.2, 3.8, and 2.7 times, respectively—compared with the control strain TFc (Fig. 4).
These results indicate that these four TFs function
positively in regard to TAG accumulation in C. merolae.
4. Comparison of expression levels of TAG andfatty-acid synthesis-related genes in
CMK212Cox, CML101Cox, CML277Cox, andCMO347Cox strains
The results described in the previous section indicate that the increase in TAG accumulation due tooverexpression of the four TFs strains (CMK212Cox,
CML101Cox, CML277Cox, and CMO347Cox) is
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TAG (% of dry matter)

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*

0.2

*

*
*

0.1

0
TFc

CMK212Cox

CML101Cox

CML277Cox

CMO347Cox

Fig. 4. A ccumulation of TAGs induced by CMK212C, CML101C, CML277C, or CMO347C overexpression. (Figure modified
from Takahashi et al. 2021)

Table 1. A list of the predicted fatty acid- and TAG synthesis-related genes and their expression ratios in CMK212Cox,
CML101Cox, CML277Cox, and CMO347Cox strains.
Gene number*
CMS299C
CMT420C
CMM286C
CML329C
CMD118C
CMS393C
CMI240C
CMT381C
CMJ027C
CMA017C
CMK217C
CME109C
CMF185C
CMJ021C
CMR054C
CMR488C
CMQ199C
CME100C
CMJ162C
CMB069C

Predicted enzyme
Biotin carboxylase, chloroplast precursor
Malonyl-CoA ACP transacylase
3-ketoacyl-ACP synthase3-ketoacyl-ACP synthase3-ketoacyl-ACP synthase3-keotacyl-ACP reductase
3-hydroxyacyll-ACP dehydratase
Enoyl-ACP reductase
Glycerol-3-phosphate acyltransferase
Glycerol-3-phosphate acyltransferase
Glycerol-3-phosphate acyltransferase
Lysophosphatidic acid acyltransferase
Lysophosphatidic acid acyltransferase
Lysophosphatidic acid acyltransferase (LPAT1)
Phosphatidic acid phosphatase
Phosphatidic acid phosphatase
Diacylglycerol acyltransferase
Diacylglycerol acyltransferase
Diacylglycerol acyltransferase
Diacylglycerol acyltransferase

Ratio (vs TFc)
CMK212Cox

CML101Cox

CML277Cox

CMO347Cox

0.9
1.2
1.9
1.0
0.9
1.0
0.6
1.3
1.1
1.0
1.1
1.0
0.9
39.9
0.8
1.5
1.0
0.7
1.1
1.1

1.2
1.5
2.2
1.0
1.4
1.4
1.0
1.5
1.5
1.4
1.2
1.3
1.0
2.0
1.2
1.6
1.2
0.8
1.4
1.3

1.5
1.4
2.4
1.0
1.1
1.5
0.8
1.6
1.9
1.4
1.3
1.4
1.2
133.5
1.7
2.3
1.3
0.8
1.5
2.4

1.2
2.0
1.9
1.0
1.5
1.7
1.2
1.6
2.1
1.8
2.1
1.9
1.8
13.0
1.6
1.8
1.9
1.4
2.1
1.9

* Gene number in C. merolae database, http://czon.jp(Table modified from Takahashi et al. 2021)

caused by changes in the expression of genes regulated by those TFs, particularly a group of genes
involved in TAG synthesis. Therefore, we conductedtranscriptome analysis (using a next-generation
sequencer) on ribonucleic acid (RNA) isolated from
cells of each overexpression strain and control strain
TFc under normal culture conditions. Table 1 shows
the variation in gene-expression ratios of the genegroups involved in synthesis of TAGs and synthesisof fatty acids, which are materials for TAG synthesis,
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with the relative value of TFc taken as 1. As shown in
Table 1, only CMJ021C, which encodes lysophosphatidic acid acyltransferase (labelled “LPAT1” on
the table), showed enhanced expression for all four
overexpression strains, i.e., expression ratio increased(compared with that of TFc) 39.9 times for CMK212Cox, 2.0 times for CML101Cox, 133.5 times for
CML277Cox, and 13.0 times for CMO347Cox.
These results suggest that the TAG accumulation
observed in the four overexpression strains may be
62Regular Articles(a)Brightfield

Calnexin

LPAT1 Chlorophyll Merged

TFc

LPAT1ox(b)
Bright field

BODIPYTFc

LPAT1ox

TAG (% of dry matter)

(c)

