NTT Technical Review

July 2017 Vol. 15 No. 7
Feature Articles: A New Era in Quantum Information Processing Technologies


The Arrival of a New Era in Quantum Information Processing Technologies



Quantum Neural Network for Solving Complex Combinatorial Optimization Problems



Lossless Wavelength Conversion of Single Photons



Demonstration of Realism Violation on a Macroscopic Scale

Coherent Coupling between 4300 Superconducting Flux Qubits and a Microwave ResonatorCreating New Two-dimensional Topological Insulators for Fault-tolerant Quantum Computing

Implementation Security of Quantum Key Distribution

Regular Articles
Heart Rate Measurement with Video Camera Based on Visible Light Communication

Path Loss Model to Evaluate Interference for Small Cells between Different Floors

Global Standardization Activities


Report on Progress of ITU-T Study Group 3 at the World Telecommunication Standardization
Assembly 2016 (WTSA-16)

Short ReportsReport on NTT Group Exhibits at CeBIT 2017—Showcasing Cutting-edge Use of ICT at theWorld’s Largest Information Technology Trade FairNTT Communications to Deploy 400-Gbit/s Optical Transmission System for Datacenter
Network Connections

External Awards/Papers Published in Technical Journals and Conference ProceedingsExternal Awards/Papers Published in Technical Journals and Conference Proceedings

Feature Articles: A New Era in Quantum Information
Processing Technologies

The Arrival of a New Era in
Quantum Information Processing
Technologies
Hiroshi Yamaguchi and Tetsuomi Sogawa
Abstract

Research on quantum information processing has progressed astoundingly in the past several years.
Concepts totally different from before such as adiabatic computation, quantum measurements, and topological states have been introduced, expanding the possibility of applications. This article introduces
recent developments in quantum information processing and discusses NTT’s research achievements as
examples of the advances in this field.
Keywords: quantum information processing, quantum computing, quantum key distribution

1. Introduction
In the last ten years, the application of quantum
information processing technologies has expanded toareas outside of our own field of specialty. In ourwork, we are often asked about quantum computing
by non-specialists. When we began research on quantum technologies at NTT, we had to start by answering the question, “What does ‘quantum’ actually
mean?” Now, however, we are more often asked, “Iknow the background story. How far have you progressed?” There have been times when news about
quantum information processing would suddenly pop
up when we were watching television or listening to
the radio. When watching science-fiction movies, wewould be surprised when a character suddenly uttereda line about “quantum teleportation.” It seems that the
subject of quantum information processing technologies has already spread widely from the realm of
specialists to the general public.
NTT’s research on quantum information processing goes back more than 30 years. We joined NTT
about 30 years ago, and it is no exaggeration to saythat our research career has unfolded alongside thedevelopment of quantum technologies in the NTT
laboratories (NTT Labs). When research began at
NTT, the usual answer to the question about when1

quantum computers would be put to practical use was“at least 30 to 50 years.” Initially, quantum computing technologies were immature, and we think theygave the impression at the time that although the field
was interesting theoretically, putting the technologies
into practical use was not a simple matter.
However, astounding progress has been made inquantum computing over the last several years. When
we first began our research, we had, in all honesty, anegative assessment of the possibility of practicalquantum computing. When we look at the state ofaffairs now, however, we seriously think that it justmight be possible that it will be used. Five years ago
we contributed an article on quantum computing to
the NTT Technical Review [1]. Re-reading the article
again, we are struck by how far the field has advanced
since then. In the US and Europe, world-famous companies have made large investments in the researchand development (R&D) of quantum information
processing technologies. It is truly an unexpectedstate of affairs.
How, specifically, has quantum computing progressed? The Feature Articles in this issue present theadvances in this field. While one cannot understandall the latest quantum information technologies in 30
minutes just by reading these articles, we think one
can understand the atmosphere in which these amazing
NTT Technical Review

Feature Articles

technologies are continually developed based on new
concepts. As a foreword to this issue, we wish to givean overview of these technologies and explain the
new ideas behind them.
2. Adiabatic computation
It can be said that a major impetus of quantum computing research was the effort to achieve high-speed
factorization to break RSA (Rivest-Shamir-Adleman)
cryptography. This claim actually demonstrates the
weakness of quantum computing. That is, the mention of factorization indicates that the problems thatcan be solved at a high speed by quantum computing
are very limited. The number of proven high-speed
algorithms, including those for simple and trivial
problems, is extremely small. Consequently, no matter how much improvement there is in hardware
development for quantum computing, it is unlikelythat quantum computers will entirely replace the allpurpose personal computers we have today.
We refer here to the conventional method of quantum computation as quantum gate computation. In
quantum gate computation, the solution to a problem
is obtained by writing bit information to quantum bits
(qubits) and then performing gate operations on themsequentially (called unitary transformation in quantum mechanics). This method is similar to thesequential computation carried out in computers
today. While this is an easy-to-understand concept,
bit information written in qubits must be correctly
maintained and operated at all times. Thus, scaling up
current hardware is not a simple matter.
In contrast to this approach, a computation method
based on a completely different concept called adiabatic quantum computation has been proposed. In
this concept, gate operations are not performed oneach bit sequentially. Instead, the necessary answer isfinally obtained by continuously changing the bit
environment that makes up the computer (such as
how bits are connected to each other). One of the biggest advantages of this method is its approach toperform calculations while constantly maintaining
the lowest energy state.
The conventional gate computation approach
involves the use of a high-energy excited state, andcomputational errors are produced by the transition toa low-energy ground state. Imagine quantum gatecomputation as passing a golf ball from person toperson in a spoon relay. If there is just one little mistake, the golf ball falls to the ground. In other words,
the computation fails.
Vol. 15 No. 7 July 2017

In contrast, adiabatic computation continues in themanner of a soccer ball being kicked from person toperson at ground level, where it cannot fall down. As
a result, computation can be carried out without losing energy (called energy relaxation in technical terminology) (Fig. 1).
Another major advantage of adiabatic computationis that it can be applied to more problems than quantum gate computation. Although rigorous proof of its
high-speed processing capability remains a future
topic of research, adiabatic computation has been
proposed as a method for handling many combinatorial optimization problems such as the traveling salesman problem and the knapsack problem, for whichconventional computers require computation times
that are exponential to the complexity of the problems.
The classical approach of obtaining a solution bychanging the system while maintaining the lowest
energy state—called simulated annealing—actually
already exists. The proposed application of thismethod to quantum mechanistic conditions is called
quantum annealing [2]. D-Wave Systems, a Canadiancompany, made news when it offered a commercial
quantum computing product utilizing this approach.
A joint research team that includes NTT Labs recently developed a quantum neural network that carries
out the annealing using laser and optical fibers, anddemonstrated a 2000-node calculation [3]. Although
the use of an approach different from quantum
annealing and the detailed roles of quantum mechanical superposition states should be clarified in the
future, the experiment confirmed the high-speed
capability (compared with the conventional computation) to solve a combinatorial optimization problem.
3. Quantum sensing technologies andexperimental verification of quantum
mechanical principles
As introduced in past articles of the NTT Technical
Review, NTT Labs have developed various qubits tocreate quantum computers. Research on these qubits
was conducted with multiplexing and circuit integration in mind and the assumption that in the future, itwill be necessary to connect many qubits—several
thousands and tens of thousands or more. The reason
that developing a quantum computer is reportedly
difficult is that it is not a simple matter to integrate
such a large number of qubits.
However, there have been recent efforts to pioneer
technologies that apply quantum computing without
2

Feature Articles

(a)

(b)

Fig. 1. Pictorial representation of (a) conventional quantum gate computation and (b) adiabatic quantum computation. Gatecomputation is similar to a spoon relay race. Superposition states in qubits (|ψ > in the figure) are manipulated onsequentially. Because a high-energy excited state is used, calculations are not possible if error (energy relaxation)
occurs when the ball is inadvertently dropped. The adiabatic computation (b) is equivalent to dribbling a ball while
weaving through the dips in the ground. Computation is performed while constantly passing through the bottom ofenergy distribution.

using qubits or by using a small number of qubits.
Until now, a linear progression of R&D was assumedfor quantum information processing technologies.
First, the problem of quantum key distribution would
be tackled, and when that was successfully solved,
the next step would be to tackle quantum computing.
Integrating many qubits is essential in this linear
development map.
However, nowadays there is growing awarenessthat quantum technologies can be used for a variety ofgoals that can be achieved with fewer qubits (Fig. 2).
Quantum sensing technology is a representative
example. The application of the superposition principle in quantum mechanics means that sensing technologies that are orders of magnitude higher in sensitivity than current technologies can be created. This
technology exploits the strong interaction between
qubits (the so-called quantum correlation) to create astate that is very sensitive to changes in the external
environment.
Meanwhile, technologies to manipulate the quantum state and read it accurately have made great
strides as a result of advances in quantum computing
technologies. A new development is the use of thesetechnologies to experimentally verify the principles
of physics. In the past, because technologies to3manipulate and detect quantum states were insufficient, it was impossible to experimentally answer
questions involving the nature of physics, such as “To
what size of objects can quantum mechanics beapplied?” However, the technology has recently
advanced to the point where accurate experiments
could be carried out. In these Feature Articles, we
introduce research on superconducting quantum circuits, which are applied to quantum sensing and theexperimental verification of physical principles [4,5].
4. Quantum cryptography and single
photon manipulation
Quantum cryptography is a quantum information
processing technology that is often discussed together with quantum computing. This technology is properly referred to as quantum key distribution. Rather
than being an encryption technology, it provides solutions to everyday issues in encryption technology
such as how to safely deliver a private key to the user.
Disturbing a pure quantum state is an intrinsic part of
the action of observing the state and cannot be avoided. Quantum key distribution secures the safe delivery of private keys by exploiting the fact that an act of
NTT Technical Review

Feature Articles

Quantum computer

Quantum communicationand network
Quantum
repeater

Quantum information
for chemistry

Quantum repeater
Quantum
standard
Quantum
light source(a) Conventional roadmap

Medical testing
equipment
Quantum devices

Quantum simulation

Quantum key
distribution

Quantum computer

Quantum sensing

Quantum key
distribution(b) Present roadmap

Fig. 2. Schematic showing the new expansion of quantum information processing. (a) The conventional roadmap shows alinear progression with the final goal of realizing quantum computing. (b) The current roadmap holds promise for theapplication of quantum technologies to a variety of fields, from quantum sensing to diagnostic tools.

eavesdropping during the stage of key distribution
leaves an effect that cannot be fundamentally erased.
Compared with quantum computing, the prospectsof quantum cryptography as a practical technology
have always abounded, and in fact, experiments using
optical fiber networks are being conducted in countries throughout the world. Although the technology
is expected to be commercialized soon, there are still
many challenges that must be solved, such as security
proofs and the development of quantum repeater
technologies. In this issue, we give an overview of thecurrent state of quantum cryptography and challenges
in its commercialization [6].
In addition to quantum cryptography, single photon
generation technology also plays a critical role inquantum measurement and quantum computing.
Quantum computing requires multiplexing. Thus,
while the idea of manipulating a single quantum(photon) may sound contradictory, the question of towhat extent each quantum can be manipulated withfine control is critical to carry out multiplexing in the
future. Technology to finely control a single quantum(photon) is extremely important because a few quantum states play a critical role in quantum key distribution and quantum repeater technologies.
In this issue, we introduce the results of successfully controlling a wavelength of a single photon,
which plays an important role in quantum information processing technologies. We accomplished thisVol. 15 No. 7 July 2017

by applying the technique of cross-phase modulation,
which is widely used in the field of optical communications, to a single photon for the first time [7].
5. Development of topological materials forquantum information processing
Finally, we touch on material research for topological quantum computers. The Nobel Prize in Physics
in 2016 was awarded to three theoretical physicists at
American universities for their theoretical discoveries of topological phase transitions and topologicalphases of matter. Topology is a discipline that seeks
to understand the conditions of an object from theconnection of things. Consider, for example, four
shapes: a ball, coffee cup, donut, and wine glass. In
topology, the size and curvature of a shape is notimportant. What is important is how connections are
made. A coffee cup can be transformed into a donutby continuous deformation, as can a wine glass into aball. However, to transform a donut (or coffee cup)
into a wine glass (or ball), deformation of the former
object by filling in its penetrating hole must be carried out somewhere. At this stage, the manner inwhich things are connected changes (Fig. 3).
In this way, it is known that different connection
states, in other words, different topological states,
appear not only in physical objects but also in a variety of physical systems. Electronics is a representative4

Feature Articles

Continuous deformation (with a hole)Continuous deformation (no hole)Fig. 3. Conceptual representation of topology. The ball and wine glass have completely different shapes. However, because
they are connected through continuous deformation, they have the same topology. The cup and the donut have thesame relationship. However, to transform the donut into the ball, for example, the penetrating hole in the donut mustbe filled in. Thus, it has a different topology, as it cannot be continuously deformed.

