2021
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FEATUREInterview

Special Issue on
Quantum Technologies
Future of Quantum ICT and
Its Impact on Our Social Life
Research activities at NICT on
quantum communication technologies

Special Issue on
Quantum Technologies
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FEATURESpecial Issue on Quantum Technologies
1

Future of Quantum ICT and Its Impact on
Our Social Life
Research activities at NICT on quantum communication technologies
TAKEOKA Masahiro

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Development of Quantum Cryptography and PhysicalLayer Cryptography
FUJIWARA Mikio / ENDO Hiroyuki

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Research and Development of Photonic Quantum
Technologies
TSUJIMOTO Yoshiaki

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Trapped-ion Optical Clock and Quantum Network
HAYASAKA Kazuhiro / TANAKA Utako

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Deepening of Superconducting Photon Detection
Technology
MIKI Shigehito

Cover photo: Quantum key distribution equipment
The sender (left) and receiver (right) exchange photons and share the secret key
for quantum cryptography. These devices
are placed at two distant positions, between which the key is shared. They arehoused in a server rack to serve as practical equipment. This equipment embodies
the results of our long-term basic researchusing the quantum optical system built in
our laboratory shown at the upper left ofthis page.

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Research and Development of Superconducting Quantum
Circuits for Quantum ICTYOSHIHARA Fumiki

TOPICS
1213NICT Quantum Camp
NICT’s Challengers File 15

KASHIOKA Hideki

KIM Sunmi
Toward a Breakthrough in Quantum Technology through Research on Superconducting Nitride Qubits

Special Column Prime Minister SUGA visited NICT headquarters to hear about
the NICT’s research on next-generation information and communications technology.

2020.12.23

On Wednesday, December 23, 2020, Prime Minister SUGA Yoshihide visited the National Institute of Information andCommunications Technology (NICT) headquarters located in Koganei City, Tokyo. Guided by Minister for Internal Affairs
and Communications TAKEDA Ryoji and President of the NICT TOKUDA Hideyuki, he was shown around our research on
next-generation information and communications technology.
The Prime Minister listened to the explanations of our researches including quantum cryptography, Beyond 5G,
multilingual translation, and cybersecurity. After the tour, he commented to the press, “The NICT and the private sector
are working hard on research and development toward the next-generation digitalization. I have got a strong belief thatJapan will lead this field.” Regarding Beyond 5G, he said, “It will become the social and industrial infrastructure in 2030 andbeyond, and the government intends to boost research and development, and extend the outcomes to the world.”

Future of Quantum ICT and Its
Impact on Our Social Life
Research activities at NICT on quantum communication technologies
There is a growing trend towards the application of quantum mechanics, a sciencefor describing the behavior of microscopic particles like photons and electrons, to engineering such as communication, computing, and sensing. IBM and Google are actively
developing quantum computers. In Japan, Toshiba announced a practical quantum
cryptography system that ensures the security of the core systems of the government
and private companies. The NICT has a long history of research and development on
quantum communication technologies, and has made significant contributions to theprogress of quantum technology in Japan, including quantum cryptography. The heartof our research and development is the Quantum ICT Advanced Development Center.
This article features an interview with the director, TAKEOKA Masahiro.

TAKEOKA Masahiro

─What is the situation of quantum technology in the world and in Japan?

quantum life science, and quantum security,
among others.

TAKEOKA Looking back on the progress of research in quantum communication
technology, research continued throughout
the 1990s and the beginning of 2000s at
the scientific level. However, around 20042010, Japan, Europe, and the United States
succeeded in giving field demonstrations ofquantum cryptography one after another,which were followed by practical researchand development by private companies. In
the mid-2010s, IBM and Google started fullfledged development of quantum computers,
spurring the application of quantum physics,
one of the most difficult-to-understand fields
in physics, to engineering.
China invested heavily in quantum cryptography research, built a quantum cryptography network over 2,000 km long in 2017,
and succeeded in an experiment with quan-tum cryptography between a satellite and theground. It is considered that, in principle,
quantum cryptography is unbreakable by any
computer, and many nations including Japanare working hard on development. This is a
new cryptographic technology and there arehigh hopes for both its social applications aswell as national security.
In January 2020, the Integrated Innovation
Strategy Promotion Council of the Japanese
government announced the “Quantum Technology Innovation Strategy," which definedquantum technology as an important strategic
technology, like AI. Also, a project for establishing a beachhead for quantum innovation
has been started. The main applications ofquantum technology include quantum computing, quantum sensing, and quantum communication. And through merging and collaboration with conventional technologies,
quantum technology is expected to spawn
unprecedented innovations in quantum AI,

Director, Quantum ICT Advanced Development
Center, Advanced ICT Research Institute
He joined Communications Research Laboratory (currently
NICT) in 2001 after receiving his Ph. D. degree. His current research area includes quantum optics, quantum information
theory, and quantum cryptography. Ph.D. (Engineering).