*

0.2

0.1

0
TFc

LPAT1oxFig. 5. Accumulation of TAGs by LPAT1 overexpression. (Figure modified from Takahashi et al. 2021)

due to increased expression of LPAT1.
5. Enhancement of LD formation and TAG
accumulation by enhancing functionalityof LPAT1
To investigate the role of C. merolae LPAT1 in
TAG synthesis and its subcellular localization, we
generated an overexpression strain of LPAT1, i.e.,
LPAT1ox. In accordance with the results of theimmunostaining analysis shown in Fig. 5(a), a signal
derived from LPAT1 (green in the figure) was detected at the same location as calnexin (yellow in the
figure) localized in the endoplasmic reticulum, whichis the site of TAG synthesis. Chlorophyll (red in the
figure) indicates chloroplast autofluorescence and
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chloroplast localization, and “Merged” is a superimposed image of the calnexin, LPAT1, and chlorophyll
images in which the scale bar indicates 2 μm.
In accordance with the results of the analysis of LD
formation and TAG accumulation in the LPAT1ox
strain shown in Figs. 5(b) and (c), respectively, LD
formation enhanced, and TAG accumulation increased
significantly (3.3 times compared with that for TFc).
These results indicate that the reaction catalyzed by
LPAT1, which localizes to the endoplasmic reticulum, is the rate-limiting step of TAG synthesis in C.
merolae and is a key factor in TAG accumulation(Fig. 6).

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Nitrogen depletion or
TOR-inactivation conditions
CO2
+
O2
TFs
• CMK212C• CML101C• CML277C• CMO347C

Carbohydrate

Acyl-CoA
+
G3P
LPA

+
+
+

LPAT1

PA
DAG
+

TAG

Unidentified
target genes

BIOFUEL
Fig. 6. Possible model for the regulation of TAG synthesis in C. merolae.

6. Conclusion and future directions
Analysis based on transcriptomic and phosphoproteomic data identified four TFs involved in TAG
accumulation. No other large-scale analysis has identified multiple TFs involved in TAG accumulation ina single algal species. Moreover, the results of analyzing those target genes revealed that (i) LPAT1
localized in the endoplasmic reticulum and (ii)enhancement of its function enhanced accumulationof TAGs. On the basis of the above results, Fig. 6
shows a possible model in which G3P, LPA, PA,
DAG, and TAG respectively stand for glycerol3-phosphate, lysophosphatidic acid, phosphatidic
acid, diacylglycerol, and triacylglycerol, and the circled + indicates a positive effect. Conditions underwhich TAG accumulates, such as -N conditions or
inhibition of TOR activity, the expression of four
TFs, CMK212C, CML101C, CML277C, andCMO347C, is enhanced, and subsequent upregulation of LPAT1 of their target genes leads to TAG
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accumulation [19].
Target genes other than LPAT1 of the four TFs(labelled “Unidentified target genes” in Fig. 6) are
unknown at this time and should be further analyzed.
Simultaneous enhancement of these genes (after they
are revealed) in addition to the function of LPAT1 is
expected to further enhance TAG accumulation. It isthus concluded that identification of TFs involved in
TAG accumulation is an excellent strategy forincreasing TAG content and will provide important
information for biofuel production using microalgae.
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Miyagishima, M. Shimojima, H. Ohta, and K. Tanaka, “Target ofRapamycin (TOR) Plays a Critical Role in Triacylglycerol
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2017. https://doi.org/10.1016/j.cell.2017.02.004[7] J. Heitman, N. R. Movva, and M. N. Hall, “Targets for Cell Cycle
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org/10.1111/tpj.13144[19] S. Takahashi, R. Okubo, Y. Kanesaki, B. Zhou, K. Takaya, S.
Watanabe, K. Tanaka, and S. Imamura, “Identification of Transcription
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merolae,” Plants, Vol. 10, No. 5, 971, May 2021. https://doi.
org/10.3390/plants10050971

Sousuke Imamura

Distinguished Researcher, Sustainable System
Group, Zero Environmental Impact Research
Project, NTT Space Environment and Energy
Laboratories.
He received a Ph.D. from Tokyo University ofAgriculture and Technology in 2005. He worked
as a JSPS Research Fellow at The University ofTokyo from 2005 to 2009. He joined Chuo University as an assistant professor in 2009. In 2012,he was appointed as an associate professor atTokyo Institute of Technology. In 2019, he was avisiting professor at Institute for Plant Biochemistry, Heinrich-Heine-University Düsseldorf. In
March 2021, he joined NTT Space Environment
and Energy Laboratories as a distinguishedresearcher. He has received prestigious awards
from the Society of Genome Microbiology,
Japan, in 2010 and Nagase Science and Technology Foundation in 2017. He is a member of theJapan Society for Bioscience, Biotechnology,
and Agrochemistry, the Japanese Society of Plant
Physiologists, the Society of Genome Microbiology, Japan, and the Japanese Society of Photosynthesis Research. His research interests
include regeneration of the global environment
using photosynthetic organisms, especiallyalgae, with molecular biology approaches.