example. The Nobel Prize mentioned above demonstrates the extremely important role that electronstates with different topologies play in solid-statephysics. If it is possible to carry out quantum computing using such states, stable and highly accurate calculations can be performed since changes between
different topological states do not occur easily.
Research on topological insulators applying theabove findings has increased markedly in recent
years. A topological insulator is a material in whichelectronic states with different topologies naturally
manifest themselves due to the material’s special
properties. The heterostructure of two materials,
indium arsenide (InAs) and gallium antimonide(GaSb), has attracted attention as an example of a
topological insulator. NTT Labs have pioneered amethod of crystal growth that dramatically improves
the characteristics of this material. In this issue of the
NTT Technical Review, we introduce the details of
this research and discuss future prospects [8].
6. Conclusion
This article presented an overview of recent developments in quantum information processing and discussed NTT Labs’ research achievements as examples of advances in this field. As mentioned in the
Introduction, the progress in research on quantum
information processing in the past several years has
been astounding. New concepts such as adiabatic
computation, quantum measurements, and topological states have been introduced, expanding the possibility of applications as a result of approaches that5differ from conventional methods. In particular, while
the adiabatic method has aroused debate about theextent to which quantum properties can be exploitedin actual hardware, its future development is drawing
attention because it solves computation problems
with architecture different from the conventional
computation architecture that uses quantum gates.
The most important properties of conventional
digital computers are accuracy, which is achieved
with binary encoding, and versatility, which isachieved with software. Originally, continuous analog values were encoded as digital information.
Although inaccuracies generated in this process are
ignored, calculations after encoding are executedwithout any mistakes. Also, the specifications ofcomputation could be freely changed with software
instead of being fixed in hardware. Present-day information processing technology has developed greatly
based on the completeness and versatility of such a
computational approach. However, we think the situation is gradually changing with the spread of theinternet and the arrival of the big data era.
In the area of search technology, for example, howimportant is it to get a unique and rigorous answer toa search query? Rather, what is important is getting
answers as quickly as possible from an extremely
large number of choices in exchange for a modest
level of certainty. In this way, adiabatic computation,
which solves specific combinatorial optimization
problems, has the possibility of truly being a powerful computing tool for a particular type of fuzzy problem. In this sense, the new technologies introduced
here not only possess high-speed performance by
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Feature Articles

exploiting quantum properties, they also have thepossibility of changing how information processing
technologies are fundamentally used.
Finally, while not introduced here, we would like tomention that great progress has also been made in the
research on conventional gate model quantum computing hardware. We cannot predict which model ofquantum information processing technology willchange the world in the future, but we definitely wish
to experience such a world beyond our wildest expectations.
References
[1]
[2]
[3]

“Feature Articles: Quantum Computing,” NTT Technical Review, Vol.
10, No. 9, 2012.
https://www.ntt-review.jp/archive/2012/201209.html
T. Kadowaki and H. Nishimori, “Quantum Annealing in the Transverse Ising Model,” Phys. Rev. E, Vol. 58, No. 5, pp. 5355–5363,
1998.
H. Takesue, T. Inagaki, K. Inaba, and T. Honjo, “Quantum Neural
Network for Solving Complex Combinatorial Optimization Problems,” NTT Technical Review, Vol. 15, No. 7, 2017.

Hiroshi Yamaguchi

Senior Distinguished Researcher and Group
Leader of the Hybrid Nanostructure Physics
Research Group, NTT Basic Research Laboratories.
He received a B.S. and M.S. in physics and a
Ph.D. in engineering from Osaka University in
1984, 1986, and 1993. He joined NTT Basic
Research Laboratories in 1986. His current interests are micro/nanomechanical devices using
semiconductor heterostructures. He has been aguest professor at Tohoku University since 2006.
He is a fellow of the Institute of Physics and theJapan Society of Applied Physics (JSAP) and amember of the Physical Society of Japan, theAmerican Physical Society, and IEEE (Instituteof Electrical and Electronics Engineers).

Vol. 15 No. 7 July 2017

[4]

[5]

[6]

[7]

[8]

https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201707fa2.html
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, 2017.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201707fa4.html
Y. Matsuzaki, K. Kakuyanagi, H. Toida, H. Yamaguchi, W. J. Munro,and S. Saito, “Coherent Coupling between 4300 Superconducting
Flux Qubits and a Microwave Resonator,” NTT Technical Review,Vol. 15, No. 7, 2017.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201707fa5.html
K. Tamaki, “Implementation Security of Quantum Key Distribution,”
NTT Technical Review, Vol. 15, No. 7, 2017.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201707fa7.html
N. Matsuda, “Lossless Wavelength Conversion of Single Photons,”
NTT Technical Review, Vol. 15, No. 7, 2017.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201707fa3.html
T. Akiho, “Creating New Two-dimensional Topological Insulators for
Fault-tolerant Quantum Computing,” NTT Technical Review, Vol. 15,
No. 7, 2017.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201707fa6.html

Tetsuomi Sogawa

Vice President, Head of NTT Basic ResearchLaboratories.
He received a B.S., M.S., and Ph.D. in electrical engineering from the University of Tokyo in
1986, 1988, and 1991. He joined NTT Basic
Research Laboratories in 1991. From 1999 to
2000, he was a guest scientist at Paul Drude
Institute in Berlin, Germany, where he investigated acoustic spin transport phenomena insemiconductor quantum structures. From 2004 to
2006, he worked for the Council for Science andTechnology Policy, Cabinet Office, Japan, as a
deputy director for policy planning. His current
research interests include fabrication technology
of low-dimensional nanostructures, optical properties of quantum dots/wires and photonic crystals, spin-related phenomena, and surface acoustic wave application on nanostructures. He is amember of JSAP.

6

Feature Articles: A New Era in Quantum Information
Processing Technologies

Quantum Neural Network for Solving
Complex Combinatorial
Optimization Problems
Hiroki Takesue, Takahiro Inagaki, Kensuke Inaba,
and Toshimori Honjo
Abstract

Optimization of various social systems, including communication, traffic, and social networking systems, requires the solving of complex combinatorial optimization problems. In this article, we describe
the recent progress achieved in developing a quantum neural network, a computer that finds solutions to
combinatorial optimization problems using a network of degenerate optical parametric oscillators.
Keywords: Ising model, combinatorial optimization problem, optical parametric oscillator

1. Introduction

2. Ising model

As various systems in our society grow larger andmore complex, it becomes increasingly important toanalyze and optimize such systems. These complex
problems are classified as combinatorial optimization
problems, which cannot be solved efficiently withconventional digital computers.
It is known that such problems can be converted to
ground-state-search problems of the Ising model,
which is a theoretical model proposed by E. Ising that
describes a group of interacting spins. Several institutions recently reported artificial spin networks thatwere applied to simulate the Ising model in order tosolve combinatorial optimization problems [1, 2].
Our team has been studying such an Ising-type computer called a quantum neural network (QNN) [3]. In
a QNN, Ising spins are represented by degenerate
optical parametric oscillators (DOPOs), which arecoupled with mutual injections of DOPO light. In thisarticle, we describe the basic working principle of a
QNN and report the recent experimental progress
made in this area at NTT.

The Hamiltonian of the Ising model is given as follows.

1

H = − Σi<jJijσiσj.

(1)Here, σi and Jij respectively represent the ith spinstate and the coupling coefficient between the ith andjth spins. The ground state of the Ising model corresponds to a set of spins {σi} that minimize Eq. (1) fora given matrix {Jij}. A QNN, also known as a coherent Ising machine, is a network of artificial spinsbased on quantum optical oscillators that simulates
the Ising model [3]. In our QNN, a spin state is represented by a DOPO, and the spin-spin interaction coefficient Jij is implemented by adjusting the length andtransmittance of an optical path between the ith andjth DOPO (Fig. 1). Since the networked DOPO tends
to oscillate with a phase configuration that minimizes
the total energy, we can obtain the ground state of agiven Ising model by simply reading the phases of the
DOPOs that are above the threshold.

NTT Technical Review

Feature Articles

Spin: Degenerate optical parametric oscillator (DOPO)

Pump laser

Spin-spin interaction:
Mutual injection of light

Pump light

Fig. 1. Concept of quantum neural network (QNN).

3. Phase sensitive amplification
An important physical phenomenon for generating
a DOPO is a nonlinear optical effect called phasesensitive amplification [4]. When we input a pumplight (angular frequency ωp) and a signal light ωs to a
medium with the second or third order optical nonlinearity, we can generate an idler light with an angularfrequency ωi = ωp − ωs (second order case) and an
initial phase −θ, where θ represents the initial phasedifference between the pump and the signal. In particular, when the signal and idler frequencies degenerate (ωs = ωi), the amplification coefficient of thesignal amplitude is proportional to cosθ, whichmeans that the light is efficiently amplified when its
initial phase difference from the pump is either 0 or π.
To achieve such a phase sensitive amplifier (PSA),
we can utilize four-wave mixing in an optical fiber [5,6] or parametric down-conversion in a periodicallypoled lithium niobate waveguide [4, 7]. If we pump a
PSA placed in an optical cavity, the 0- or π-phasecomponent of the spontaneous emission noise is mostefficiently amplified by the PSA, which eventuallyleads to an optical parametric oscillation with either
one of the binary phase states (Fig. 2). In addition, if
we use a pulsed pump whose temporal interval is set
at 1/N of the cavity round-trip time, we can generate
N time-multiplexed DOPOs using a single cavity.
We succeeded in generating more than thousands of
DOPOs using a 1-km fiber cavity, paving the way
towards achieving a large-scale QNN [5–7]. Morerecently, we reported the generation of more than 1
million DOPOs using a 10-GHz clock pump and a
20-km fiber cavity [8].
We can implement couplings between time-multiVol. 15 No. 7 July 2017

plexed DOPOs by inserting delayed interferometers.
In an experiment reported in an earlier paper [6], we
implemented nearest-neighbor coupling between
DOPOs by placing a 1-bit delayed interferometer inthe fiber cavity so that we could realize a one-dimensional Ising model. With the optically coupled
DOPOs, we successfully observed that the DOPOs
showed ferro- and antiferromagnetic behavior when
we changed the phase difference between the twoarms of the interferometer, suggesting that the
DOPOs well simulated the characteristics of lowtemperature spins.
4. Measurement-feedback scheme
The above one-dimensional Ising model experiment was important for understanding the physics ofa QNN. However, it is difficult to solve real-worldcombinatorial optimization problems that are usually
represented by large and complex spin networks
using an optically coupled DOPO network. For
example, in order to achieve all-to-all connection
between 2000 DOPOs, we need as many as 1999
interferometers placed in the cavity, which is very
hard to do experimentally.
We employed the measurement-feedback scheme
shown in Fig. 2 in order to achieve flexible couplings
among thousands of DOPOs. In this scheme, we measure the amplitudes of all N DOPOs {ci} while the
DOPOs are circulating in the cavity and while controlling the evolution of their amplitudes. The measurement results are fed to a field programmable gatearray (FPGA), where the spin-spin coupling matrix{Jij} for a given problem is set in advance. In each
DOPO circulation in the cavity, the FPGA calculates2Feature Articles

Pump pulses (0.77 µm)

DOPO pulses(1.54 µm)
Optical fiber
cavityCoherent
measurement

PPLN waveguide module(1) Phase/amplitude
measurementof each DOPO
1-km fiber
DOPO
pulsesInjection pulses
Modulator(3) Optical pulses with feedback
signals are injected to DOPOs(completion of DOPO couplings)

FPGA(2) Calculation of feedback signals for each DOPOFPGA: field programmable gate array
PPLN: periodically poled lithium niobate

Fig. 2. QNN configuration.

(a)

(b)

Fig. 3. Solution searching of a 2000-node graph problem with QNN. (a) Graph problem (random graph with 19,990 edges).
The pink dots and white lines respectively show the nodes and edges. (b) Solution obtained with the QNN
experimentally. The pink dots are divided into red and blue dots; as a result, we can cut the edges shown by the green
lines. Larger dots exhibit nodes with a larger number of edges.

si = ΣjJijcj, the feedback signal for the ith DOPO in thenext round trip. The feedback signal si is used tomodulate a light pulse whose frequency is exactly thesame as that of the DOPOs, and the pulse is launched
to the ith DOPO. As a result, any pair among N
DOPOs can be coupled with this scheme. The num3

ber of such combinations is N(N−1)/2 when the problem is given by a directed graph.
We used the measurement-feedback scheme to construct a QNN with 2048 DOPOs [9], with which we
succeeded in finding the solutions to maximum cut
problems for a 2000-node graph (Fig. 3). In particular,
NTT Technical Review

Feature Articles−3.5 104−4 104

Ising energy−4.5 104
QNN

Simulated annealing−5 104−5.5 104−6 104−6.5 104
10−5

10−4

10−3

10−2

10−1

Time (s)The blue and red curves show the energies obtained with the QNN and simulated annealing
run on a CPU. The dotted line denotes the reference energy obtained with an algorithm
called GW-SDP. It took 3.2 ms to reach the reference energy using simulated annealing,
while the QNN delivered the solution with the same Ising energy in only 0.7 ms.
GW-SDP: semidefinite programming relaxation algorithm by Goemans and Williamson

Fig. 4. Ising energy as a function of computation time for a maximum cut problem of a 2000-node complete graph (K2000).

when we applied the QNN to a maximum cut problem of a 2000-node complete graph, we obtained asolution that corresponds to a reference Ising energy
in less than 100 µs, which is approximately 50 times
shorter than the computation time obtained withsimulated annealing run on a CPU (central processing unit) (Fig. 4).