─ NICT has a long history of researching
quantum cryptography technology.
TAKEOKA We collaborated with privatecompanies including NTT, Mitsubishi Electric, NEC, and Toshiba, and universities tolaunch the Tokyo QKD Network in 2010.
QKD stands for “quantum key distribution”,
and the Tokyo QKD Network is a testbed for
conducting developments of practical quantum cryptography technologies. It is the longest-running testbed in the world. Throughvarious experiments with private companies
and universities, we have developed manytechnologies, and as a result, Japan is leading the world in terms of the performanceof QKD tranceivers. We are also in the vanguard of application technologies such as
controlling and managing networks, and how
NICT NEWS 2021 No.2

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Special Issue on Quantum Technologies

Future of Quantum ICT and Its Impact on Our Social Life
Research activities at NICT on quantum communication technologies

to exploit QKD networks.
─ Could you talk about your research on
satellite QKD?
TAKEOKA In QKD, it is necessary totransmit photons, which are optical particles.
However, photons scatter to some extent ina fiber, which restricts the distance they canreach. In contrast, space is a vacuum and solight does not scatter, meaning that in principle, we can transmit photons over long
distances. To demonstrate this, we are working with the Wireless Networks ResearchCenter to develop technology for QKD between a satellite and the ground. NICT has
world-leading technology in satellite opticalcommunication. Using this technology, we
succeeded in a world-first basic experimenton using quantum communication to exchange information at the level of a single
photon between a micro-satellite, what wecall SOCRATES, and an optical ground station in 2017. The Chinese experiment useda large satellite weighing about 600 kg, but
ours weighs just 50 kg. Micro-satellites arecheaper and therefore easier to commercialize. The development has been conducted aspart of a five-year project by the Ministry ofInternal Affairs and Communications (MIC).
In fiscal 2020, we started new research anddevelopment involving collaboration amongindustry, academia, and government in Japanto create a global quantum cryptography network that integrates the ground networks andsatellite QKD under a new MIC project.

■ Quantum research hubs
─What is the quantum research hub
mentioned in the Quantum Technology
Innovation Strategy of the Japanese government?
TAKEOKA Hubs are being established as
strategic centers for research and develop-

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NICT NEWS 2021 No.2

ment on major quantum technologies including quantum computing, quantum sensing,and quantum communication. The explicit
intention is to attract players into this fieldof research. The NICT is in charge of the“quantum security hub,” which pursues thecommercialization and dissemination of newtechnologies created by combining quantum
security technologies such as QKD, modern
security technologies, as well as the cutting-edge communication technologies. Researchers and engineers from academia andindustry gather here. The facility is the center of research and development, and also aplace for nurturing human resources and social implementation.
Through collaboration between the government and private sector, we can clarify
the vision for the practical stage of quantum
cryptography. It also enables us to identify
technology fields where we are weak, how
the technology will be used, and how to makeit more useful. For example, we are currently
working with medical professionals to carryout demonstration experiments on protecting
medical records and genetic information,
which is important private information forpatients. We also started collaboration withfinancial institutions and government organizations.

■ Japanese leadership ininternational standardization
─ Could you talk about your efforts on
international standardization?
TAKEOKA From the technological viewpoints, quantum cryptography network is
a communication network similar to telephones and the Internet. If each countrybuilds a network based on its own standard,
it cannot connect to others. This is why wehave to establish an international standard.
When building a standard across theglobe, each country tries to take advantage of

it by making it compatible with its own development. We have made a head start overothers by launching the Tokyo QKD Network, and have gained practical experiencethrough 10 years of development and operation. Another great advantage is the close ties
between research institutions like the NICT
and the manufacturers involved in actual operation. A network is not just a collection ofequipment; its elements must be connectedwith each other and controlled in an integrated manner. Those nations that have acquired
such techniques have an advantage.
In October 2019, the International Telecommunication Union Telecommunication
Standardization Sector (ITU-T) published arecommendation labelled as Y.3800, the firstinternational standard for QKD networks, atthe initiative of Japan. Backed by our knowledge and experience accumulated throughoperating the Tokyo QKD Network, we wereable to prepare a persuasive draft. The adoption of our proposal will bring significant
advantages to Japanese industry when their
QKDs are disseminated in worldwide commercial applications.

■ Educating “quantum natives”
─Do we have enough human resources
for quantum ICT development?
TAKEOKA Because this field of technology
is very new, we do not have enough humanresources. Quantum science and technology
such as quantum communication have been
considered to be a part of physics, mathematics, and basic computer science. There
has been little exchange between academia
and industry on implementing communication and computing in the real world. Veryfew people understand both worlds. Since
2020, the NICT has been operating a project, NICT Quantum Camp (NQC), for educating “quantum natives,” a new generation
speaking quantum technology as native lan-

guages. This fiscal year, due to the spread of
Covid-19, the main activities have been limited to online lectures with around 30 participants. However, we will expand the activities
to include hands-on experiments and tours toresearch and development facilities, as wellas lectures.
Most participants are graduate students
interested in quantum technology, but there
are also company employees and undergraduate students. In the future, we hope to develop human resources who will be involved inquantum technology as a business, not just as
researchers.

Figure 1

Overview of global quantum network

Figure 2

Basic configurations of the QKD network and user network specified in
the international standard recommendation, ITU-T Y.3800

─Are there any differences between thestudents studying quantum physics as cutting-edge physics and the people looking atengineering applications?
TAKEOKA Quantum physics itself is abroad field of study that cannot be fully explored in a lifetime. However, from the viewpoint of its engineering, it is not necessary toknow everything. I think we will introduce a
new form of study in which we start by efficiently acquiring only the essence of quantum mechanics required for engineering, theninvestigating the details when it is required.
In fact, the basics can be understood with justknowledge of undergraduate-level linear algebra.

■ The future of quantum
communication
─What future do you see for quantum
communication technology?
TAKEOKA When Toshiba started introducing practical quantum cryptography last year,
it marked the dawn of the industrializationof quantum communication technology inJapan. Other quantum-related technologies,
such as quantum computing, have been attracting public attention, but these are newtechnologies with many issues to be solved.
Rather than being swayed by the latest trends,
we will take a practical approach and deliver
what society really wants.
To make this happen, I think it is important to steadily nurture the existing technologies into practical forms. Another important
approach is to think beyond the conventional frame of reference. We intend to conduct
research and development while striking thebest balance between these approaches.
In the 1960s, when several researchers
started experiments on the first Internet, could
they have imagined the services we enjoy today, such as online shopping and SNS? The
early Internet was nowhere near as good as

the telephone networks already in existence
around the world at the time. Simply thinking
beyond the conventional frame of reference
and challenging new technologies led to thedevelopment of technologies that changed
the world in 20 or 30 years. The same can besaid for quantum network technology. Rather than keeping up with the latest trends, bycontinuing to tackle difficulties diligently andboldly, I believe we will be able to contributeto society in a completely new way.