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65

Global Standardization Activities

Recent Activities of QoE-relatedStandardization in ITU-T SG12
Yoichi Matsuo, Kazuhisa Yamagishi, and Masanori Koike
Abstract

This article introduces recent standardization activities related to the evaluation of the quality of experience (QoE) of speech, audiovisual, and other new services such as extended reality and chatbots, focusing on the activities of the Study Group 12 of the International Telecommunication Union - Telecommunication Standardization Sector (ITU-T SG12), which is responsible for standardization work on
performance, quality of service, and QoE.
Keywords: quality of experience, adaptive bitrate streaming, extended reality, chatbots

1. ITU-T Study Group 12
The Study Group 12 of the International Telecommunication Union - Telecommunication Standardization Sector (ITU-T SG12) is a lead SG on network
performance, quality of service (QoS), and quality ofexperience (QoE) in the worldwide standardizationof speech and video quality evaluation that takes into
account achievements in regional standardization
bodies such as ETSI (European Telecommunications
Standards Institute) and ATIS (Alliance for Telecommunications Industry Solutions). Since standardization work is carried out in various standardization
organizations regarding network performance parameters, all these organizations have confirmed that
their work matches that of SG12.
2. Full-band and super-wideband
E-model (G.107.2)
Recommendation G.107, called the E-model, hasbeen standardized as a quality-planning tool for telephony services and is used worldwide. For instance,
in Japan, JJ201.11, which specifies the quality ofInternet Protocol (IP) telephony services, uses R values calculated on the basis of the E-model. In Question 15/12 (Parametric and E-model-based planning,
prediction and monitoring of conversational speech
quality), the extension of the E-model was studied for
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evaluating full-band (20 to 20,000 Hz) speech communication services, and the basic algorithm wasstandardized as Recommendation G.107.2 in 2019.
As the next step, the revision of G.107.2 was studied
for enabling the full-band E-model to be used undervarious conditions. Therefore, G.107.2 was revised
by updating the calculation algorithm of the effective
equipment impairment factor (le, eff), delay impairment factor (ld), basic signal-to-noise ratio (Ro), andsimultaneous impairment factor (ls) so that it canhandle background noise, burst packet loss, anddelay.
3. Quality-estimation model anddegradation-analysis procedure for adaptive
bitrate streaming (P.1203, P.1204,
and P.DiAQoSE)
For monitoring adaptive-bitrate-streaming quality,
Recommendation P.1203, which specifies qualitymonitoring techniques for high-definition resolution
coded in H.264/AVC (Advanced Video Coding), andRecommendations P.1204.3, P.1204.4, and P.1204.5,
which specify quality-monitoring techniques for 4K
video and H.265/HEVC (High Efficiency Video Coding), have been standardized. Adaptive bitratestreaming using AV1 codec (AOMedia Video 1) has
been increasing. Thus, the extension of P.1203 andP.1204 to support the new codec is being studied.
66

Global Standardization Activities

HD: high definition
SD: standard definition

Fig. 1. Overview of P.DiAQoSE.

A procedure for interpreting the quality-degradation factors is also being studied for monitoring theadaptive-bitrate-streaming services (Fig. 1). A work
item, P.DiAQoSE, calculates the degree to which
quality parameters (i.e., bitrate, resolution, frame
rate, and stalling information), which are inputs toaudiovisual quality-estimation models such as
P.1203, P.1204.3, P.1204.4, and P.1204.5, reduces theaudiovisual quality estimated using audiovisualquality-estimation models. Specifically, the amountof degradation due to the quality parameters for agiven session is calculated by distributing the difference between the maximum estimated audiovisualquality, which is calculated by selecting the highest
quality parameters, and the current estimated audiovisual quality to each quality parameter on the basisof Shapley theory. In the example in Fig. 1, the mean
opinion score (MOS) estimated by P.1203 for thevideo viewing of Users 1 and 2 are 2.95 and 2.98,
respectively. However, when the degradation amountfor each quality parameter is calculated using P.
DiAQoSE, the MoS of User 1 is more affected byvideo-quality level 2, while the MOS of User 2 issignificantly reduced by stalling. Thus, in addition toaudiovisual quality-estimation models, the amount ofdegradation for each quality parameter makes it easier to take action to improve service quality.

4. Object-recognition-rate-estimation model insurveillance video of autonomous driving(P.obj-recog)
SAE International (formerly, Society of Automotive Engineers) defines six levels of automated driving, from Levels 0 to 5, depending on the driver andarea where the vehicle can be driven [1]. For Level 2,
a remote monitoring center detects objects on thebasis of video from an in-vehicle camera and assists
driving in emergency situations. In this service, anobserver in a remote monitoring center checks forobjects on the road on the basis of the video from thein-vehicle camera that is encoded and transmitted to
the remote monitoring center. Therefore, the video
quality transmitted from the in-vehicle camera should
be clear enough for observers to recognize objects. To
confirm that the video is always transmitted in sufficient quality for object recognition, P.obj-recog waslaunched as a work item to study a technique toderive the probability that an observer can recognize
an object from the encoded video, as shown in Fig. 2.
This will enable quality monitoring of video transmitted from in-vehicle cameras.
5. QoE factors for augmented reality services
and objective quality-evaluation method forextended reality services(G.1036, PSTR-OQMXR)
Recommendation G.1036, which specifies QoE

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Global Standardization Activities

Fig. 2. Estimation method for object-recognition ratio in P.obj-recog.