[2]

[3]
[4]

5. Future prospects
Although QNN research is still in a very earlystage, our experimental results suggest that the QNN
may outperform conventional digital computers forcertain types of problems. We are currently planning
to increase the number of DOPOs of our QNN tofurther widen the performance advantage over digitalcomputers. We also plan to search for applications of
QNN by collaborating with researchers from various
fields such as statistical physics, mathematics, andcomputer science.
References[1]

M. W. Johnson, M. H. S. Amin, S. Gildert, T. Lanting, F. Hamze, N.
Dickson, R. Harris, A. J. Berkley, J. Johansson, P. Bunyk, E. M.
Chapple, C. Enderud, J. P. Hilton, K. Karimi, E. Ladizinsky, N. Ladizinsky, T. Oh, I. Perminov, C. Rich, M. C. Thom, E. Tolkacheva, C. J.
S. Truncik, S. Uchaikin, J. Wang, B. Wilson, and G. Rose, “Quantum

Vol. 15 No. 7 July 2017

[5]

[6]

[7]

[8]
[9]

Annealing with Manufactured Spins,” Nature, Vol. 473, pp. 194–198,
2011.
M. Yamaoka, C. Yoshimura, M. Hayashi, T. Okuyama, H. Aoki, and
H. Mizuno, “A 20k-spin Ising Chip to Solve Combinatorial Optimization Problems with CMOS Annealing,” IEEE J. Solid-State Circuits,Vol. 51, No. 1, pp. 303–309, 2015.
S. Utsunomiya, K. Takata, and Y. Yamamoto, “Mapping of Ising Models onto Injection-locked Laser Systems,” Opt. Express, Vol. 19, No.
19, pp. 18091–18108, 2011.
T. Umeki, O. Tadanaga, T. Kazama, K. Enbutsu, Y. Miyamoto, and H.
Takenouchi, “Advances in Phase Sensitive Amplifiers Based on PPLN
Waveguides for Optical Communication,” NTT Technical Review,Vol. 14, No. 1, 2016.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201601fa3.html
H. Takesue, T. Inagaki, K. Inoue, and Y. Yamamoto, “Coherence Property of >2500-pulse Multiplexed Degenerate OPO,” The 61st JSAP
(Japan Society of Applied Physics) Spring Meeting, 17p-PA1-1, Saga­
mihara, Kanagawa, Japan, Mar. 2014 (in Japanese).
H. Takesue, T. Inagaki, T. Umeki, O. Tadanaga, and H. Takenouchi,“Large-scale Time-division Multiplexed OPOs Using PPLN Waveguide,” The 62nd JSAP Spring Meeting, 12a-P6-1, Shonan, Kanagawa, Japan, Mar. 2015 (in Japanese).
T. Inagaki, K. Inaba, R. Hamerly, K. Inoue, Y. Yamamoto, and H.
Takesue, “Large-scale Ising Spin Network Based on Degenerate Optical Parametric Oscillators,” Nat. Photonics, Vol. 10, pp. 415–419,
2016.
H. Takesue and T. Inagaki, “10 GHz Clock Time-multiplexed Degenerate Optical Parametric Oscillators for a Photonic Ising Spin Network,” Opt. Lett., Vol. 41, No. 18, pp. 4273–4276, 2016.
T. Inagaki, Y. Haribara, K. Igarashi, T. Sonobe, S. Tamate, T. Honjo,
A. Marandi, P. L. McMahon, T. Umeki, K. Enbutsu, O. Tadanaga, H.
Takenouchi, K. Aihara, K. Kawarabayashi, K. Inoue, S. Utsunomiya,and H. Takesue, “A Coherent Ising Machine for 2000-node Optimization Problems,” Science, Vol. 354, No. 6312, pp. 603–606, 2016.

4

Feature Articles

Hiroki Takesue

Senior Distinguished Scientist, Group Leader,
Quantum Optical State Control Research Group,
Optical Science Laboratory, NTT Basic ResearchLaboratories.
He received a B.E., M.E., and Ph.D. in engineering science from Osaka University in 1994,
1996, and 2002. In 1996, he joined NTT Basic
Research Laboratories, where he was engaged inresearch on lightwave frequency synthesis, optical access networks using wavelength divisionmultiplexing, and quantum optics. He is currently pursuing research on communication andcomputation using quantum optics technologies.
He is the recipient of several awards, including
the ITU-T Kaleidoscope Conference 2nd Best
Paper Award in 2008 and The Commendation for
Science and Technology by the Minister of Education, Culture, Sports, Science and Technologyof Japan (The Young Scientists’ Prize) in 2010.
He was a visiting scholar at Stanford University,
California, USA, from 2004 to 2005, and a guest
researcher at the National Institute of Standards
and Technology (NIST), Colorado, USA, in
2014. He is a Guest Professor in the Graduate
School of Engineering Science, Osaka University, and a member of the Institute of Electrical andElectronics Engineers (IEEE) and the Japan
Society of Applied Physics (JSAP).

Takahiro Inagaki

Researcher, Quantum Optical State Control
Research Group, NTT Basic Research Laboratories.
He received a B.E., M.E., and Ph.D. in engineering science from Tohoku University, Miyagi,
in 2007, 2009, and 2012. At Tohoku University,
he conducted research on quantum state transfer
between photon polarization and an electron spinin a semiconductor. He joined the Quantum Optical State Control Research Group in NTT Basic
Research Laboratories in 2012. He is currently
researching quantum communication based on
entanglement and a coherent Ising machine for
combinatorial optimization problems. He is amember of the Physical Society of Japan (PSJ)
and JSAP.

5

Kensuke Inaba

Senior Research Scientist, Quantum Optical
State Control Research Group, Optical Science
Laboratory, NTT Basic Research Laboratories.
He received a B.E., M.E., and Dr.Eng. from the
Department of Applied Physics, Osaka University, in 2003, 2005, and 2007. He joined NTT
Basic Research Laboratories in 2008. Since
2004, he has been studying quantum many-body
physics in correlated electron systems in condensed matter. His current interests are correlated
cold atoms and photons and their application toquantum simulation and computation.

Toshimori Honjo

Senior Research Scientist, Quantum Optical
State Control Research Group, Optical Science
Laboratory, NTT Basic Research Laboratories.
He received a B.S. and M.S. in information
science from Tokyo Institute of Technology in1996 and 1998, and a Ph.D. in engineering from
Osaka University in 2007. In 1998, he joined
NTT Software Laboratories, where he conducted
research on secure communication systems.
Since 2003, he has been researching quantum
optics and quantum cryptography at NTT Basic
Research Laboratories. His current researchinterests include physical computer systems andquantum communication. In 2009, he was a visiting researcher at the University of Vienna, Austria. He is a member of the Information Processing Society of Japan (IPSJ).

NTT Technical Review

Feature Articles: A New Era in Quantum Information
Processing Technologies

Lossless Wavelength Conversion ofSingle Photons
Nobuyuki Matsuda
Abstract

We demonstrated a new scheme for wavelength conversion of a single-photon wave packet for use inquantum information communication technologies. The scheme enables us to deterministically change
the color and shape of a single-photon wave packet in a lossless manner. The scheme is directly applicable to single photons traveling in an optical fiber network and will be a key technology in developinga photonic wavelength interface for quantum networking.
Keywords: quantum communications, single photons, nonlinear optics

1. Introduction
Wavelength (frequency, color) is an important
physical parameter of light, and wavelength conversion is essential in current photonics technologies.
Applications of wavelength conversion range from
simple devices such as laser pointers to the fields ofcommunications, precision measurement, and biomedical sciences.
Wavelength conversion of single photons is crucial
for quantum communication, which holds promise
for applications such as quantum cryptography andquantum teleportation. For example, the telecommunications wavelength is the wavelength that is suitable for long-distance transmission of photons in
optical fiber networks. However, many physical systems such as atoms are sensitive to light with wavelengths in the visible band. In addition, even within
the same wavelength band, the wavelengths and spectral shape of photons generally differ from each otherdepending on the physical properties of the photon
sources or on the wavelength channel used for thetransmission. Hence, the ability to harness the wavelength of photons, ideally in a lossless manner for
scalability, is required in order to distribute quantum
information between distant sites via interactions
between light and matter or between different lightsources.

1

2. Methods of single-photon
wavelength conversion
The wavelength conversion of intense optical pulses induced by themselves (i.e., self-induced) in an
optical fiber is shown in Fig. 1. The optical pulses,
whose initial center wavelength was 1 µm in thisexample, acquired new wavelength components via
nonlinear optical effects (self-phase modulation,
four-wave mixing, stimulated Raman scattering,etc.), which can be easily induced by intense opticalfields. However, the intensity of a single-photon wave
packet is more than 10 orders of magnitude lower
than that of ordinary laser pulses.
The first experimental demonstration of wavelength conversion was conducted in 1992. Since then,wavelength conversion of nonclassical light has been
widely investigated using nonlinear three- or fourwave mixing (Fig. 2(a)), where photon wavelengths
can be converted with mediation provided by theenergy of other input pump fields [1]. While theintense pump required for a highly efficient conversion tends to simultaneously create noise photons, thedemand for scalable quantum networking provides
strong motivation for researching the simultaneous
realization of lossless and noiseless conversion.
Another nonlinear optical interaction that enables
frequency conversion is cross-phase modulation(XPM) (Fig. 2(b)), which is a third-order nonlinear
NTT Technical Review

Feature Articles

(a)

(b)

(c)

Fig. 1. Self-induced wavelength conversion of intense optical pulses with a center wavelength of 1 μm in an optical fiber. The
input power was increased from (a) to (c). The output field was diffracted onto the screen by an external grating (notshown).

Frequency mixing

Cross-phase modulation (XPM)(Sum frequency generation etc.)

Output photon
SignalSignalIntensity

EnergyControl
ωpControl pulse (synchronizedwith signal photons)
dP/dt
Outputω + ωpωωω + ∆ω(∆ω ∝ dP/dt)• Translates the center frequency by mixing• Large wavelength shift possible (between
visible and telecom wavelengths)• Conversion efficiency depends on the powerof the control field• Adiabatic wavelength conversion induced bydynamic refractive index modulation• Wavelength conversion up to 100 nm• Deterministic (no photon loss)

(a)

(b)

Fig. 2. Nonlinear optical effects for single-photon wavelength conversion.

optical effect that enables the phase of an optical(signal) field to be controlled by another (control)
field. XPM provides a function for the wavelength
conversion via dynamic phase alteration of an opticalfield in the following way. The presence of a control
pulse with an intensity profile P(t) leads to an intensity-dependent variation in the refractive index variation of the material, which is experienced by the signal field as a phase shift φ (t) = 2γP(t)L, where γ is thenonlinearity strength of a medium and L is the device
length. This gives rise to the instantaneous frequency
shift

γ
φ
∆v(t) = − 1 d (t) = − L dP(t)
π dt
2π dt

(1)

of the signal photons. Hence, we can add a wavelength shift, which is proportional to the time derivative of the control field intensity, to the single phoVol. 15 No. 7 July 2017

tons.
Unlike frequency-mixing based schemes, wavelength conversion can be accomplished without any
photon loss because the XPM process deterministically occurs regardless of the control field intensity.
For example, if we consider standard nonlinear optical fiber (γ = 0.01 /W/mm, L = 10 m) and standard
picosecond optical pulses (dP/dt = 600 W/1 ps), we
can obtain ∆v(t) = 19 THz, which corresponds to awavelength shift over 150 nm.
The quantum field to be converted now has a considerably lower intensity than that of the control
pulses. Accordingly, a large frequency separation isneeded between the two interacting fields in order toeliminate potential noise caused by the nonlinear
spectral broadening of the control pulse. Moreover,
the two interacting fields must propagate at the same
pace; otherwise, φ (t) will become constant over the2

Feature Articles

Photonic crystal fiber

SignalGroup delay (5 ps/m/div)

Control pulse

Control

0.8
Single (Signal) photon wave packet
(a)

1.0
1.2
1.4
Wavelength (µm)

1.6

(b)

Fig. 3. (a) Schematic of the experimental method. (b) The measured group-delay spectrum of the photonic crystal fiber.