NICT NEWS 2021 No.2

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Special Issue on Quantum Technologies

cryptography

Development of Quantum Cryptography and Physical Layer
Cryptography
Figure 2

he NICT has been conducting research and development of a technology for a cryptographic communication
system that can not be broken even withthe most advanced computers in the future, or a quantum cryptography networkthat ensures information-theoretically secure communication, as well as a technology for building a distributed storage on the
network that is also theoretically secure. We
have been operating a quantum cryptography network, Tokyo QKD Network, covering
an area within 100 km from the center ofTokyo since 2010, and research on spaceborne implementation is also ongoing. This
article introduces our efforts and the progress towards global deployment.

T

FUJIWARA Mikio
Research manager, Quantum ICT
Advanced Research Center, Advanced
ICT Research Institute
He joined the Communications Research Laboratory, Ministry of Posts andTelecommunications (currently NICT) in
1992. He has been engaged in researchon satellite-mounted far-infrared detectors, photon number classifiers,
cryogenic electronics, and quantum key
distribution. Ph.D. (Science).

ENDO Hiroyuki
Researcher, Quantum ICT Advanced
Research Center, Advanced ICT
Research Institute
After completing doctorial course, hejoined NICT in 2017. He has been engaged in research on physical layer
cryptography, quantum random number generation source. Ph.D. (Science).

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NICT NEWS 2021 No.2

■Background
RSA and DH are currently the most popular public key cryptography, and are usedin cryptographic communication through
TLS and digital signatures. However, they
are known to be breakable in polynomial
time using a quantum computer, and so there
is an urgent need to make them stronger. In
August 2015, the National Security Agency(NSA) announced a migration to post-quantum cryptography based on a mathematicalalgorithm resistant to quantum computers.
Then, in February 2016, the National Institute of Standards and Technology (NIST)
released a plan for standardizing post-quantum cryptography, which is now in its thirdround as of December 2020. The migration
is expected to start in around 2025. However,the post-quantum public key cryptography,
which is used to exchange encryption keys,
is only computationally secure, which does
not guarantee that it will remain unbreakablein the future. Communication that is considered secure today could be broken someday.
If genomic information that is encrypted today is decrypted in 30 years, it would cause

serious damage, and so it is vital to take measures against information leakage today. A
quantum cryptography technology frees us
from the threat of decryption and is readily
available. However, it has some technicalissues such as the transmission distance andkey generation rate. It is being implemented
in society, along with research for overcoming the above weakness and introducing new
functions.

Figure 1

Performance of quantum key distribution (top) andkey relay schematic (bottom)

Figure 3 oncept of quantumC
secure cloud

Schematic of ground-satellite integrated global quantum secure cloud■QKD network
Quantum cryptography is a combinationof two techniques: quantum key distribution(QKD) that enables sharing a secret key (random number) and Vernam’s one-time pad(OTP) cipher that XORs the transmission
data with a random number shared by thesender and receiver bit by bit. Based on the
key distribution speed and distance of QKD
as shown in Figure 1 (top), with typical optical fiber (having a transmission loss of 0.2
dB/km), a key generation rate of around 100kbps to 1 Mbps is achievable over a distanceof 50 km (transmission loss of 10 dB). Since
the service distance is limited with only asingle link, the encryption key informationis stored in a trusted node as classical information and the key is distributed by encapsulation relay to extend the key distribution
distance, or service area (Figure 1 bottom).
QKD links and trusted nodes comprise a network, which is defined as a QKD network.
Since 2010, the NICT has been operating Tokyo QKD Network, which was developed toenable reliable key relay even between QKD
links of different vendors. The skills acquired
through the development led us to establish
Y.3800, the first ITU-T recommendation in
the field of QKD, and the following seriesof recommendations, seven in total, demonstrating Japan’s initiative in standardization.
In Europe, the operation of a networkwith a similar size is starting. China has built
a QKD network with a total distance of 2,000

km through key relay between Shanghai and
Beijing.
Even though our QKD network is smaller
than the Chinese one, it is world-leading interms of the performance of the QKD equipment, and the reliability and applications of
the network.

■Quantum secure cloud technology
A quantum secure cloud is defined as a
QKD network and a distributed storage built
on a quantum cryptography network that enables secure communication using the QKD
network (Figure 2). The distributed storageis based on a protocol called secret sharing,which enables information-theoretically secure data storage. Secret sharing converts
the original data into several pieces of data
called “share” and transmits them throughcommunication secured with quantum cryptography to distant places, where they are
stored. The owner of the data can restore
the original data by collecting a predefined
number (threshold) of shares. If a number
less than the threshold of shares is obtainedby someone, the information in the original
data is never leaked, which is mathematically guaranteed. The concept of secret sharing
was first proposed in 1979. However, thesecure transmission of shares was merely anassumption. The information-theoreticallysecure distributed storage of data, which does
not rely on hand delivery, became available

only after the advent of a quantum cryptography network. In secret sharing, it is also
possible to implement a secure computation
function that calculates statistical information about the stored data while maintaining
its security. This means that secure transmission, storage, and secondary use of data are
guaranteed. We are also conducting researchand development of various other functions
such as user authentication and integrity-ensuring technology. With these technologies,
we succeeded in demonstrating the distributed storage of genome analyses, electronic
health records (EHR), and biometrics data.
Experiments using actual data are underway.