factors for augmented reality (AR) services, wasstandardized to enable the evaluation of the quality of
AR services. This Recommendation specifies QoE
factors of two AR services that add objects around theuser’s face and objects to the real space. In addition
to quality-influencing factors used in conventional
audio and video quality factors such as bitrate, resolution, frame rate, transmission delay, and device
size, the factors related to the degree of integration
between virtual objects and objects in the real space,such as detection performance of user faces andobjects in the real space, and response performance ofAR services are described. The quality related to theinteraction between the user and AR service is also
specified.
In addition, PSTR-OQMXR has been launched as awork item for a technical report on objective quality
modelling for extended reality (XR) services. The
purpose of this technical report is to identify the current status and issues of current objective qualityassessment methods for XR services as well as issues
that need to be addressed to construct an estimation
model.
6. Guidance for the development of machinelearning-based solutions for QoS/QoE prediction
and network-performance management intelecommunication scenarios (P. 1402)
Recommendation P.1402, which specifies guideNTT Technical Review

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lines for applying machine-learning methods to theprediction of QoS/QoE, has been standardized,
enabling machine-learning methods to be applied tostudied work items in SG12. This Recommendation
describes the basics of using machine learning,
including how to construct training and evaluation
data, categorization of machine-learning methods,
and how to avoid over-fitting. Specific use cases,
which are input/output examples and methods whenmachine learning is used for voice and video QoS/QoE prediction, are also described such as in the
P.565 series.
7. Subjective quality evaluation of text-based
chatbots (P.852)
Recommendation P.852, which specifies a subjective-quality-evaluation method for text-based chatbots, has been standardized. This Recommendation
describes the quality factors for chatbots, which areoverall impression, system information, systembehavior, and user impression of the system. It also
describes the subjective experiments for chatbots andthe evaluation procedure.
8. Outlook
This article described subjective assessment andquality-estimation models for speech, video streaming, XR, and chatbots. SG12 has recently studied the68Global Standardization Activities

extension of recommendations, such as support for a
new codec for adaptive-bitrate-streaming services
and revision of the full-band E-model. A study on thequality of transmitted monitoring video for automated driving services has also been launched.
Since various services are expected to be launchedalong with the deployment of the fifth-generation
mobile communication system (5G)/6G, the design

Yoichi Matsuo

Research Engineer, NTT Network Technology
Laboratories.
He received an M.E. and Ph.D. in appliedmathematics from Keio University, Kanagawa,in 2012 and 2015. Since joining NTT in 2015, hehas been engaged in research on network management. He has been contributing to ITU-T
SG12 since 2019.

and management of QoS/QoE for various services
should be considered. Therefore, it will be important
to investigate the activities of SG12.
Reference[1]

SAE International, “Taxonomy and Definitions for Terms Related to
Driving Automation Systems for On-road Motor Vehicles,”J3016_201806, pp. 1–5, 2018.

Masanori Koike

Researcher, NTT Network Technology Laboratories.
He received a B.E. in mathematical engineering and information physics in 2015 and M.S. in
information science and technology in 2017 fromthe University of Tokyo. Since joining NTT in
2017, he has been engaged in research related tovideo quality. He has been contributing to ITU-T
SG12 since 2023.

Kazuhisa Yamagishi

Senior Research Engineer, NTT Network
Technology Laboratories.
He received a B.E. in electrical engineeringfrom Tokyo University of Science in 2001 and
M.E. and Ph.D. in electronics, information, andcommunication engineering from Waseda University, Tokyo, in 2003 and 2013. In 2003, hejoined NTT, where he has been engaged in thedevelopment of objective quality-estimation
models for multimedia telecommunications. He
has been contributing to ITU-T SG12 since 2006.
He has been a rapporteur of Question 13/12 since2017, a vice-chair of Working Party 3 in SG12
since 2021, and is a vice-chair of SG12 for the2022–2024 study period.

NTT Technical Review

Vol. 21 No. 6 June 2023

69

Practical Field Information about
Telecommunication Technologies
Practical Field Information about Telecommunication Technologies

Deterioration of Telecommunication
Equipment and Facilities in Saltdamage Environments—Case Studies
of Corrosion in Guy Wires andMaintenance Holes
Technical Assistance and Support Center, NTT EAST
Abstract

This article presents examples of equipment and facility deterioration due to corrosion of guy wires
and maintenance holes installed in coastal areas as well as examples of countermeasures to protect
equipment and facilities from salt damage. This is the seventy-sixth article in a series on telecommunication technologies.
Keywords: salt damage, corrosion, maintenance hole

1. Introduction

2. Factors causing salt damage

NTT’s telecommunication equipment and facilities
are installed throughout Japan in a variety of naturalenvironments. Metals, concrete, and plastics are usedas materials for telecommunication equipment andfacilities, which deteriorate over time due to theeffects of the surrounding environment such as ultraviolet rays, rainwater, and seawater.
In response to requests from the field, the Technical
Assistance and Support Center (TASC) has been
investigating the causes of telecommunication failure
due to salt damage in outdoor facilities and indoor
equipment. Examples of equipment deterioration due
to corrosion of guy wires and maintenance holes
installed in coastal areas as well as examples of countermeasures to protect equipment and facilities fromsalt damage are presented in this article.