entire wave packet and cancel the frequency shift.
To fulfil the criterion, we used a photonic crystal
fiber (PCF) as a Kerr medium with the dispersion
property shown in Fig. 3. It exhibits two widely separated wavelengths with the same group velocity,
thanks to the dispersion property, which is managed
by arranging the air-hole claddings that surround thesilica core.
3. Experimental results [2]We converted the wavelength of heralded single
photons created via type-II spontaneous parametricdown-conversion (SPDC) in a periodically poled
potassium titanyl phosphate crystal. The center wavelength of the control field λc was chosen to be 756
nm, which satisfies vg(λc) ~ vg (2λc), where vg is thegroup velocity of the PCF (Fig. 3(b)). In this way,
both the XPM control pulses and SPDC excitation
pulses can be fed from a pulsed laser source. The
peak power of the control pulses coupled to the PCF
was approximately 350 W.
The dependence on signal delay of the signal photon spectra heralded by the detection of idler photons
is shown in Fig. 4(a). The heralded spectrum wasmarkedly modified as the delay time varied. For
instance, at the negative delay, where the signal photons are mainly synchronous at the falling edge of thecontrol pulses in the PCF, the spectrum was entirely
red shifted because dP/dt > 0. In contrast, the spectrum was blue shifted at the positive delay.
Numerical simulation of the frequency modulation
was performed based on coupled nonlinear Schröding3

er equations including XPM, self-phase modulation,and dispersion. The simulation result (not shown)well described the characteristics of the spectral
shifts seen in the experimental data. The maximum
wavelength shift was 3.2 nm (0.4 THz), which was
limited by the length and the group velocity dispersion (GVD) of the PCF used. Further dispersion
engineering of PCFs will be useful for reducing theGVD, which will lead to a larger wavelength shift.
The sum of the photon counts for each delay time isplotted in Fig. 4(b). The total coincidence countremains constant regardless of the delay, demonstrating that the conversion occurred without an observable photon loss. Hence, we have successfully
achieved lossless wavelength conversion of single
photons.
We applied the scheme to further control the quantum correlation of pairs of photons. The demonstrations included modulation of nonclassical frequency
correlation, interference, and entanglement between
distant photons [2]. These results demonstrate ourscheme’s applicability to a wide range of quantum
information science and technologies, including
computation and metrology.
4. Outlook
We are attempting to further improve the bandwidth and controllability of wavelength conversion.
This can be achieved by further engineering the dispersion property of the photonic crystal fiber or engineering the temporal shape of the control pulses. The
scheme, achievable in an optical fiber, is compatible
NTT Technical Review

Feature Articles

2

0

200

−1

100

Signal photon delay (ps)300
1Photon counts (/5s)

Signal photon delay (ps)

2

1
0
−1
−2

−2
1.50

1.51

1.52

1.53

0

6000

9000

Signal wavelength (µm)

Total photon counts

(a)

(b)

Fig. 4. Experimental results. (a) Delay dependence of the marginal distribution of heralded single (signal) photons. (b) Total
photon counts as a function of the delay time.

with the equipment in optical communications networks. With this feature, we aim to develop a photon
wavelength interface for flexible quantum networking.

References[1][2]

M. G. Raymer and K. A. Srinivasan, “Manipulating the Color andShape of Single Photons,” Phys. Today, Vol. 65, No. 11, p. 32, 2012.
N. Matsuda, “Deterministic Reshaping of Single-photon Spectra
Using Cross-phase Modulation,” Sci. Adv., Vol. 2, No. 3, e1501223,
2016.

Nobuyuki Matsuda

Distinguished Researcher, Quantum Optical
State Control Research Group, NTT Basic
Research Laboratories.
He received a B.E., M.E. and Ph.D. in electrical engineering from Tohoku University, Miyagi,
in 2005, 2006, and 2009. In 2009 and 2014 he
was a visiting research scholar at University ofBristol, UK. He joined NTT Basic ResearchLaboratories in 2010 and has since been engaged
in research on integrated quantum photonics andsingle-photon nonlinear optics. He is a memberof the Optical Society (OSA), the Physical Society of Japan, and the Japan Society of Applied
Physics.

Vol. 15 No. 7 July 20174

Feature Articles: A New Era in Quantum Information
Processing Technologies

Demonstration of Realism Violation
on a Macroscopic Scale
Kosuke Kakuyanagi, Yuichiro Matsuzaki, Hiraku Toida,
Hiroshi Yamaguchi, Shiro Saito,
and William John Munro
Abstract

In quantum mechanics, we can realize quantum superposition states, which cannot be understood by
the usual common sense in our living world. In the microscopic world, such as that experienced by asingle electron spin, superposition behavior has been experimentally confirmed. However, it is still controversial as to whether or not we can in principle observe such quantum phenomena in the macroscopic world. At NTT Basic Research Laboratories, we have experimentally tested such macroscopic quantum phenomena by using a macroscopic quantum device.
Keywords: macroscopic realism, quantum mechanics, superconducting flux qubit

1. Introduction
What is realism? Suppose we have a single die (oneof a pair of dice) underneath a cup. If we lift the cup
and look at the die, we can see which pips (dots) areon the top surface (1, 2, 6, etc.). We assume that thepips on the die were predetermined before we lifted
the cup (Fig. 1(a)). The idea of considering objects tobe predetermined regardless of our observation ofthem is called realism. In our daily life, we cannot
observe any phenomenon that contradicts this realism
concept, which seems to support the validity of realism in the macroscopic world.
However, it is known that microscopic particles
obey quantum mechanics, and as such, these particles
can sometimes display counterintuitive phenomena.
As an example, we can conduct an experiment wherea single electron passes through a double-slit. If wetake just a few measurements of the position of thesingle electron after passing through the double-slit,
the measurements seem to be random. However, if wetake many measurements in order to obtain statisticalresults, the electron position shows an interference
fringe, which is typically observed as a wave thatpasses through both slits. It is worth mentioning that1

although we cannot divide the single electron into
two, this result shows the possibility that the electron
spin actually passed through both slits. We can understand this counterintuitive result as a realization ofquantum superposition that is described by quantum
mechanics. The electron shows a superposition
between the state of passing through the right slit andthe left slit.
Interestingly, if we observe the path of the electronby placing a detector at one of the slits, the interference fringe disappears. This can be easily understoodas the situation in which we can determine the path ofthe electron by our observation of it, which also indicates that an object state (such as the path of an electron) is not determined until some observations are
performed. This kind of phenomenon has been
observed in many microscopic systems that obey
quantum mechanics (Fig. 1(b)).
Because macroscopic objects are composed of
microscopic systems such as atoms and electrons, we
may think that macroscopic objects should also obey
quantum mechanics (why would they not do this?).
However, as the example of the die illustrates, realism
seems to be valid in the macroscopic world from ourexperience. Whether the application of quantum
NTT Technical Review

Feature Articles

Electron passing
through double
slit

Dice pips are fixed without anyone looking at them.
e−Observation
Readout(a) Macroscopic realism

We cannot say whichslit the electron passed
through without observation.
(Origin of quantum computationand communication)

Observation(b) Breaking of microscopic realism (quantum behavior)

Fig. 1. (a) Realism in macroscopic world and (b) realism breaking in microscopic world.

mechanics has a limitation based on size or not has
been an unsolved problem ever since the discovery ofquantum mechanics. NTT has now demonstrated theviolation of realism caused by quantum mechanics inthe macroscopic world.
2. Testing the violation of realism
In the macroscopic world, it is thought that the stateof objects is not disturbed if we implement optimal
observations. For example, we can easily observe thepips on a die without disturbing the die. If we observe
objects at different times, we can calculate temporalcorrelations between the observation results. It is
known that if the observation does not induce a disturbance and realism is true, then the temporal correlation obtained by the experimental results should
satisfy a specific condition called the Leggett-Garg
inequality. This means that if the temporal correlation
obtained by experimental observation without disturbance shows a violation of the Leggett-Garg inequality, this result indicates that realism is no longer valid.
At NTT Basic Research Laboratories, we found an
experimentally more feasible condition that is mathematically equivalent to the Leggett-Garg inequality,
so we conducted tests to determine whether our condition was violated as a macroscopic realism challenge.
3. Superconducting flux qubit
We tested macroscopic realism using a superconducting flux qubit, which has been used in many
investigations of quantum mechanics. The flux qubit
is composed of a number of Josephson junctions. A
Josephson junction can be regarded as an inductanceVol. 15 No. 7 July 2017

similar to an induction coil. This inductance is dependent on the current, so Josephson junctions are anonlinear inductance. If we fabricate a loop structurewith these Josephson junctions, the circuit can form
an effective two-level system (qubit) by applying
optimal bias magnetic fields. The state of this artificial qubit is then composed of clockwise and anticlockwise currents. Interestingly, the current of thisstate is around 100 nA, which corresponds to a current of 1012 electrons per second. This is an ideal
device for checking whether or not quantum superposition can exist at the macroscopic level.
4. Quantum state observation
It is worth mentioning that even if the realism istrue, a disturbance induced by the observation could
provide us with a violation of the Leggett-Garg
inequality. For example, if a die is underneath a cup
and we lift up the cup, the cup itself may touch thedie. Consequently, the position of the die (and thus,
the pips that are apparent) could be changed by thisobservation, which also demonstrates a violation of
the Leggett-Garg inequality. Therefore, to test macroscopic realism, it is crucial to carry out observations
with as little disturbance as possible.
We used a superconducting quantum interference
device (SQUID) embedded in a microwave resonatorto observe the flux qubit with minimal disturbance.
The states of the superconducting flux qubits can be
distinguished by the magnetic fields from the device
because the currents in the two states flow in opposite
directions. The SQUID is magnetically coupled withthe flux qubit, and the state of the flux qubit changes
the inductance of the SQUID. The change in the
inductance can be detected by the transmitted
2

Feature Articles

SQUID structure

Flux qubit|+1

Microwave pulse
for observation|−1

Readout+1 or −1

Qubit control pulse

Fig. 2. Readout and control of superconducting flux qubit.

microwave of the resonator. Since this readout does
not change the Josephson junctions into voltage
states, we can implement a quantum non-demolition
measurement*1 that is known to have a small disturbance on the system (Fig. 2).
5. Experimental method
If the superconducting flux qubit obeys quantum
mechanics, we should be able to generate a superposition state with a specific gate operation, which wecall a quantum state operation. Microwave pulses areused to perform the quantum state operations in our
experiments. We set the microwave frequency to be
the same as the resonance frequency of the flux qubit,
enabling us to control the flux qubit state by changing
the amplitude or length of the pulse. We define |−1〉and |+1〉*2 as the states for the flux qubit. A long
period of thermal relaxation initializes the flux qubit
state into the |−1〉 state. When our quantum state
operations are performed, our state will evolve as |−1〉
operation
operation|−1〉 + |+1〉

operation|+1〉

operation|−1〉 − |+1〉|−1〉 as shown in Fig. 3(a). When the quantum state operations are applied four times, the state
returns to the initial state, which is similar to a rotation with a 2π angle. We call this operation a π/2pulse. Next, |−1〉 + |+1〉 represents a specific quantum
superposition state. The objective of our experimentis to check if such a superposition actually exists
where the state is not determined until carrying out
our observation.
If our qubit obeys quantum mechanics, we shouldbe able to realize the superposition state of |−1〉 + |+1〉
by applying the π/2 quantum pulse to the |−1〉 state.
Without any observation, we can keep the superposition state; thus, an additional quantum operationinduces a state of |+1〉. The measurement result on
3

this |+1〉 state is always +1. In contrast, if we observethe |−1〉 + |+1〉 state, the observation stochasticallyprojects our state into a |−1〉 or |+1〉 state. In this case,an additional quantum operation after the observation
provides us with the states of |−1〉 + |+1〉 or |−1〉 −|+1〉, where the expectation value of the measurement
result is 0.
We can also consider a case when the realism iscorrect and perform two experiments (Fig. 3(b)). Oneof them contains two π/2 operations; we observe thesystem between these operations. The other experiment is composed of two π/2 operations without anobservation between them. Since we can observe
without any disturbance (or only a negligible disturbance), any observation between the operations
should not change the state of the objective. Therefore, the measurement results for these two experiments should be the same.
For the reasons explained above, we should observea difference between these two experiments if our
superconducting flux qubit obeys quantum mechanics, while there should be no difference if the realism
is correct. We refer to this as our main experiment.
However, since any observation must have a finite
disturbance, we need to quantify the disturbance inthe other experiment, which we call the control
experiment. This can be done as follows. We prepare
the state |−1〉 or |+1〉, observe the state, and read it out.
Then we repeat the same experiment without theobservation in the middle. By comparing the difference in the readout results, we can estimate the degreeof disturbance that occurs with our observation.
*1 Quantum non-demolition measurement: A measurement in which
the state does not change if we prepare an eigenstate.
*2 |−1〉, |+1〉: A description method to represent a quantum state.
|−1〉 and |+1〉 correspond to clockwise and anticlockwise current
states. To represent a superposition state, we use for example,|−1〉 + |+1〉.