■Future prospects —Towards
globalization—
With only a QKD network and a quantum
cryptography network using an optical fiber
network, it is too costly to cover the whole ofJapan, let alone the entire world. Considering
the above quantum secure cloud technology
as a function like a database, accessing thecloud with an encryption key transmitted bysatellite would dramatically expand the service area. Since 2018, the NICT has been
participating in the “Research and Development of Quantum Cryptography Technology
in Satellite Communication,” a research directly controlled and commissioned by the
Ministry of Internal Affairs and Communications, and proceeding with the developmentof small-satellite-borne equipment and portable ground stations. QKD is designed to be
secure even against eavesdroppers by using aquantum computer or quantum memory, in a
trade-off with restrictions on throughput. On
the other hand, in line-of-sight communication such as that between a satellite and theground, eavesdroppers in the communication
channel can be identified by various means.
The attacks available to an eavesdropper canbe rationally restricted to passive tapping.
We distinguish the technology for securely
sharing an encryption key in such a communication channel from QKD and call it“physical layer cryptography.” Research anddevelopment is under way on technology forsharing an encryption key at speeds that areorders of magnitude higher than that of QKD.
Assuming that either QKD or physical layer
cryptography will be used depending on thetarget, efforts will focus on developing practical technology. It is considered possible
to build an information-theoretically secure
global network using this technology (Figure3). We are the only organization in the world
which is conducting comprehensive research
on a globalization strategy and killer applications, based on feedback from those involvedin the NICT with various backgrounds. To
achieve social implementation, we will strive
to develop this technology into a truly meaningful art, not just a technique for dealing
with quantum technology.

NICT NEWS 2021 No.2

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Special Issue on Quantum Technologies

Research and Development of Photonic Quantum Technologies

quantum network is the ultimate

A network which aims to connect

TSUJIMOTO Yoshiaki
Researcher (Tenure-Track), Quantum
ICT Advanced Development Center,
Advanced ICT Research Institute
After getting his Ph.D. in Physics, hejoined NICT in 2017. He is engaged in developing entangled photon pair sourcesand experimental demonstration of thequantum protocols. Ph. D. (Science).

quantum devices such as quantum computers in order to derive the maximum
functionality based on the principles ofphysics. This network uses a correlation
specific to quantum mechanics called“quantum entanglement” as a resource torealize a protocol that is not attainable withconventional technology based on classical physics. This article explains the latest
results of our research and developmenton the fast generation and application ofentangled photon pairs that have such aquantum correlation, and a proof-of-principle experiment of the quantum protocol
made possible using those entangled photon pairs.

■Creating a quantum network
The Internet has brought new values to society by connecting people and things. Similarly, the concept of a quantum network is a
network that connects quantum devices suchas quantum computers and quantum sensors.
Distributed/blind quantum computing and ultra-precise time synchronization and various
other protocols using quantum networks havebeen proposed, and many other applications
will emerge in the future. However, muchtechnical development remains to attain thebenefits that are expected of quantum networks. NICT has been conducting researchand development on technologies for generating and controlling entangled photon pairs,
which will provide a basic resource for quantum networks.

■Entanglement
Entanglement is a quantum correlationthat cannot be explained by classical physics. Before exploring quantum entanglement,
let’s look at a classical correlation by taking
the polarization of photon pairs as an exam-

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NICT NEWS 2021 No.2

ple. Here, we have a device that generates
photon pairs polarized vertically or horizontally, which is selected at random. By distinguishing whether each photon is polarizedvertically or horizontally using a polarization
filter, a correlation in which one photon isvertically (horizontally) polarized and theother photon is also vertically (horizontally)polarized, should be observed. However, if
we replace the filter to distinguish between
clockwise and counter-clockwise circularpolarization, no correlation is detected. A
correlation obtained by only a specific polarization is called a classical correlation. On
the other hand, if we use entangled photon
pairs, both the measurement that distinguishes between vertical and horizontal polarization and the measurement that distinguishes
between clockwise and counter-clockwise
circular polarization will detect a correlation.
This behavior cannot be explained using theconcepts of classical physics. A quantum network exploits such a quantum correlation.

■Development of a source of ultrafast entangled photon pairs
NICT has been developing a high-brightness / high-quality entangled photon pairsource using non-linear optical effects called
spontaneous parametric down-conversion(SPDC). Since SPDC generates photon pairs
with a probability proportional to the intensity of a pumped laser, it should be possible toraise the brightness by increasing the intensity of the pumped laser. However, it is known
that the higher the intensity of a pumped laser, the larger the probability of generating
two or more photon pairs in a single pulse, oran error event, which degrades entanglement.
One way to avoid this trade-off is to raise
the pulse repetition frequency of the pumped
laser. This decreases the energy per pulse,which increases the rate of generating entangled photon pairs without an increase in error
events. We have succeeded in developing a

Figure 2

Figure 1a) Schematic of the experiment system to measure
the correlation parameter, S, of entangled
photon pairsb) R elationship between the average photon
number of entangled photons and Sa) Configuration of ultra-fast entangled photon pairs: The entangled photon pairs are
emitted from the waveguide-type nonlinear crystal. The pump beam is obtained bydoubling the frequency of the high-speed optical pulses generated by electro-opticalmodulation (EOM).
b) Relationship between the detection rate of entangled photon pairs and pumping powerc) Relationship between the quality of entangled photon pairs and pumping power

new ultra-fast entangled photon pair sourceby combining a frequency comb source anda high-efficiency waveguide-type non-linear
crystal (see Figure 1(a)). In this frequency comb source, the repetition frequency
can be varied up to 50 GHz. As Figure 1(b)shows, the maximum detection rate of entangled photon pairs was 1.6 MHz, which wasapproximately a hundred times higher than
the rate obtained in a previous experimentconducted by the Quantum ICT Advanced
Development Center. In addition, as shownin Figure 1(c), we achieved high levels of fidelity, purity, and entanglement of formation(EoF), which represents the quality of quantum entanglement.