As shown in Fig. 1, salt damage, which is one of thecauses of deterioration of materials, is a phenomenon
by which particles of sea salt are blown into the air bystrong winds and adhere to equipment containing
metal, and the adhered salt accelerates the corrosionof that metal. Corrosion reactions caused by salt damage are accelerated by electrolytes containing salt,
which increase electrical conductivity and facilitate
the flow of the corrosion current. Since salt is hygroscopic, it absorbs moisture from the atmosphere andbecomes wet, which increases risk of equipment corrosion. Since Japan has a long coastline, salt damage
is an unavoidable phenomenon. Sea-salt particles
originating from sea spray fall in coastal regions dueto the limited distance they can travel, so the effectsof salt damage are more pronounced on equipment
closer to the shoreline, thus prematurely deteriorating
the equipment.

NTT Technical Review

Vol. 21 No. 6 June 2023

70

Practical Field Information about Telecommunication Technologies

Sea spray(salt water)

Carriedby wind

Evaporationof water
Sea-salt particles
Adhesion of salt to buildings andstructures on the groundIn some cases, inflow of seawater
occurs when the bottom of amaintenance hole is lower than
the surface of the adjacent river.

Inflow ofseawater

Maintenance hole
ConduitRiversFig. 1. Causes of salt damage (airborne salt and inflow of seawater).

Bedrock wall

45CR guy
wire

Severe corrosion area
(point of measurementof residual strength)ble

Metal ca

Trunk line A,
No. 1 utility pole

30CR
ger wire
messenLegendCable
Messenger wire
Guy wire
Utility poleMetal cableTrunk line A,
No. 2 utility pole30
messe CR
nger wire

Trunk line A,
No. 3 utility poleFig. 2. Facility configuration.

3. Cases of equipment and facility deterioration
due to salt damage
3.1 Corrosion of a guy wire due to salt damage
First, we present a case study of guy wire deterioration along the coastline. The middle part of a guy wireof a utility pole had significant corrosion. The guy
wire is installed about 30 m from the sea, and theregion is designated as a high-salt-damage area on a“salt-damage map” developed and provided by
NTT Technical Review

Vol. 21 No. 6 June 2023

TASC [1, 2]. The configuration of the facility in thiscorrosion case is shown in Fig. 2.
3.1.1 Investigation method
We investigated the guy wire recovered from thesite from two perspectives: (i) residual strength of thesevere corrosion area and (ii) progress of corrosion inareas other than the severe corrosion area.
3.1.2 Results of investigation
The cross section of the severe corrosion area of theguy wire is shown in Fig. 3. We measured the residual71Practical Field Information about Telecommunication Technologies

Cross section of 45CR guy wire
2.9 mm in diameter (with plating)Breakage

Corrosion wastage
2.9 mmConfirmation of breakage of one of the six
outer wires and corrosion wastage of theremaining five wires

BrokensurfaceFig. 3. Cross section of the severe corrosion area of the guy wire and the state of corrosion of the outer wires.

Table 1. Residual strength of wires of the severe corrosion area.
Target wire

Residual
strength [kN]Standard
value [kN]

Degradation rate [%](1 − residual strength /standard value × 100)Outer wire (1)2.8Outer wire (2)

2.8

−65
−69

Outer wire (3)

2.6

−68
8.1

Outer wire (4)1.8Outer wire (5)

1.4

−78
−83

Outer wire (6)

0

−100

(Reference) Center wire

6.4

−21

Average residual strengthof outer wires

1.9

–

−77

Break point of outer wires
Length of guy wire (length when severe corrosion area is taken as 0 m)
2m

4m

6m

8m

10 m

× 0-m point (measurement point of residual strength)
× 6.7-m point
× 1.45-m point
× 6.8-m point× 2.1-m point× 7.85-m point
× 2.5-m point
× 7.95-m point× 2.75-m point
× 2.85-m point
× 3.6-m point

12 m

14 m

16 m

18 m

Increase in the numberof breakage points

20 m

Utility-pole side

Bedrock side

0m
(Relevant point)

Fig. 4. P rogression of corrosion in areas other than the severe corrosion area of the guy wire (partially broken outer wires:
11 locations in total).

strength of the wires within the guy wire. As listed inTable 1, the residual strength of the wires had
decreased by an average of 77%. The progression of
corrosion in areas other than the severe corrosion area
is shown in Fig. 4, which shows the number of parNTT Technical Review

Vol. 21 No. 6 June 2023

tially broken outer wires (“×” in the figure) increases
from the utility-pole side to the bedrock side. Thisconfirms that corrosion areas are more prevalent on
the bedrock side than on the utility-pole side.

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Practical Field Information about Telecommunication Technologies

N

Meteorological data near Trunk line A, No. 2 utility pole
• Distribution of wind direction in 1 hour at the maximum wind speeds• Hourly observations in 10 years from 2007 to 2016aSe

North northwest

Northnortheast

Northwest

ze
ee
br

Hill
(upwind sideof location A)

North

Location ofseverecorrosion areaB

Guy wire

Northeast

North northwest

North

North
northeast

Northwest

NortheastWestnorthwest

EastnortheastWestnorthwest

Eastnortheast

West

East

West

East

West
southwest

EastsoutheastWestsouthwest

Eastsoutheast

Southeast

Southwest

Trunk line A, No. 2 utility pole

Enlarged

South
South
southwest South southeastA(a) Aerial photograph

Southeast

Southwest
South
South
southwest South southeast: All wind speeds: Wind speed of 5 m/h or more
: Wind speed of 10 m/h or more(b) Meteorological data (wind direction)

Fig. 5. Local aerial photograph and meteorological data (wind direction).