NTT Technical Review

Feature Articles

State evolution|−1|−1 − |+1
Superposition stateState control
microwave

Quantum
operation |−1 + |+1(θ = π/2)
Superposition state
|+1
(a)

|−1
Preparation|−1

Operation(θ = π/2)

Operation(θ = π/2)

Observation

Readout

Q3
Difference: dρOperation(θ = π/2)

Operation(θ = π/2)

Readout= 0: realism possible≠ 0: realism impossible

Q3

(b)

Fig. 3. (a) Quantum state operation and (b) sequence of experiment.

We can now examine how the observation between
the quantum operations affects our readout results.
For an initial state of |−1〉 (|+1〉), we define the difference in the readout between two experiments as dg(de) in the control experiment and dρ for our main
experiment. If realism is true, dρ should be between
dg and de, because the state is predetermined before
the observation. On the other hand, if dρ is outside theregion between dg and de, we can conclude that therealism is not valid in this system.
6. Experiment
In our experiment, the energy difference between
the two states of the superconducting flux qubit isseveral gigahertz. To avoid an unwanted thermal
effect, we need to reduce our fridge temperature until
the thermal energy was much lower than the qubit
energy. For this purpose, we used a specific apparatus
called a dilution refrigerator that realizes a temperature around 10 mK in our experiment.
By performing our experiment using a superconducting flux qubit, we obtained the values of dρ, dg,

Vol. 15 No. 7 July 2017and de as displayed in Fig. 4(a). Interestingly, dρ
appears beyond the region between dg and de. This
clearly shows that the dynamics of the superconducting flux qubit cannot be explained by the realism
despite the macroscopicity of the flux qubit (Fig. 4(b)).
Furthermore, our experimental results demonstrated
the breaking of realism in the superconducting flux
qubit current states by 84 times the standard deviation[1].
7. Future plans
We demonstrated that quantum mechanics can beapplied on a macroscopic scale with a large supercurrent, where the device itself can be observed with an
optical microscope. Our results are crucial in understanding the basics of quantum mechanics. In the
future, we aim to conduct measurements with muchless observation disturbance and to increase the macroscopic nature of the system by using a larger supercurrent qubit or an ensemble of superconducting flux
qubits in order to understand quantum mechanics ona more macroscopic scale.

4Feature Articles

Difference by imperfection of observation
Main experiment

dρ

dg

de

Control experiment
Macroscopic
realism
possible

Macroscopic realism
impossible
−0.10

−0.08

−0.06

−0.04

−0.02

0.00

0.02

0.04

Difference depending on existence of observation
(a)

2 µm

=

2 µm

+
|+1

|−1

(b)

Fig. 4. (a) Experimental result and (b) quantum superposition of current state.

Reference[1]

5

G. C. Knee, K. Kakuyanagi, M-C. Yeh, Y. Matsuzaki, H. Toida, H.
Yamaguchi, S. Saito, A. J. Leggett, and W. J. Munro, “A Strict Experimental Test of Macroscopic Realism in a Superconducting Flux
Qubit,” Nat. Commun., Vol. 7, 13253, 2016.

NTT Technical Review

Feature Articles

Kosuke Kakuyanagi

Research Scientist, Hybrid Nanostructure
Physics Research Group, NTT Basic ResearchLaboratories.
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 the PhysicalSociety of Japan (JPS) and the Japan Society ofApplied Physics (JSAP).

Hiroshi Yamaguchi

Senior Distinguished Researcher and Group
Leader of the Hybrid Nanostructure Physics
Research Group, NTT Basic Research Laboratories.
He received a B.S. and M.S. in physics and a
Ph.D. in engineering from Osaka University in
1984, 1986, and 1993. He joined NTT Basic
Research Laboratories in 1986. His current
research interests are micro/nanomechanicaldevices using semiconductor heterostructures.
He has been a guest professor at Tohoku University since 2006. He is a fellow of the Institute ofPhysics and JSAP, and a member of JPS, theAmerican Physical Society, and the Institute ofElectrical and Electronics Engineers (IEEE).

Yuichiro Matsuzaki

Research Scientist, Hybrid Nanostructure
Physics Research Group, NTT Basic ResearchLaboratories.
He received his B.Sc., M.Sc., and D.Phil. inphysics from Waseda University, the Universityof Tokyo, and University of Oxford in 2005,
2007, and 2011. He joined NTT Basic ResearchLaboratories in 2011. Since then, he has been
researching quantum information theory such as
hybrid quantum systems, quantum metrology,
and quantum computation. He is currently a visiting associate professor at Kyoto University(2016–2017).

Hiraku Toida

Research Scientist, Hybrid Nanostructure
Physics Research Group, NTT Basic ResearchLaboratories.
He received a B.E., M.E., 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 and asuperconducting flux qubit. From April to Juneof 2014, he was with NTT Microsystem Integration Laboratories, where he studied numericalsimulation of plasmonic waveguides. He is currently studying highly sensitive spin sensing
using superconducting quantum circuits. He is amember of JPS and JSAP.

Vol. 15 No. 7 July 2017

Shiro Saito

Distinguished Researcher/Senior Research
Scientist, Supervisor, Hybrid Nanostructure
Physics Research Group, Physical Science Laboratory, NTT Basic Research Laboratories.
He received his 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, where he has
been engaged in the study of quantum information processing using superconducting circuits.
He was a guest researcher at Delft University ofTechnology in the Netherlands from 2005 to
2006. He has been a guest associate professor atTokyo University of Science since 2012. He wasappointed a Distinguished Researcher of NTT in
2012. He received the Young Scientist Award
from JSAP at the Spring Meeting in 2004. He isa member of JPS and JSAP.

William John Munro

Senior Distinguished Researcher and Group
Leader of the Theoretical Quantum Physics
Research Group, NTT Basic Research Laboratories.
He received a B.Sc. in chemistry and an M.Sc.
and D.Phil. in physics from the University ofWaikato, New Zealand, in 1989, 1991, and 1995.
He was a research fellow at the University of
Queensland, Australia, from 1997 to 2000 andthen a staff scientist at Hewlett Packard Laboratories in Bristol, UK (2000–2010). He joined
NTT Basic Research Laboratories in 2010 as a
research specialist. Since then, he has researchedseveral areas of quantum physics ranging fromfoundational issues of quantum theory through toquantum information processing and its practicalrealization. He has been a visiting professor at
the National Institute of Informatics in Japan(2006–2015), the University of Leeds in the UK(2009–2015), and the University of Queenslandin Australia (2012–2015). He was appointed a
distinguished scientist of NTT in 2015 and iscurrently a senior distinguished researcher andthe group leader of the Theoretical Quantum
Physics Research Group. He was made a fellowof the Institute of Physics (UK) in 2009, theAmerican Physical Society in 2013, and the
Optical Society (OSA) in 2014. He is also amember of SPIE, the International Society for
Optical Engineering.

6

Feature Articles: A New Era in Quantum Information
Processing Technologies

Coherent Coupling between 4300
Superconducting Flux Qubits and a
Microwave Resonator
Yuichiro Matsuzaki, Kosuke Kakuyanagi, Hiraku Toida,
Hiroshi Yamaguchi, William John Munro,and Shiro Saito
Abstract

A superconducting flux qubit can be used as a magnetic field sensor, and this has potential applications
in many fields such as medical science and biology. If we could realize an entanglement between the
superconducting flux qubits, it is expected that such a device would show much better sensitivity thanany existing magnetic field sensors. We introduce here our recent research along this direction.
Keywords: superconducting qubit, quantum computation, quantum sensing

1. Introduction

2. Superconducting flux qubit

Sensitive magnetic field sensing has many potential
applications in medical science and biology. Magnetic resonance imaging (MRI) is a standard technique to obtain three-dimensional (3D) images ofmaterials by detecting their magnetic field information. Magnetoencephalography provides a way tomeasure the electrical activity of the brain, and thishas an important role in medical science. If we can
improve the sensitivity of magnetic field sensors, we
could characterize materials from high-resolution 3D
imaging, or we could target the location of tumors
before surgery by detecting small signals.
There are many potential candidates to achieve asensitive magnetic field sensor. One promising example is the quantum bit (qubit*1), which has been thesubject of much theoretical and experimental
research. The technology for fabricating qubits has
mainly been developed for the purpose of implementing quantum computation. However, some types of
qubits can be strongly coupled with external fields
such as magnetic fields and electrical fields, so it isalso possible to use qubits for sensing technology.

Among the many possible candidates for qubitbased field sensing, we have investigated a superconducting flux qubit for quantum metrology. A superconducting qubit is an artificial atom, so we can easily design and characterize the device with the desired
parameters (a significant advantage to achieve scalability [1]).
The flux qubit is composed of a loop structure containing three Josephson junctions*2 (Fig. 1(a)). With
the application of magnetic flux (n+0.5)ϕ0, where ϕ0
denotes the quantized magnetic flux and n an integer
number, this device acts as a qubit. The resonant frequency of this qubit can be tuned by applying magnetic flux, so a change in magnetic fields can bedetected by measuring the resonant frequency. A

1*1 Qubit: In classical computation, a bit to store information can
represent either 0 or 1. However, a quantum bit (qubit) obeys
quantum mechanics, so the state of a qubit can contain a superposition of 0 and 1.
*2 Josephson junction: A structure composed of two superconductors coupled by a thin insulator. This induces a supercurrent thatflows without applied voltage due to a tunneling effect.

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Feature Articles

Josephson junctions

Qubit energy (GHz)
3.0
2.5
2.0
1.5
1.0
0.5
−0.03 −0.02 −0.01

Tunnelingenergy0.01 0.02 0.03

Superconducting circuit

Applied magnetic flux (mΦ0)(a) Josephson junctions(b) Qubit energy

Fig. 1. Superconducting flux qubit.

superconducting flux qubit has two states corresponding to clockwise and anticlockwise persistentcurrent states. When we apply ϕ0 magnetic flux, thetransition energy from the ground state to the excitedstates reaches a minimum (Fig. 1(b)). The difference
between the ground and excited state is called the tunneling energy.
It is also possible to improve the sensitivity if weincrease the number of superconducting flux qubits.
In particular, if we create an entanglement*3 that does
not exist in the classical world, we can achieve a sensor that has in principle much better performance
than any existing devices. For example, superconducting quantum interference devices (SQUIDs)*4
are used in the medical sciences, but the sensitivity ofan entanglement sensor composed of millions ofsuperconducting flux qubits would be a few orders ofmagnitude higher than that of a SQUID [2].
3. Coupling of microwave resonator andsuperconducting flux qubits
A microwave resonator is a natural way to achieve
large-scale entanglement between multiple superconducting qubits [3]. When an LC (L = inductor, C =capacitor) circuit is fabricated using superconducting
materials, the energy levels become quantized, so we
can control a few microwave photons inside the resonator. By initiating an interaction between the microwave resonator and the superconducting qubits, we
can generate an entanglement between the superconducting qubits, where the microwave photons mediate the interaction between them (Fig. 2(a)) [4]. Thus,
it is important to achieve strong coupling between the
superconducting flux qubit and the microwave resonator.
Vol. 15 No. 7 July 2017

One way to increase the coupling strength between
the superconducting qubits and the microwave resonator is to use collective quantum effects*5. By fabricating multiple superconducting flux qubits near the
microwave resonator, the superconducting flux qubits
show wave-like properties. The collective effect is
one such property. This means that by coupling L
superconducting qubits with the microwave resonator, the coupling strength is enhanced by a factor of
L0.5. In fact, 20 superconducting flux qubits were
fabricated near a microwave resonator [5], and a frequency shift to indicate such a collective effect wasobserved in spectroscopic data (Fig. 2(b)). However,to achieve a practical quantum device, we need toincrease the number of superconducting flux qubits
collectively coupled with the resonator, which isimportant to create a large entanglement between the
superconducting flux qubits.
We fabricated a device consisting of 4300 superconducting flux qubits embedded in a microwave
resonator (Fig. 3) [6]. By spectroscopically analyzing
this device, we observed the enhancement of the coupling strength due to the collective effect between the
superconducting flux qubits and the microwave resonator. This represents the largest number of coupled*3 Entanglement: In quantum mechanics, it is possible to create an
entanglement that is a stronger correlation than any classical correlation. Entanglement has been shown to play an important role
when a quantum computer solves a difficult problem much fasterthan classical computers.
*4 Superconducting quantum interference device (SQUID): A device composed of a superconducting loop that contains Josephson junctions. This can be used as a magnetic field sensor.
*5 Collective quantum effect: When quantum bits are coupled witha photon or a microwave photon, the ensemble of the qubits behaves like a wave. This phenomenon is called the collective effect.

2

Feature Articles

Resonant frequency without a qubit
Output microwave

Resonator frequency>>Qubit frequency

Resonator frequency<<Qubit frequency

Transmitted photon intensity

Microwave resonator

1.0

Qubit
ensemble
Coupling

0.8

0.6

0.4

0.2

6.2

Input microwave

6.3
6.4
6.5
Microwave frequency (GHz)(a) Microwave resonator

6.6

6.7

(b) Frequency shift

Fig. 2. Coupling of microwave resonator and superconducting flux qubits.