■Quantum protocol demonstration
experiment
A protocol that uses such entangled photon pairs as a resource is device-independent
quantum key distribution (DIQKD). DIQKD
is a next-generation QKD protocol that canobtain the secret key without any knowledgeof the QKD device by monitoring the degreeof quantum correlation between entangled
photon pairs. The quantum correlation can beevaluated with the correlation parameter S,

which is obtained from measurement, and if
S exceeds two, the security of the private key
is guaranteed. However, when performing
DIQKD using entangled photon pairs generated with SPDC, the average photon numberthat maximizes S has not been clearly known.
Using the experimental setup illustrated in
Figure 2(a), we experimentally confirmedthat S takes the maximum in a region of theaverage photon number that is significantly
higher than conventionally thought (Figure 2(b)). In addition, we experimentally demonstrated that long-distance DIQKD is possible
by using a technique called entanglement
swapping. The experimental setup shown in
Figure 3(a) forms a long-distance quantum
entanglement by generating two pairs of entangled photons and concatenating them. We
applied entanglement swapping after adding
the loss corresponding to a 50 km-long optical fiber to entangled photon pairs. The final
state estimated from the result indicated thatS > 2 (Figure 3(b)).

Figure 3a) Schematic of the experiment systemb) Quantum state obtained by entanglement
swapping

can be demonstrated by properly controllingand measuring generated photons. To create
a quantum network, it is essential to mutually exchange quantum information by linking
matter quantum memory and entangled photon pairs, as well as to sophisticate the above
optical quantum control technology. This
will require the research and developmentof various elemental technologies including
quantum memory and a quantum media converter. The Quantum ICT Advanced Development Center will continue to study thesetechnologies from various perspectives.

■Future prospects
The research done so far has shown thepossibility of fast generation of entangled
pairs, and a quantum protocol such as DIQKDNICT NEWS 2021 No.2

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Special Issue on Quantum Technologies

Trapped-ion Optical Clock and Quantum Network
Figure 3 35 calcium ions trapped by the magic potential

n ion trap is a device for capturing

A ions in a closed space with con-

HAYASAKA Kazuhiro
Research Manager, Quantum ICT
Advanced Development Center,
Advanced ICT Research Institute
Joined Communications Research Laboratory (currently NICT) in 1990 after finishing his graduate studies. His research
activities cover quantum optics withtrapped ions. He is a specially appointed
associate professor at Graduate Schoolof Engineering Science of Osaka University. Ph.D. (Science).

TANAKA Utako
Senior Researcher, Quantum ICT
Advanced Development Center,
Advanced ICT Research Institute
Joined Communications Research Laboratory (currently NICT) in 1994 after
finishing her graduate studies. In 2003,
she started working on quantum metrology with trapped ions at Graduate
School of Engineering Science of Osaka
University and she was appointed to an
associate professor in 2006. In 2017, she
was appointed to the current position
under the cross-appointment system.
Ph.D. (Science).

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NICT NEWS 2021 No.2

trolled magnetic fields. In combination
with the laser cooling technique, it cankeep ionized atoms stationary in a space.
In addition, optical clocks, which are atomic
clocks using optical frequencies, and quantum computers are known to be feasible
by controlling the quantum states inside
the atoms and of motion. This article introduces the research and development of atrapped-ion optical clock conducted by the
NICT and the prospects for its application toa quantum network of optical clocks.

■Background
Quantum mechanics textbooks introduce
many thought experiments that involve manipulating the quantum states of each atom.
However, even E. Schrödinger, one of thefathers of quantum mechanics, stated in the
1950s that experiments on an individual atom
are unrealistic. However, in the late 1970s, it
became possible to observe the quantum statesof each ion in an ion-trap experiment. In 1982,
a single-ion optical clock with a frequency accuracy of 18 digits was proposed. Then, basedon the quantum state control technology of asingle-ion optical clock, an ion-trap quantum
computer was proposed in 1995. These technologies have translated into the recent realization of 19-digit frequency accuracy opticalclocks and 32-qubit quantum computers.

■Trapped-ion optical clock
The NICT has been conducting researchand development on ion-trap technology
since the 1990s. We have been leading theworld, creating a calcium ion optical clock in2008 and an indium ion optical clock basedon sympathetic cooling in 2017. A single-ion
optical clock generates an accurate opticalfrequency with the clock transition of anion as the reference frequency. However, ittakes a long time to determine the precise
frequency due to the weak signal from a single ion. This issue was solved by an opticallattice clock that uses atomic ensembles. In
this clock, ten thousand or more atoms arearranged in optical lattices, which are generated by light with a specific wavelength thatcancels frequency fluctuations, to amplify
the signal. Attempts have been made to use
many ions in trapped-ion optical clocks, but
the situation is different from a neutral atomthat has no charge. In the conventional ion
trap, frequency fluctuations called electric
quadrupole shifts differ between ions, whichleads to a different transition frequency byeach ion, lowering the advantage of the useof many ions (Figure 1). While working toovercome this problem, the NICT has discovered a trap potential at which all ions generate the same transition frequency. Following
the practice of calling the optical wavelengththat suppresses the frequency fluctuations inan optical lattice clock a magic wavelength,
we call this trap potential a magic potential.
However, with the conventional ion trap, it
is not possible to generate a magic potential
due to the limited degree of freedom of controlling the potential.