3.1.3 E stimation of the cause of corrosion andcountermeasures
The reason that the guy wire had severe corrosion
on the bedrock side but light corrosion on the utilitypole side was estimated as follows and illustrated inFig. 5(a). On the bedrock side, there are no obstacles
blocking the air containing sea-salt particles above
the sea from being carried by the sea breeze to thesevere corrosion area of the guy wire. In other words,
sea-salt particles in the air above the sea are blown inas they are. On the utility-pole side, however, the hillacts as an obstacle blocking the sea breeze and captures sea-salt particles blown in by the sea breeze. It
can therefore be concluded that the difference in the
corrosion state in those two sides of the guy wire isdue to the difference in the amounts of sea-salt particles in the air blowing on those sides. This estimation
is supported by the meteorological data (wind direction) shown in Fig. 5(b), that is, the hill on the upwindside of location A is located in a position where it
obstructs sea breeze from the most-frequent winddirection (northwest).
For a countermeasure against salt damage, it is firstnecessary to identify areas where corrosion progresses rapidly and requires careful inspection by referencing salt-damage maps, etc. Second, when conducting inspections, it is necessary to ensure that theentire facility is thoroughly inspected since the corrosion rate may vary—even for the same facility—
owing to differences in environmental conditions
such as wind exposure, which makes it possible toreliably detect unsafe equipment. When messenger
wires and guy wires are renewed, implementing corNTT Technical Review

Vol. 21 No. 6 June 2023

rosion countermeasures such as replacement withproducts with stronger anti-corrosion treatment (e.g.,
powder coating [3]), as shown in Fig. 6, can be
expected to extend the service life of equipment.
3.2 M etal corrosion in a maintenance hole due tosalt damage
Cases of metal corrosion in a maintenance hole in asection located along a river are shown in Fig. 7. As
shown in Fig. 8, the maintenance hole is locatedabout 20 m from the river and about 400 m from thesea. It was previously confirmed that the bottom of
the maintenance hole is 0 m above sea level or lower
and that the water level inside the maintenance holerises and falls due to the ebb and flow of the tide, so
it was suspected that river water flows into the maintenance hole.
3.2.1 Investigation method
Water retained in the maintenance hole, corrosion
fixtures, and river water were collected. Ion concentrations of the maintenance-hole water and riverwater were first analyzed by ion chromatography.
The presence or absence of river-water inflow and thecause of corrosion were then analyzed by conducting
X-ray structural analysis of the fixtures from themaintenance hole: mounting hardware with loose rustremoved, a metal fixture with significant loose rust(metal-fixture adhering substance 1), and metal fixture with minor loose rust (metal-fixture adhering
substance 2).
3.2.2 Results of investigation
Regarding the analysis of ion concentration contained in maintenance-hole water and river water, as
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Practical Field Information about Telecommunication Technologies

Not powder coated

Powder coatedFig. 6. Example of anti-corrosion measure for messenger wires and guy wires (powder-coated messenger wires).

Reddish-brown rust has developed
and the surface is uneven.

Fig. 7. Appearance of metal corrosion on fixtures inside a maintenance hole.

Sea
Acquisition locationof Sample 1

River

River

×
20 m
Acquisition locationof Sample 2

×Maintenance
hole

400 m
Maintenance holeFig. 8. Location of maintenance hole.

shown in the “Sample 1: river water” row in Table 2,high concentrations of chlorine and sodium ions were
found in the river water. As shown in the “Sample 2:maintenance-hole water” row of the same table, theNTT Technical Review

Vol. 21 No. 6 June 2023

maintenance-hole water has higher concentrations ofchloride and sodium ions than the maintenance-holewater from inland areas, which suggests inflow ofriver water into the maintenance hole.
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Practical Field Information about Telecommunication Technologies

Table 2. Ion concentrations in maintenance-hole water and river water [mg/L].
ID

Chlorine ionsCl−

Sodium ionsNa+

Potassium ions
K+

Magnesium ions
Mg2+

Calcium ions
Ca2+

Sample 1:river water

4200

1800

140

220

78

Sample 2:maintenance-hole water

170

71

5.9

6.8

17

(Reference)
Maintenance-hole waterfrom inland areas

42

19

4.2

7.9

43

(Reference)
Seawater

19000

11000

380

1300

11000

Intensity (count per second)

1.5e+004

Measurement data

1.0e+004

Spectrum of FeO(OH)
Spectrum of Fe3O4
Spectrum of SiO2 (silicon dioxide)

5.0e+003

0.0e+0002 (deg)

Fig. 9. Results of X-ray structural analysis of metal-fixture adhering substance 1.