Output microwave

Output microwave
CapacitanceInductance

Qubit

10 µm

Capacitance

Input microwave

Input microwave
(a)

(b)

Fig. 3. Schematic of our device.

superconducting qubits realized so far and is a crucial
step to realize a large entanglement between superconducting qubits.

4. Spectroscopic measurements of the
microwave resonator transmission properties
We placed our sample (sample A) in a dilution
refrigerator operating at 20 mK and conducted spectroscopic measurements of the microwave resonator’s

3

NTT Technical Review

Feature Articles

Simulation
Resonant frequency (GHz)

Microwave frequency (GHz)

Experiment
5.6
5.5
5.4
5.3
5.2
−0.02 −0.01 0.00 0.01 0.02
Applied magnetic flux (mΦ0)−0.02 −0.01 0.00 0.01 0.02
Applied magnetic flux (mΦ0)

Experiment
6.42

50 mK
100 mK
150 mK
200 mK
223 mK

6.41
6.40
6.39

(a) Sample A

Resonant frequency (GHz)

Microwave frequency (GHz)

Simulation

Simulation

Experiment
6.50
6.45
6.40
6.35
6.30
−0.02 −0.01 0.00 0.01 0.02
Applied magnetic flux (mΦ0)

6.42
6.41
6.40
6.39

−0.02 −0.01 0.00 0.01 0.02
Applied magnetic flux (mΦ0)(b) Sample B−0.02 −0.01
0
0.01
0.02
Applied magnetic flux (mΦ0)(c) Thermal effectFig. 4. Spectroscopic measurements of the microwave resonator’s transmission properties.

transmission properties. By changing the appliedmagnetic fields so they were perpendicular to the flux
qubits, we observed a frequency shift of approximately 250 MHz (Fig. 4(a)). Such a frequency shift
is observed due to the collective coupling between the
superconducting qubits and the microwave resonatorwhen the resonant frequencies between them aredetuned (Fig. 2(b)). We succeeded in reproducing
these experimental results using a simple theoreticalmodel that includes collective coupling. We can inferfrom our numerical simulations that thousands offlux qubits were collectively coupled with the microwave resonator in our experiment.
Next, by using the second sample (sample B) fabricated using different qubit conditions, we observed
both positive and negative frequency shifts of tens ofmegahertz. These frequency shifts are known tooccur when the tunneling energy of the superconducting flux qubit is smaller than the energy of the microwave resonator (Fig. 2(b)). The observed frequency
shift of sample B is one order of magnitude smaller
Vol. 15 No. 7 July 2017

than that of sample A (Fig. 4(b)). This is due to thefact that the tunneling energy of sample B is comparable to the thermal energy of the environment, andthe thermal effect suppresses the collective enhancement of the coupling strength. By including the effectof the temperature, we succeeded in reproducing our
experimental results with numerical simulations. We
measured the temperature dependence of the resonator frequency (Fig. 4(c)) and confirmed that anincrease in the temperature suppressed the frequency
shift of the resonator. Our results confirm that we cancontrol the strength of the collective coupling bychanging the temperature.
5. Conclusion
We fabricated 4300 superconducting flux qubits
embedded in a microwave resonator and achievedcoherent coupling between them. We observed alarge frequency shift of the resonator, indicating collective quantum behavior of the superconducting
4

Feature Articles

qubits. It is worth mentioning that we could notobserve vacuum Rabi splitting*6 that is an indicationof strong coupling between the two systems. With theexisting technology, we cannot fabricate homogeneous superconducting flux qubits, so the inhomogeneous resonant frequency of the qubits makes it difficult to observe the vacuum Rabi splitting.
To address this problem, it is not possible to createa large entanglement between the qubits for the quantum enhanced magnetic field sensor in the current
device. One way to overcome the problem of theinhomogeneous broadening is to fabricate control
lines to apply magnetic flux on the qubits, which
would enable us to tune the resonant frequency ofeach qubit. Therefore, as a next step, we aim toachieve strong coupling between the superconducting
flux qubits and a microwave resonator by tuning thefrequency of the qubits. We will also attempt to generate a large entanglement between the qubits via themicrowave resonator, which is useful for constructing
an ultrasensitive magnetic field sensor.

References
[1]
[2]

[3]

[4]

[5]

[6]

J. Clarke and F. Wilhelm, “Superconductiong Quantum Bits,” Nature,
Vol. 453, pp. 1031–1042, 2008.
T. Tanaka, P. Knott, Y. Matsuzaki, S. Dooley, H. Yamaguchi, W. J.
Munro, and S. Saito, “Proposed Robust Entanglement-based Magnetic Field Sensor beyond the Standard Quantum Limit,” Phys. Rev.
Lett., Vol. 115, No. 17, 170801, 2015.
A. Blais, J. Gambetta, A. Wallraff, D. I. Schuster, S. M. Girvin, M. H.
Devoret, and R. J. Schoelkopf, “Quantum Information Processing
with Circuit Quantum Electrodynamics,” Phys. Rev. A, Vol. 75, No. 3,032329, 2007.
J. M. Fink, R. Bianchetti, M. Baur, M. Göppl, L. Steffen, S. Filipp, P.
J. Leek, A. Blais, and A. Wallraff, “Dressed Collective Qubit States
and the Tavis-Cummings Model in Circuit QED,” Phys. Rev. Lett.,Vol. 103, No. 8, 083601, 2009.
P. Macha, G. Oelsner, J.-M. Reiner, M. Marthaler, S. André, G. Schön,
U. Hübner, H.-G. Meyer, E. Ilichev, and A. V. Ustinov, “Implementation of a Quantum Metamaterial Using Superconducting Qubits,” Nat.
Commun., Vol. 5, 5146, 2014.
K. Kakuyanagi, Y. Matsuzaki, C. Déprez, H. Toida, K. Semba, H.
Yamaguchi, W. J. Munro, and S. Saito, “Observation of Collective
Coupling between an Engineered Ensemble of Macroscopic Artificial
Atoms and a Superconducting Resonator,” Phys. Rev. Lett., Vol. 117,
No. 21, 210503, 2016.

*6 Vacuum Rabi splitting: When the resonant frequency of a microwave resonator is the same as that of a qubit, an observed spectroscopic peak can be split into two, which indicates strong coupling between the two systems. This phenomenon is called vacuum Rabi splitting.

5

NTT Technical Review

Feature Articles

Yuichiro Matsuzaki

Research Scientist, Hybrid Nanostructure
Physics Research Group, NTT Basic ResearchLaboratories.
He received his B.Sc., M.Sc., and D.Phil. inphysics from Waseda University, the Universityof Tokyo, and University of Oxford in 2005,
2007, and 2011. He joined NTT Basic ResearchLaboratories in 2011. Since then, he has been
researching quantum information theory such as
hybrid quantum systems, quantum metrology,
and quantum computation. He is currently a visiting associate professor at Kyoto University(2016–2017).

Hiroshi Yamaguchi

Senior Distinguished Researcher and Group
Leader of the Hybrid Nanostructure Physics
Research Group, NTT Basic Research Laboratories.
He received a B.S. and M.S. in physics and a
Ph.D. in engineering from Osaka University in
1984, 1986, and 1993. He joined NTT Basic
Research Laboratories in 1986. His current
research interests are micro/nanomechanicaldevices using semiconductor heterostructures.
He has been a guest professor at Tohoku University since 2006. He is a fellow of the Institute ofPhysics and JSAP, and a member of JPS, theAmerican Physical Society, and the Institute ofElectrical and Electronics Engineers (IEEE).

Kosuke Kakuyanagi

Research Scientist, Hybrid Nanostructure
Physics Research Group, NTT Basic ResearchLaboratories.
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 the PhysicalSociety of Japan (JPS) and the Japan Society ofApplied Physics (JSAP).

Hiraku Toida

Research Scientist, Hybrid Nanostructure
Physics Research Group, NTT Basic ResearchLaboratories.
He received a B.E., M.E., 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 and asuperconducting flux qubit. From April to Juneof 2014, he was with NTT Microsystem Integration Laboratories, where he studied numericalsimulation of plasmonic waveguides. He is currently studying highly sensitive spin sensing
using superconducting quantum circuits. He is amember of JPS and JSAP.

William John Munro

Senior Distinguished Researcher and Group
Leader of the Theoretical Quantum Physics
Research Group, NTT Basic Research Laboratories.
He received a B.Sc. in chemistry and an M.Sc.
and D.Phil. in physics from the University ofWaikato, New Zealand, in 1989, 1991, and 1995.
He was a research fellow at the University of
Queensland, Australia, from 1997 to 2000 andthen a staff scientist at Hewlett Packard Laboratories in Bristol, UK (2000–2010). He joined
NTT Basic Research Laboratories in 2010 as a
research specialist. Since then, he has researchedseveral areas of quantum physics ranging fromfoundational issues of quantum theory through toquantum information processing and its practicalrealization. He has been a visiting professor at
the National Institute of Informatics in Japan(2006–2015), the University of Leeds in the UK(2009–2015), and the University of Queenslandin Australia (2012–2015). He was appointed a
distinguished scientist of NTT in 2015 and iscurrently a senior distinguished researcher andthe group leader of the Theoretical Quantum
Physics Research Group. He was made a fellowof the Institute of Physics (UK) in 2009, theAmerican Physical Society in 2013, and the
Optical Society (OSA) in 2014. He is also amember of SPIE, the International Society for
Optical Engineering.

Shiro Saito

Distinguished Researcher/Senior Research
Scientist, Supervisor, Hybrid Nanostructure
Physics Research Group, Physical Science Laboratory, NTT Basic Research Laboratories.
He received his 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, where he has
been engaged in the study of quantum information processing using superconducting circuits.
He was a guest researcher at Delft University ofTechnology in the Netherlands from 2005 to
2006. He has been a guest associate professor atTokyo University of Science since 2012. He wasappointed a Distinguished Researcher of NTT in
2012. He received the Young Scientist Award
from JSAP at the Spring Meeting in 2004. He isa member of JPS and JSAP.

Vol. 15 No. 7 July 20176

Feature Articles: A New Era in Quantum Information
Processing Technologies

Creating New Two-dimensional
Topological Insulators for
Fault-tolerant Quantum Computing
Takafumi Akiho
Abstract

Two-dimensional topological insulators (2DTIs) pave the way to explore novel quasiparticles for
fault-tolerant quantum computing and to create devices utilizing spin degree of freedom. However, hostmaterials that realize 2DTIs have so far been limited to two systems. In this article, we propose a strained
quantum well structure as a new 2DTI candidate and demonstrate that strain enhances the energy gap
and leads to superior bulk insulation properties.
Keywords: two-dimensional topological insulators, fault-tolerant quantum computing, strain band engineering

1. Introduction
Topological insulators (TIs) featuring insulating
bulk and conducting edge states originating fromtopologically nontrivial inverted band structures havebeen attracting much interest. Topology is a field ofmathematics; objects with shapes that can be made toconform to each other by a continuous deformation
are said to have the same topology. TIs are insulators
but have a different topology than normal insulators(NIs), as their band structures cannot be transformed
continuously into each other [1]. Therefore, at theinterface between a TI and NI (which includes avacuum), the band gaps must close, which results in
conducting edge states at the interface (Fig. 1). The
Nobel Prize in Physics 2016 was awarded to US scientists for “Theoretical discoveries of topologicalphase transitions and topological phases of matter.”This further fueled research interest in topologicalmaterials science.
TIs were first proposed for two-dimensional (2D)
systems. The theory predicted ballistic and quantizedtransport at the edge of TIs. Moreover, helical spincurrent in which the spin direction is locked to thedirection of momentum (e.g., up spins to the left anddown spins to the right) was predicted. The helical1spin current can be used as a spin source with semiconductors to create novel spintronic devices.
Additionally, it is theoretically predicted that Majorana fermions show up in topological superconductors where the edge states of a TI are proximitized bya superconductor. Majorana fermions have a different
nature than conventional particles of fermions andbosons, and they are promising for use in fault-tolerant quantum computing because they are protectedfrom most types of decoherence. Thus, TIs have been
proposed as a platform for exploring spintronic applications and exotic quasiparticles.
In this article, we introduce a strained quantum well
(QW) structure as a new 2DTI candidate and demonstrate that strain enhances the energy gap and leads tosuperior bulk insulation properties [2].
2. New 2DTI candidate
Although various materials have been theoretically
proposed as 2DTI candidates, experimental reports oftransport properties clearly indicative of 2DTIs haveso far been limited to two systems, cadmium telluride/mercury telluride (CdTe/HgTe) QWs and indium
arsenide/gallium antimonide (InAs/GaSb) QWs, bothof which are lattice-matched semiconductor
NTT Technical Review

Feature Articles

Edge at TI
Conducting

NI (vacuum)
Insulating

EnergyInside TI
Insulating

Wave vector

Wave vector

Wave vector

Fig. 1. Schematic energy dispersion of a topological insulator. TIs have different topology than normal insulators (including
vacuum). Therefore, the band gaps must close at the interface between a TI and NI, resulting in conducting edge
states.