■On-chip ion trap
Since the conventional ion trap has theelectrode arranged in three dimensions, it is
not easy to generate arbitrary trap electricfields. This problem can be solved by an onchip ion trap. In this ion trap, the electrodes
approximate a flat shape and are placed on
the same plane. By dividing these electrodes
and applying different voltages, it is possible
to generate a trap potential with a high degreeof freedom (Figure 2). With this feature, onchip ion traps are also widely used for quantum computers. In collaboration with Osaka
University, the NICT created a prototype ion
trap with 13 pairs of electrodes, and tried togenerate a magic potential. As a result, weFigure 1

 onventional ion trap (left) and trapped calcium ion (right)
C
It is recognized from the uneven intervals of the ions that the electric quadrupole shifts
are not uniform.

Figure 2

Configuration of ion-trap electrode(A) Conventional ion-trap (linear trap) and (B) On-chip ion trap

confirmed that a magic potential could be
generated with 35 ions (Figure 3). Moreover,collaboration within the Advanced ICT Research Institute accomplished the in-housemanufacturing of an on-chip ion trap witha superior surface accuracy and voltage resistance (Figure 4). In addition, we verified
signal enhancement by several ions in an emulator with a conventional ion trap, and cameclose to creating a multi-ion optical clock using an on-chip ion trap.

■Quantum networks with ion traps
and photons
When ions generate photons, the quantum states of the ions and photons produce
a correlation called “quantum entanglement.”
While an ion has a mass and cannot be transported, photons can be transported over long
distances in free space or through an opticalfiber with relative ease. It is thought that byusing ion traps and photons, we can attainquantum clock synchronization that quickly
determines time by connecting optical clocks
quantum-mechanically, and also create aphotonically interconnected quantum computer by connecting quantum computers. The

NICT has focused on quantum networks with
ion traps and photons since an early stage,and, by teaming up with the Max-Planck-Institute of Quantum Optics, in 2004 succeeded in generating a single photon source with
controlled time waveforms from a single ion.
The wavelength of these photons was 866 nm,
which limited the length of transmission
through an optical fiber. However, in 2018,
in collaboration with the University of Sussex and Osaka University, we demonstrated
transmission through a 10-km optical fiber
by converting these photons into photons
having a wavelength for optical communication while preserving the quantum mechanical properties of the original photons. This
achievement brought us much closer to therealization of a quantum network with ion
traps connected through optical fibers.

Figure 4 n-chip trap manufactured by theO
NICT
Manufac tured by the Frontier
Research Laboratory using the
NICT’s in-house facility

distributing and sensing time-space information obtained from such a network to the limit
allowed by quantum mechanics. In addition
to ion-trap technology, the NICT is conducting research and development on many element technologies required for quantum networks, such as optical frequency referencetechnology and micromachining technology,
with the aim of pioneering the realization ofthese technologies.

■Future prospects
Forming a network of optical clocks is
expected to improve time-space informationand the sensing of local gravity fluctuations,which will lead to the deployment of fiber
links in Japan and other nations. A quantum
network should reduce the time required for
NICT NEWS 2021 No.2

9

Special Issue on Quantum Technologies

Deepening of Superconducting Photon Detection Technology

hoton detection is a basic technolo-

P gy in various fields such as quantum

MIKI Shigehito
Senior Researcher, Frontier ResearchLaboratory, Advanced ICT ResearchInstitute
After Completing doctorial course, hebecame a researcher of JST. Joined NICT
in 2005. He has been engaging in research of superconducting single photon detectors. Ph. D. (Engineering).

Figure 1 O ptical microscopic photo of a prototype
1024-pixel SSPD*

10

 ttps://www.nict.go.jp/en/data/nict-news/NICT_h
NEWS_1312_E.pdf

NICT NEWS 2021 No.2

communication, very long-distance laser
communication, laser ranging, and fluorescence correlation spectroscopy. For morethan 10 years, the NICT has been conducting research and development of superconducting-nanowire single photon detectors(SSPDs) that outperform other types of detectors by using a superconducting material. Following past technological revolutions
which delivered outstanding performance,
work has started on refining the technology
further.
Our research was introduced in the December 2013 issue of the NICT news under
the title “Revolution of superconducting
photon detection technology.”* An SSPD isformed with superconducting nanowire witha thickness of several nm (nanometers) and awidth of 100 nm or less. Our research project at that time succeeded in forming opticalresonance structures above and below thesenanowires, which boosted the detection efficiency at the communication wavelengthband (wavelength of 1550 nm) from 20% to80%. Then, the employment of a new structure further improved the performance. The
detection efficiency of our SSPDs is now
close to 90%. The dark count rate (numberof misdetections per unit time), which is an
important performance index of an SSPD, is
10 counts per second, which is much better
performance than other types of detector.

■Development of SSPD array fordeepening of the technology
The conventional SSPD can only detect
the incidence of photons, but there is growing demand in various fields of advanced
technologies for more sophisticated detection such as resolving the number of photons,
their wavelength, and identifying the positionof incidence. We believe that multi-pixeliz-ing of the SSPD is the key technology toobtain such functions, and hence we are engaged in research and development. Since alarge number of superconducting nanowires
with a width of 100 nm or less have to befabricated on a single chip, the difficulty ofdevice fabrication technology increases dramatically. A technology for multiplexing theoutput signals from many SSPD pixels in acryogenic environment (multiplexing signal
processing) is also required. To solve thesechallenges, we have proposed SSPD multiplexed signal processing using a superconducting single-flux quantum (SFQ) circuit,and have demonstrated its operation. The
SFQ circuit is a digital circuit, and so has
flexibility in processing signals. With a multipixel SSPD, the various functions mentioned
previously can be implemented by combining a variety of circuits including multi-input
logical OR, coincidence counting, address
encoding, and so on. In addition, with theprogress of technology for fabricating superconducting nanowires, starting from 4 pixels,
we have succeeded in creating and demonstrating the operation of 8, 16, and 64-pixel SSPDs. We have also started to develop
a prototype of a 1024-pixel SSPD as shownin Figure 1. Although it has not yet reached
the stage of a complete operational demonstration, partial operation of the 1024-pixel
SSPD has been confirmed, paving the way
for a complete demonstration of a combination of SFQ circuits.
The number of pixels will continue togrow. Although many technological advances are still needed, we believe that a one million-pixel SSPD is no longer a dream.