The results of the X-ray structural analysis of themounting hardware and metal-fixture adhering substances are shown in Fig. 9. They indicate that metalfixture adhering substance 1 contains iron-corrosion
products, indicated by peaks at the spectral positionsof hydroxides (goethite: FeO(OH) and magnetite:
Fe3O4), which are formed when iron corrodes. As
shown in Table 3, the other samples (the mounting
hardware and metal-fixture adhering substance 2)
were confirmed to contain a hydroxide, FeO(OH),
which is an iron-corrosion product. Since iron corrosion is accelerated as the concentration of chloride
ions increases and FeO(OH) generates Fe3O4, theabove results suggest the effect of salt damage.
3.2.3 Estimation of the cause of corrosion andcountermeasures
From the results of the above investigation, we
estimated that salt-containing river water flowed into
the maintenance hole and increased the salinity of thewater retained in the maintenance hole, which in turnaccelerated metal corrosion. Effective countermeasures against corrosion include (i) installing a stopNTT Technical Review

Vol. 21 No. 6 June 2023

Table 3. Identification results of X-ray structural analysis
of mounting hardware and metal-fixture adhering
substances.
SampleCompounds

Mounting hardware

FeO(OH), SiO2

Metal-fixture adhering substance 1

FeO(OH), Fe3O4, SiO2

Metal-fixture adhering substance 2

FeO(OH), SiO2

cock in ducts (Fig. 10) to prevent inflow of seawater
into the maintenance hole in question and surrounding maintenance holes in the same installation environment; (ii) replacing metal fixtures inside maintenance holes with ones that have powder coating [3]providing robust corrosion resistance (Fig. 11); and(iii) attaching a galvanic anode to the metal fixture,i.e., by connecting a metal with lower electricalpotential than the metal of the target fixture, which
sacrificially corrodes instead of the target fixture.

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Practical Field Information about Telecommunication Technologies

For 50-mm dia. conduit

For 25-mm dia. conduit

Fig. 10. Examples of stopcock.

Non-powder-coated mounting hardware

Galvanic anode

Powder-coated mounting hardware

Fig. 11. Examples of corrosion countermeasures for metal fixtures.

4. Conclusion
We conclude by suggesting countermeasures toprotect telecommunication equipment from corrosion caused by salt damage.
It is necessary to apply appropriate robust corrosion-resistant coatings because salt comes into directcontact with steel surfaces. It is also necessary tothoroughly inspect infrastructure equipment in accordance with the corrosion rate of the region while
using salt-damage maps. For the interior of maintenance holes, it is necessary to prevent the inflow ofsalt-containing water from the outside and implement
corrosion countermeasures for metal fixtures. Applying these countermeasures will prolong the service
life of telecommunication equipment and facilities,
especially in salt-affected regions along the coast.
TASC will continue to engage in technical coopera-

NTT Technical Review

Vol. 21 No. 6 June 2023

tion to solve problems in the field, such as issues
related to equipment and facility deterioration due tosalt damage and other types of corrosion and contribute to improving the quality and reliability of telecommunication services.
References
[1]

[2]

[3]

Technical Assistance and Support Center, NTT EAST, “Introductionof Salt-damage Maps,” NTT Technical Review, Vol. 17, No. 4, pp.
31–35, 2019.
h ttp s ://n tt- r e v ie w. jp /a r c h iv e /n ttte c h n ic a l. p h p ? c ontents=
ntr201904pf1.html
Technical Assistance and Support Center, NTT EAST, “Salt Damage
in RT-BOXes: Investigation and Countermeasures of Salt-damage
Environments,” NTT Technical Review, Vol. 20, No. 6, pp. 52–56,
2022. https://doi.org/10.53829/ntr202206pf1
Technical Assistance and Support Center, NTT EAST, “Sulfur Damage and Its Countermeasures in Telecommunication Equipment andFacilities,” NTT Technical Review, Vol. 19, No. 6, pp. 110–114, 2021.
https://doi.org/10.53829/ntr202106pf1

76

External Awards
Best Paper
Winner: Mehdi Tibouchi, NTT Social Informatics Laboratories
Date: December 6, 2022
Organization: The International Association for Cryptologic
Research (IACR)

Using an 80W Yb:KGW Laser.”
Published as: T. Okamoto, Y. Kunihashi, Y. Shinohara, H. Sanada,
M. Chen, and K. Oguri, “MHz-repetition-rate Generation of 1.7cycle Intense Pulses Using an 80W Yb:KGW Laser,” The 83rd JSAP
Autumn Meeting 2022, 22p-C206-11, 2022.