heterostructures. Historically, the CdTe/HgTe system
was first predicted to be a 2DTI, which was subsequently followed by the InAs/GaSb system.
InAs and GaSb are conventional semiconductormaterials that are easier to work with than HgTe. The
InAs/GaSb system has a key advantage of being insitu tunable between normal and topological insulating phases. However, the residual bulk conductivity
in the topological insulating phase has been an obstacle to investigating edge transport. In this article, we
propose InAs/indium gallium antimonide (InGaSb)
strained QWs as a new 2DTI candidate that can overcome this problem.
3. Strain band engineering
The band-edge profile of an InAs/In0.25Ga0.75Sbstrained QW is shown in Fig. 2(a). Electrons andholes are separately confined to the InAs andIn0.25Ga0.75Sb QWs, where they form subbands E1
and HH1, respectively. The conduction band (CB)bottom of InAs is located ~0.17 eV below the valenceband (VB) top of In0.25Ga0.75Sb (Fig. 2(b)). Note that
this ordering of the band extrema is opposite to thatin ordinary semiconductors. This band inversion is aprerequisite for topological insulators.
Additionally, In0.25Ga0.75Sb has a larger lattice constant than aluminium antimonide (AlSb), which
induces compressive strain in the In0.25Ga0.75Sb layer.
This splits the VB edge of the heavy hole (HH) andlight hole (LH) by 0.06 eV (Fig. 2(b)), strengthening
Vol. 15 No. 7 July 2017

(weakening) the interaction between CB and HH(LH). From these relations, we expect larger bandinversion for strained InAs/InGaSb QWs thanunstrained InAs/GaSb QWs.
We calculated the energy dispersion relation byusing the k∙p method because the energy gap Δ originating from the interaction between E1 and HH1
opens at a finite wave vector k. The band structure ofan unstrained InAs/GaSb QW is shown in Fig. 2(c) incomparison to that of a strained InAs/In0.25Ga0.75Sb
QW (Fig. 2(d)) for InAs and (In)GaSb thicknesses of
10 and 6 nm, respectively. As the calculations show,
the InAs/In0.25Ga0.75Sb strained QW has larger band
overlap and larger Δ than the unstrained InAs/GaSb
QW, suggesting favorable bulk insulation properties
in the strained InAs/InGaSb QW.
The heterostructures studied were grown by molecular beam epitaxy on Si (silicon)-doped (001) GaAs
substrates (Fig. 3(a)). The QWs consist of InAs and5.9-nm-thick In0.25Ga0.75Sb, sandwiched between
AlSb barrier layers. Four heterostructures with different InAs layer thickness (8.5, 9.1, 10.0, 10.9 nm)
were prepared.
The heterostructures were processed into Hall bar
devices (width: 50 μm, voltage-probe distance: 180μm) by using lithography (Fig. 3(b)). We applied current I and obtained longitudinal resistance Rxx fromvoltage Vxx and Hall resistance Rxy from voltage Vxy.
Gate voltage VFG was used to vary the carrier density.
Magnetic field B was applied perpendicular to thesample plane.
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CB

AlSb

InAs

InGaSbAlSb
CB
VB (HH)
VB (LH)

0.06 eV

0.17 eVE1

VB(a) Band-edge profile of InAs/In0.25Ga0.75Sb QW(b) Close-up of band alignment at InAs/In0.25Ga0.75Sb interface
10

0

HH1

Energy (meV)

Energy (meV)

20
10
−10
−20

InGaSb

InAs

HH1

E1

−30

HH1

0
−10
−20
−30
−40

E1
−50
−0.3 −0.2 −0.1 0.0 0.1 0.2 0.3−40−0.3 −0.2 −0.1 0.0 0.1 0.2 0.3
k (nm−1)

k (nm−1)(c) Energy dispersion of unstrained InAs/GaSb QW(d) Energy dispersion of strained InAs/In0.25Ga0.75Sb QW

Fig. 2. Band structures of InAs/(In)GaSb QWs.

GaSb 5 nm

Vxx

AlSb 50 nm
VFG

InAs QW

Vxy

In0.25Ga0.75Sb QW
AlSb 800 nm

n-GaAs substrate

I(a) Schematic image of layer structure(b) Picture of Hall bar device and measurement configuration

Fig. 3. Sample structure.

4. Quantitative analysis of band inversion
In Fig. 4(a), Rxx vs. B is plotted for different VFG
values at 2 K. The sample contained a 10.9-nm-thick
InAs QW. Due to the quantization of cyclotron
motion of electrons and holes by a strong magnetic
field, Rxx oscillates as a function of B (SdH (Shubnikov-de Haas) oscillation). Fast Fourier transform
3(FFT) analysis of the SdH oscillations revealed two
frequency components f (Fig. 4(b)). The one with thecorresponding carrier density increasing (decreasing)with VFG can be identified as being associated withelectrons in InAs (holes in InGaSb). The coexistenceof electrons and holes indicates that the band isinverted and the Fermi level intersects both electronlike and hole-like bands.
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Feature Articles

80

Rxx (kΩ)

60
40
20
0

0

2

4

6

8

10

12

Density (Rxy)Density (SdH)
gs = 1

−5
−10
−1.2

gs = 2
−0.8

−0.4

0.0

20

30

40

(b) Results of FFT analysis of SdH oscillations

Density ( x1015 m−2)

Density ( x1015 m−2)

0

10

f (T)

(a) SdH oscillations with different gate voltages VFG

5

VFG
0.2 V
0.3 V
0.5 V

nh

0

B (T)

10

ne

Intensity (arb. unit)

VFG
T=2K
0.2 V tInAs = 10.9 nm
0.3 V
0.5 V

0.4

0.8

VFG (V)(c) Comparison of carrier densities deduced fromHall resistance Rxy and SdH oscillations

8
7
Hole: nh
6
5
4
3
2
1
Electron: ne
0−1.2 −0.8 −0.4 0.0

ncross0.40.8

VFG (V)(d) Electron density ne and hole density nh vs. VFG

Fig. 4. Quantitative evaluation of band inversion.

Carrier density n is related to f as n = gs f(e/h). Here,h is Planck’s constant, e is elemental charge, and gs isspin degeneracy. Moreover, gs can take a value of 1 or
2; gs = 1 (2) means that the energy levels are spin-split(degenerate). To determine whether gs = 1 or 2, wecompared the total carrier densities deduced from Rxywith those deduced from SdH oscillations for bothspin-degenerate and spin-split cases. Interestingly,
we found the energy levels were spin split (gs = 1) in
the InAs/InGaSb strained QW (Fig. 4(c)). Such characteristics have not been observed in the conventional
InAs/GaSb QWs. One possible origin of the spin
splitting is spin-orbit interaction; however, further
investigation is necessary to clarify the origin.
A quantitative measure of band inversion is given
by kcross [= (4πncross/gs)1/2], the wave vector at which
the anti-crossing of E1 and HH1 occurs. From thelinear fit of the dependence of electron density ne andhole density n h on gate voltage (Fig. 4(d)), we
obtained the density ncross at the charge neutrality
point (where ne = nh) of 3.6 × 1015 m−2. Using thencross and gs values, we obtained kcross = 0.21 nm−1.

Vol. 15 No. 7 July 2017

5. Enhanced bulk resistivity
The Rxx peak height at the charge neutrality point
measured at 0.25 K, which mainly reflects in-gap
states in the bulk, increased with decreasing InAs
thickness and reached 889 kΩ (corresponding resistivity of ~10 h/e2) for the 8.5-nm-thick QW (Fig. 5(a)).
Higher peak resistance indicates superior bulk insulation properties. The obtained Rxx peak values are plotted as a function of kcross (Fig. 5(b)). The data for
InAs/GaSb QWs are also plotted as a reference. The
InAs/InGaSb strained QWs exhibited higher bulk
resistivity in the deep band inverted regime than thatof the reference. Generally, the residual bulk conductivity problem becomes more significant as the bandinversion increases. Therefore, it was hard to satisfy
high bulk resistivity and deep band inversion at thesame time with unstrained InAs/GaSb QWs. The k∙pcalculation predicts that an even larger energy gap
can be obtained by exploiting higher strain, that is, ahigher In composition in the InGaSb layer. Thismeans that there is much room to improve the bulk
insulation properties by optimizing the strained QW
4Feature Articles

8.5 nm

9.1 nm

10.0 nm10.9 nm106
Rxx peak (Ω)

Rxx (Ω)

106
105
104

105
104
InAs/InGaSb
InAs/GaSb(reference)

103

103
−1 0

1

−1 0

1

−1 0

1

−1 0

0.0

1

VFG (V)0.1

0.2

kcross (nm−1)(a) Rxx vs. VFG with different InAs thicknesses. Peak heightreflects bulk resistance.

(b) Rxx vs. kcrossFig. 5. Demonstration of high bulk resistance in deep band inverted regime.

structure.
6. Conclusion
High bulk resistivity in the deep band inverted
regime was obtained by using InAs/InGaSb strained
QWs. Our results demonstrate that InAs/InGaSb
strained QW structures are a promising platform forrobust 2DTIs, where intrinsic edge physics can bestudied with the advantage of good bulk insulation
deep in the band-inverted regime. As the next step, weplan to demonstrate quantized edge transport by optimizing InAs/InGaSb strained layer structures. The
origin of the spin-splitting transport properties should

also be clarified. Demonstration of the quantizededge transport in strained QWs will be an important
step in studies of superconductor/TI heterojunctions
for fault-tolerant quantum computing.
References
[1]

[2]

K. Suzuki and K. Onomitsu, “Creating a Topological Insulator Using
Semiconductor Heterostructures,” NTT Technical Review, Vol. 13,
No. 8, 2015.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201508fa7.html
T. Akiho, F. Couëdo, H. Irie, K. Suzuki, K. Onomitsu, and K. Muraki,“Engineering Quantum Spin Hall Insulators by Strained-layer Heterostructures,” Appl. Phys. Lett., Vol. 109, p. 192105, 2016.

Takafumi Akiho

Researcher, Quantum Solid State Physics
Research Group, Physical Science Laboratory,
NTT Basic Research Laboratories.
He received his B.E., M.E., and Ph.D. in engineering from Hokkaido University in 2010,2012, and 2015. He joined NTT Basic ResearchLaboratories in 2015. He has investigatedmolecular beam epitaxy growth of semiconductor heterostructures and studied magneto-transport properties of the grown structures. He is amember of the Japan Society of Applied Physics
and the Physical Society of Japan.

5

NTT Technical Review

Feature Articles: A New Era in Quantum Information
Processing Technologies

Implementation Security of Quantum
Key Distribution
Kiyoshi Tamaki
Abstract

Quantum key distribution is the ultimate cryptography in that it is theoretically secure against any
possible eavesdropping. This ultimate security is an attractive feature, and companies and organizations
in several countries, including Japan, are working on the deployment of networks for quantum key distribution. However, further research is needed to achieve the ultimate practical security, rather thansecurity in principle. In this article, we introduce our recent research activities toward achieving theultimate practical security.
Keywords: quantum key distribution, information-theoretic security, implementation security