Research and Development of Superconducting Quantum
Circuits for Quantum ICT

n recent years, the research and development of quantum computers
has gained momentum. About ten years
ago, the feasibility of quantum computers
was highly uncertain, but the number of
qubits has grown rapidly in the last several
years, and for specific problems, quantum
computers have been demonstrated to offer faster processing speed than supercomputers.

I

YOSHIHARA Fumiki
Senior Researcher, Macroscopic Quantum Physics Research Project, Frontier
Research Laboratory, Advanced ICT
Research Institute
After completing his doctorial courseof Nuclear Engineering, he worked at
RIKEN as a researcher. He joined NICT in
2014. He has been engaged in researchof quantum physics using superconducting quantum circuits. Ph.D. (Engineering).

The minimum building block of a quantum computer is called a qubit. It is the quantum version of a bit, which represents 0 or 1.
A qubit can take a superposition of the 0 and1 states. If there are two qubits, they can takea superposition of the state in which qubit A
is 0 and qubit B is 0, and the state in which
qubit A is 1 and qubit B is 1. In this state, thestate of one qubit is determined by the stateof the other qubit. Because of the strong correlation between the two qubits, this is called
an entangled state. A quantum computer
exploits superposition and entangled states
specific to quantum mechanics to conduct
quantum calculations. Such states are applicable to various types of quantum information processing, such as quantum communication, quantum measurement, and quantum
simulation, as well as quantum computing.

■Superconducting quantum circuits

FigureS uperconducting quantum bit (in the redrectangle) and a superconducting resonancecircuit*1 “Superconducting qubit-oscillator circuit beyond theultrastrong-coupling regime,"
DOI：https://doi.org/10.1038/NPHYS3906*2 “Inversion of qubit energy levels in qubit-oscillator
circuits in the deep-strong-coupling regime,"
DOI：https://doi.org/10.1103/PhysRevLett.120.183601

Among the various physical systems forquantum information processing, a superconducting electric circuit is thought to be one of
the most promising. When a superconducting electric circuit is in an ultra-low noiseand very-low temperature (several tens ofmK) environment, states specific to quantum
mechanics such as superposition and entanglement appear. A superconducting electric
circuit that shows phenomena specific toquantum mechanics is called a “superconducting quantum circuit.” For a superconducting quantum circuit to operate, it must becooled down to a very low temperature using
a dilution refrigerator. However, a superconducting quantum circuit has the following advantages that more than offset the drawbackof very-low operation temperature. A superconducting qubit formed on a superconducting quantum circuit has a size of several µm
to several hundreds of µm, which is far larger
than many other qubits. Therefore, it is possible to establish a very strong coupling between superconducting qubits and between asuperconducting qubit and another circuit element, which results in high-speed operation.
Moreover, since a superconducting quantum
circuit is an electric circuit, it can be integrated, providing an infinite degree of freedom
in design. These advantages imply the unexplored potential of quantum information processing technology using superconducting
quantum circuits.

■Future prospects
The Macroscopic Quantum Physics Research Project is conducting research withthe aim of elucidating the interaction between superconducting qubits and microwaves at the level of a single photon, using
a superconducting quantum circuit, which isindispensable for quantum information processing. By fully exploiting the advantagesof a superconducting qubit, a large size andinfinite degree of freedom in design, we haveobtained various results.*1,2 We will continue
our research and development on improving
the coherence properties of qubits, the qualityof the qubit-gate operations and the measurement of qubit state, and so forth, which areessential for quantum information processing
based on superconducting quantum circuits.

NICT NEWS 2021 No.2

11

File 15KASHIOKA Hideki

NICT Quantum CampManaging Director, Strategic
Program Produce Office, Social
Innovation Unit, Open Innovation
Promotion HeadquartersThere are high expectations for quantum ICT epitomized byquantum computation and quantum communication as atechnology that provides superior performance to conventional
technologies. As an important technology for maintaining national security and economic competitiveness, research and development of basic technology, as well as trials and reviews of industrial
applications have been conducted. Many well-funded projects areunder way in various countries.
Although the field is drawing attention, human resources are
scarce. The final report on the Quantum Technology Innovation
Strategy compiled by the Cabinet Office indicates the importanceof “quantum-natives.” The NICT held the NICT Open Summit on
the theme “Next-Generation Quantum Information Processing
and Artificial Intelligence” in July 2019, which also pointed out the
shortage of researchers working on quantum ICT. In response, andin view of the requirement for developing good instructional materials and a system for developing human resources, NICT President
Dr. TOKUDA initiated a review on specific efforts for developing human resources in this field. The NICT has been conducting a humanresource development program called “SecHack365,” which aims
to foster cyber security innovators. While capitalizing on the experience of this program to establish an implementation system, weare developing “quantum natives” by inviting specialists in a wide
range of fields, not just those covered by the NICT, from universities and companies across Japan to serve as instructors. For details,
please visit https://nqc.nict.go.jp/(in Japanese).
Under the policy above, we are conducting two programs.
(See Figure below.)
1. “Hands-on Human Resource Development Program” withseminars and exercises on basic knowledge and practical skills
concerning quantum ICT
The purpose of this program is to broaden the human resources
involved in quantum ICT. In fiscal 2020, it was initially planned torecruit around 20 participants. However, among the many applicants, 30 people including technical college students, university