For “SWIFTEC: Shallue–van de Woestijne Indifferentiable Function to Elliptic Curves.”
Published as: J. Chavez-Saab, F. Rodríguez-Henríquez, and M.
Tibouchi, “SWIFTEC: Shallue–van de Woestijne Indifferentiable
Function to Elliptic Curves,” Advances in Cryptology – ASIACRYPT
2022, Lecture Notes in Computer Science, Vol. 13791, Springer,
Cham. https://doi.org/10.1007/978-3-031-22963-3_3

Young Scientist Award for an Excellent Article
Winner: Ai Ikeda, NTT Basic Research Laboratories
Date: March 15, 2023
Organization: JSAP Superconductors Division

AI2000 Most Influential Scholar Honorable Mention
Winner: Marc Delcroix, NTT Communication Science Laboratories
Date: February 24, 2023
Organization: AMiner, Tsinghua University
In recognition of outstanding and vibrant contributions in the fieldof speech recognition.
Electronics Society Activity Testimonial
Winner: Akihiro Kohno, NTT Device Technology Laboratories
Date: March 7, 2023
Organization: The Institute of Electronics, Information and Communication Engineers (IEICE) Electronics Society
For contribution as a secretary of the Polymer Optical Components
Technical Group Committee.
Electronics Society Activity Testimonial
Winner: Toru Segawa, NTT Device Technology Laboratories
Date: March 7, 2023
Organization: IEICE Electronics Society
For contribution to planning and management of IEICE Electronics Society.
Young Researcher’s Award
Winner: Hideaki Kinsho, NTT Network Service Systems Laboratories
Date: March 9, 2023
Organization: IEICE
For “Proposal of a Method for Constructing a Throughput Estimation Model for Wireless Base Stations Based on Hierarchical Bayesian Model Considering the Number of Observations.”
Published as: H. Kinsho and K. Takeshita, “Proposal of a Method forConstructing a Throughput Estimation Model for Wireless Base Stations Based on Hierarchical Bayesian Model Considering the Number of Observations,” Proc. of the 2022 IEICE Society Conference,
B-11-7, Virtual, Sept. 2022.
Young Scientist Presentation Award
Winner: Takuya Okamoto, NTT Basic Research Laboratories
Date: March 15, 2023
Organization: The Japan Society of Applied Physics (JSAP)
For “MHz-repetition-rate Generation of 1.7-cycle Intense Pulses

NTT Technical Review

Vol. 21 No. 6 June 2023

For “Designing Superlattices of Cuprates and Ferrites for Superconductivity.”
Published as: A. Ikeda, Y. Krockenberger, Y. Taniyasu, and H.
Yamamoto, “Designing Superlattices of Cuprates and Ferrites forSuperconductivity,” ACS Appl. Electron. Mater., Vol. 4, No. 6, pp.
2672–2681, 2022.
TELECOM System Technology Award
Winners: Takahiro Kashiwazaki, NTT Device Technology Laboratories; Taichi Yamashima, The University of Tokyo; Naoto
Takanashi, The University of Tokyo; Asuka Inoue, NTT Device
Technology Laboratories; Takeshi Umeki, NTT Device Technology
Laboratories; Akira Furusawa, The University of Tokyo/Institute ofPhysical and Chemical Research
Date: March 22, 2023
Organization: The Telecommunications Advancement Foundation
For “Fabrication of Low-loss Quasi-single-mode PPLN Waveguide and Its Application to a Modularized Broadband High-level
Squeezer.”
Published as: T. Kashiwazaki, T. Yamashima, N. Takanashi, A.
Inoue, T. Umeki, and A. Furusawa, “Fabrication of Low-loss Quasisingle-mode PPLN Waveguide and Its Application to a Modularized
Broadband High-level Squeezer,” Appl. Phys. Lett., Vol. 119,251104, 2021.
Best Paper
Winners: Yosuke Todo, NTT Social Informatics Laboratories; Takanori Isobe, University of Hyogo
Date: March 22, 2023
Organization: IACR
For “Hybrid Code Lifting on Space-hard Block Ciphers: Application to Yoroi and SPNbox.”
Published as: Y. Todo and T. Isobe, “Hybrid Code Lifting on Spacehard Block Ciphers: Application to Yoroi and SPNbox,” IACR Trans.
Symmetric Cryptol., Vol. 3, pp. 368–402, 2022. https://doi.
org/10.46586/tosc.v2022.i3.368-402
Best Paper
Winners: Varun Maram, ETH Zurich; Keita Xagawa, NTT Social
Informatics Laboratories
Date: March 26, 2023
Organization: IACR
For “Post-quantum Anonymity of Kyber.”Published as: V. Maram and K. Xagawa, “Post-quantum Anonymityof Kyber,” Cryptology ePrint Archive, Paper 2022/1696, 2022.

77

External Awards

CSEC Research Award
Winners: Fumihiro Kanei, NTT Social Informatics Laboratories;
Ayako A. Hasegawa, National Institute of Information and Communications Technology; Eitaro Shioji, NTT Social Informatics Laboratories; Mitsuaki Akiyama, NTT Social Informatics Laboratories
Date: April 4, 2023
Organization: Information Processing Society of Japan (IPSJ) Com-NTT Technical Review

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puter Security Group (CSEC)
For “A Survey on Public and In-house Secure Development
Guidelines in U.S. and Japanese Industries.”
Published as: F. Kanei, A. A. Hasegawa, E. Shioji, and M. Akiyama,“A Survey on Public and In-house Secure Development Guidelines
in U.S. and Japanese Industries,” 100th CSEC Meeting, Mar. 2023.

78