Figure12

NICT NEWS 2021 No.2

Toward a Breakthrough in Quantum Technology throughResearch on Superconducting Nitride Qubitsstudents, graduate school students, and employees were selected
and are receiving remote seminars and exercises. During breaks
between seminars, interaction among the participants and instructors is promoted to encourage the exchange of various ideas. Even
when describing the same technology, instructors with different
backgrounds have different aspects in their scope. Through theseseminars, the participants learn various views and aspects of thesame subject, quantum ICT.
2. “Exploring/Problem-Solving-Type Human Resource Development Program” for fostering researchers
This program aids research and development on quantum ICTwith the theme chosen by each participant. A person or group can
propose a theme for investigation, development, or research, witha grant of 1 million yen available for each theme. The program is intended to develop world-leading researchers who will support the
NICT. Research and development are now under way on the twothemes selected in fiscal 2020.
The name of the quantum ICT human resource development
program, NICT Quantum Camp (NQC), was chosen based on various ideas and visions of people at the NICT. The NQC participants
are closely linked, sometimes helping each other, sometimes
competing with each other. The NQC provides such a community
space. This is the essence of the NQC.

1975
2003

2006
2009
2013

2018

KIM Sunmi
Researcher, Macroscopic
Quantum Physics Research
Project, Frontier ResearchLaboratory, Advanced ICT
Research Institute
Ph.D. in Physics

Joined ATR after completing the doctoral
course at a graduate school, then joined the
NICT in 2006. After working as the Directorof the Spoken Language Communication
Laboratory and the Director of the Brain
Networks and Communication Laboratory,
appointed to current post. Ph.D.(Engineering).

Born in Buyeo, Korea.
After completing the Ph.D. coursein the Graduate School of Sogang
University (Korea), came to Japanand joined the National Institutefor Materials Science (NIMS) as a
postdoctoral fellow.
Postdoctoral fellow at the Institute ofLaser Engineering, Osaka University.
Postdoctoral fellow at the NIMS.
Postdoctoral fellow at the Instituteof Industrial Science, The Universityof Tokyo, and since 2014, Project
assistant professor.
Joined the NICT as a researcher.

Q What do you want to be in your next life?

A I want to be a doctor who saves people’s lives.

Q What was the most difficult time in your life?

A During an invited talk at an international conference heldin the U.K. in 2006, I got nervous and froze. I learned theimportance of presentation practice.

Q Are you hooked on anything now?

A I am a frequent visitor to a new deep-fried chicken shop in
my neighborhood.

● Awards, etc.
2020
2018

 ICT Excellence Award, “SuccessfulNfabrication of an all-nitride π-junction
flux qubit”
Best Poster Presentation Award,
Advanced ICT Device Lab, NICT,“Fabrication of a superconducting
quantum circuit with a π-junction
qubit”

am working on nitride-based superconducting quantum bits (qubits) ina macroscopic quantum physics researchgroup. Using nitride superconducting materials, I design and fabricate quantum
circuits functioning as artificial atoms, andinvestigate their microwave properties at
an extremely low temperature of 10 mK.
By establishing the nitride-based qubits tohave a long coherence time, we are trying
to create a new material platform of superconducting quantum hardware, which isimportant for quantum information processing.
Recently, superconducting qubits havebeen developed for quantum information
processing not only in academia but also
by companies such as Google and IBM. The
core part of the superconducting qubit is a
Josephson junction (JJ), which is a superconductor/insulator/superconductor junction, mainly fabricated by the aluminum(Al) oxidation process. However, it has been
pointed out that the coherence times of superconducting circuits based on Al-based
JJs are fundamentally limited by energy

I

KASHIOKA Hideki

Q&As● Biography

and phase relaxation due to microscopic
two-level systems (TLS) in the amorphous
Al-oxide layers. To solve such problems, it isessential to consider alternative materials.
In comparison to Al-based JJ, an epitaxially-grown nitride (NbN/AlN/NbN) JJ has highcrystal quality, chemical stability against
oxidization, and relatively high transition
temperature (~16 K) of NbN, making it suitable for qubit applications. In collaboration
with the group of Dr. TERAI Hirotaka in the
Frontier Research Laboratory of NICT and
Prof. YAMASHITA Taro of Nagoya University, I have successfully fabricated a superconducting nitride qubit. By employing a
Si substrate with TiN buffer layer insteadof a conventionally used MgO substrate to
reduce dielectric losses and a structure ofcapacitively-shunted flux qubit to suppress
charge fluctuations, we achieved a significant improvement in quantum coherenceof epitaxially-grown NbN-based qubits(manuscript in preparation). Also, we aredeveloping the NbN-based qubits with ferromagnetic JJs (so-called π-junction) whichmake them operate in a magnetic flux-freestate. Through these investigations withnitride superconducting qubits, we would
like to develop a new quantum hardware
platform that can be a breakthrough in thefield of quantum information processing.

FigurePhotograph of an NbN coplanar waveguideresonator coupled with a nitride flux qubits(a) without and (b) with π phase shifter

Overview of NICT Quantum CampNICT NEWS 2021 No.2

13

NICT NEWS  2021  No.2  Vol.486

Published by Public Relations Department, National Institute of Information and Communications Technology
Issue date: March 2021 (bimonthly)

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ISSN 2187-4050 (Online)