1. Introduction
We often rely on data encryption when sending
confidential information such as passwords or private
information over the Internet. Modern cryptography
is commonly used for this, and with such cryptography, we exploit the fact that it is difficult to solve
particular mathematical problems such as the factorization of large numbers by means of existing technologies and algorithms. A crucial problem is that
there is no guarantee that these problems are indeed
hard to solve. This implies that the security of modern
cryptography would be threatened by the development of new algorithms and the technologies to run
them.
Researchers have recently been working on newforms of cryptography to protect confidential information sent over a communication channel. Theseuse the laws of nature (such as the properties of aphoton) and are called quantum cryptography. In thisarticle, we discuss quantum key distribution (QKD),
a particular type of quantum cryptography. One of theremarkable features of QKD is that it is secure against
any possible eavesdropping. This is because one has
to break the laws of nature to crack QKD, which is ofcourse impossible. Therefore, unlike other modern
forms of cryptography, QKD is secure against current
eavesdropping technologies and will remain secure1

against all future technologies.
Optical communications technologies can be utilized in QKD since QKD is part of the optical communications family. However, we need to consider
how to generate, manipulate, and detect a single photon, which is far beyond the capabilities of the technologies used in standard optical communications.
Research and development of technologies for thesetasks has resulted in the commercialization of a QKD
system [1], and organizations in several countries,
including Japan [2], have started field testing it [3] todetermine how it can be integrated with current technologies.
2. Security of QKD
An Internet search for “hacking, QKD” will lead toa number of articles about the successful hacking of
QKD. These articles contradict what is mentioned inthis article, and to determine what is happening, weneed to give a more precise argument about the security of QKD. First, a theory called the security proof
of QKD guarantees the security of QKD. When we
work on a security proof, we use mathematical models for QKD devices, which are constructed by making assumptions on actual devices. We then apply
quantum mechanics and information theory based onthese models to create a security argument. For
NTT Technical Review

instance, we typically assume that a phase modulator
can achieve exact phase modulations of 0, π/2, π, and
3π/2. Quantum key distribution is shown to be secure
under such an assumption, but this security statement
cannot be applied in practice because the exact modulation cannot be achieved due to noise or imperfections. In other words, the security proof provides us
with a conditional statement such as “If a QKD
device satisfies these assumptions, then it is secure.”
A security proof does not tell us anything about what
has to be done to satisfy all the assumptions. Therefore, when some assumptions do not hold, the security argument fails, allowing a hacker to crack QKD
systems. This is exactly what is happening in the
aforementioned articles found on the Internet. In
other words, one cannot crack QKD when all theunderlying assumptions hold, but one can crack it byexploiting the gap between the assumptions made intheory and the actual properties of QKD devices.
Therefore, to guarantee the implementation security of QKD, that is, the security of QKD when it is
implemented, we need to bridge this gap. There are
two approaches for this: (1) developing QKD devices
that fulfill all the assumptions, and (2) developing asecurity proof that accommodates the imperfectionsof actual QKD devices. Researchers are working on
both of these. We have mainly taken the latterapproach, which is briefly introduced below.
3. Guaranteeing implementation securityof QKD
There are three main components in QKD: a sending device, a quantum channel, and a receiving
device, all of which need to be properly implemented
in a QKD experiment. When we conduct a security
analysis of QKD for a quantum channel, we assume
a worst-case scenario in which a quantum channel istotally under the control of an eavesdropper. This
scenario includes a case in which an eavesdropper
sneakily replaces a noisy and lossy quantum channel
with one without any noise and loss, and all the eavesdropping attempts are disguised, as they are mistaken
for noise and loss. Therefore, although a quantum
channel may be useful to increase the communication
speed, it does not provide any advantage in terms of
security.
To improve security, therefore, we have to seriously
investigate the sending and receiving devices. We
discuss here the imperfections of a phase modulator
as an example of a sending device. As we mentioned
above, with some security proofs, exact phase moduVol. 15 No. 7 July 2017log10R

Feature Articles

−2
−3
−4
−5
−6
−7
−8
0

50
100
Distance (km)150

Fig. 1. D istance dependency of communication speed (R)on a log scale. Solid, dashed, and dotted lines
respectively represent cases with perfect phasemodulation, a 3.6-degree modulation error, and a
7.2-degree modulation error.

lation is assumed. Unfortunately, such an assumption
is difficult to satisfy in reality. There is a security
proof accommodating such imperfections [4]; however, according to this security proof, a slight degradation in the modulation precision severely limits thecommunication distance, as shown in Fig. 1. This
figure shows the distance (km) dependency of thecommunication speed in log scale. As can be seen inthis figure, just a slight modulation error, such as
those of 3.6 and 7.2 degrees, reduces the communication distance significantly. For instance, an error of
3.6 degree halves the achievable communication distance compared to perfect phase modulation.
To solve this problem, we checked all the steps in a
QKD protocol and found that some data, which were
believed to be useless and were thus discarded, wereactually the key to solving the problem. We previously proposed a data-processing scheme to enable
us to make full use of the data and successfully solved
this problem [5]. The graph in Fig. 2 shows the distance (km) dependency of the communication speed
based on the same experimental setup as in Fig. 1, butwhere its data processing was based on our scheme.
The three lines are almost superposed, meaning thatphase-modulation error was not an issue.
So far, the discussion here has focused only on theeffect that manipulation with low precision has on
security, where an eavesdropper would exploit deviations of an imperfect photon from the ideal photon.
Unfortunately, there is another type of eavesdropping
strategy in which an eavesdropper actively tries toobtain information that is processed within a QKD
device. In this case, an eavesdropper shines a very
bright pulse into a QKD device and learns the internal
state of the device by monitoring the back-reflected
light.
2

log10R

Feature Articles

model of a sending device [6]. This security proof isgeneral in the sense that we do not need to redo asecurity proof every time we find a new attack, andwe are hoping that our security proof will play a keyrole in guaranteeing security when implementing
QKD.

−2
−3
−4
−5
−6
−7
−8
0

50
100
Distance (km)150

4. Future prospects

Fig. 2. Distance dependency of R of our scheme on a log
scale. Here also, the solid, dashed, and dotted lines
respectively represent cases with perfect phasemodulation, a 3.6-degree modulation error, and a
7.2-degree modulation error.

Although only a sending device was discussed here,
we have seen rapid progress on the receiving-device
side as well (see [3] for a recent review of QKD). We
expect that it will not be long until we establish practical ultimate security of QKD. We will continue to
conduct research to make this a reality.

This type of eavesdropping seems to be circumvented by preventing illegitimate pulses from entering a QKD device. This may be possible by installing
an optical isolator in front of the QKD device. However, no optical isolator can perfectly extinguish theincoming illegitimate pulses, and we have to seriously consider how to accommodate these attacks ina security proof. In our attempts to do so, we noticed
that QKD under this type of attack can be seen as
QKD with a multi-mode pulse. We therefore generalized this idea to construct a security proof that was
valid against any attack for any given mathematical

References
[1]
[2]
[3]
[4]
[5]
[6]

ID Quantique, http://www.idquantique.com/
The Project UQCC, http://www.uqcc.org/
H.-K. Lo, M. Curty, and K. Tamaki, “Secure Quantum Key Distribution,” Nature Photonics, Vol. 8, pp. 595–604, 2014.
D. Gottesman, H.-K. Lo, N. Lütkenhaus, and J. Preskill, “Security of
Quantum Key Distribution with Imperfect Devices,” Quant. Inf. Comput., Vol. 14, No. 5, pp. 325–360, 2004.
K. Tamaki, M. Curty, G. Kato, H.-K. Lo, and K. Azuma, “Loss-tolerant Quantum Cryptography with Imperfect Sources,” Phys. Rev. A,
Vol. 90, 052314, 2014.
K. Tamaki, M. Curty, and M. Lucamarini, “Decoy-state Quantum Key
Distribution with a Leaky Source,” New J. Phys., Vol. 18, 065008,
2016.

Kiyoshi Tamaki

Professor, Faculty of Engineering, Universityof Toyama.
He received an M.Sc. and diploma in theoretical physics from Tokyo Institute of Technology
in 1999 and 2001, and a Ph.D. from the Graduate
University for Advanced Studies (SOKENDAI)
in 2004. During his Ph.D. studies, he spent six
months as a visiting researcher with Prof. Norbert Luetkenhaus’s group at the University ofErlangen-Nuremberg, Germany. He also worked
at the Perimeter Institute for Theoretical Physics
in Canada under Dr. Daniel Gottesman and at the
University of Toronto as a postdoctoral fellow
with Prof. Hoi-Kwong Lo. He joined NTT Basic
Research Laboratories in 2006. He has been aprofessor in the Faculty of Engineering at the
University of Toyama since April 2017.

3

NTT Technical Review

Regular Articles

Heart Rate Measurement with Video
Camera Based on Visible Light
Communication
Takashi G. Sato, Yoshifumi Shiraki, and Takehiro Moriya
Abstract

We have developed a system based on optical camera communication and an encoding method that
can transfer the heart rates (HRs) of multiple people with high precision. The system can potentiallymeasure and transfer the HRs of hundreds of people with their positional information. The unique system
is composed of HR sensor units and a conventional camera. It is presented here along with the novel
event timing encoding method.
Keywords: visible light communication, heart rate, event timing encoding

1. Introduction
Recent advances in light emitting diodes (LEDs)
have opened up numerous possibilities for new visible light communication (VLC) technologies andsystems [1]. VLC systems typically use LEDs andphotodiodes. However, much progress has also been
made recently in image sensor technology. Consequently, optical camera communication (OCC), a
VLC system that uses LEDs and image sensors, hasbeen attracting a lot of attention. A typical OCC system is composed of a single camera with multiple
sensors distributed in sight of the camera [2]. OCC isadvantageous in that it can obtain data from multiple
points together with the sensor positions.
From another point of view, as the population ofelderly citizens increases, healthcare monitoring is
becoming an important and promising application.
For example, suppose a situation where a member ofa gym is wearing a heart rate (HR) sensor that transmits his or her heart beat (HB) in real time (Fig. 1(a)).
The gym staff could provide immediate aid to themember if they observed danger signs in the member’s HR [3]. This technology will also be beneficial
for small children and infants. One potential application is monitoring the HRs of sleeping infants in childcare centers (Fig. 1(b)). Monitoring their HRs would
1

enable a caregiver to quickly deal with abrupt HRchanges such as those associated with sudden infant
death syndrome [4].
However, with the conventional radio frequency
wireless network, this task is not as easy as one may
think. The network may not work well if there are alarge number of devices [5] and positional information is difficult to obtain.
Therefore, we have developed an OCC system [6]and an event timing encoding (ET encoding) technique [7] that can precisely transfer the intervals of
HB events with a conventional camera. The high spatial directionality of visible light enables multiple
data channels to be transmitted simultaneously
through different optical paths along with the positional information of sensors. In addition, OCC canpotentially be combined with natural image analysis
in the future.
2. Method to transfer precise HB timing
As explained in the introduction, the system is composed of a sensor unit and camera (image sensor).
The capturing speed of a commonly used camera isusually limited to 30 or 60 frames per second, which
is not high enough to observe the event timing of the
HB. Furthermore, the system only detects the ‘on/off’
NTT Technical Review

Regular Articles

180

Warning

Copyright ©2017 IEICE, [7]
(a)

(b)

Fig. 1. (a) In this scenario, a member of a gym is wearing a heart rate (HR) sensor. The gym staff can advise the member
by monitoring the HR or provide immediate aid if danger signs are observed. (b) In this scenario, the OCC system isused to support child care center caregivers in monitoring babies who are napping. Information can be easily
superimposed on the captured image by using OCC.

state of the LED for simplicity and robust processing.
If someone simply flashes the LED at a sensor unit
when an HB event occurs, the captured images on thecamera side give us only 33 ms of time resolution.
However, the requirement is less than 4-ms resolution
even for simple HR analysis.
ET encoding is designed to overcome this problem.
The idea is that it first signalizes the occurrence of the
HB event and then refines the relative timing withconsecutive LED patterns. In practice, the unit simulates the frame rate of the camera and switches theLED ‘on’ and ‘off’ using this clock (LED clock).
When the unit detects an HB event, it marks the timing location of the event within this LED clock frame.
The unit first sends an ‘on’ signal after the detection
to inform the decoder of the LED clock frame. Whenthe camera detects this first ‘on’ signal, we can determine on the camera side that the event occurredsomewhere in the previous frame shot. The unit continuously sends details about the location of the eventin the LED clock frame. From these data, we canrefine the timing of the event. In other words, ET
encoding tells the LED clock frame that the event isoccurring and sends the natural binary expression of
the event timing in the frame. The flow chart for sending the event timing by flashing an LED is shown in
Fig. 2. The encoding image is shown in Fig. 3.
The time resolution of the HB interval in ET encoding is calculated as
R1 = (TminF1z−1) ,(1)
2FzVol. 15 No. 7 July 2017

Begin

1. Occurrenceof eventnoyes
2. Switch on the LED(set it as first bit).
3. Set the decision window.
Count i=1.

no4. Event timing ismarked in first half of thedecision window.
yes
5. Switch on the LED
in next sequence.

5’. Switch off the LED
in next sequence.

6. i=i+1

7. i<=Pnoyes
8. Update the decisionwindow.

Fig. 2. F low chart for sending the event timing by flashingan LED.

2

Regular Articles

0

0.5

0.25

0.75
Time (s)IncidenceFirst or last half of window

LED pattern

Amplitude ofelectrocardiogram

HB timing

LED clock (1/Fz)
First or last half of 1/2 window
First or last half of 1/4 window
0

1/4

1/2

3/4

1
Magnified viewFig. 3. Example of LED flashing patterns. The LED is switched on immediately as the first bit of the packet after the event
detection. The precise event timing is marked on the LED clock frame (see magnified view). The LED pattern fromthe second bit is determined according to this marked location.

where Tmin is the shortest term of the HB interval andFz is the capturing speed of the camera (which is thesame as the flashing frequency of the LED). This ET
encoding is more efficient than directly transferring
the HB time interval under the condition of
Tmax − Tmin > 2/Fz, (2)where Tmax is the longest term of the HB interval.
3. Demonstration and comparison
The ET encoding was implemented in our HR
monitoring system. The unit was custom-built andcan be attached to “hitoe” sensing fabric developedby Toray Industries, Inc. and NTT. It samples an electrocardiogram signal with 500 Hz and 10 bits of resolution. A microcomputer installed in the unit searches
for the HB timing and utilizes 5 bits (five on/off
flashes of an LED) to send one HB interval.
A comparison between the HR observed fromdirect monitoring and the HR decoded from the LED
flashing patterns captured by the camera is shown in
Fig. 4. HB intervals were calculated for HRs obtainedby using four different methods: an HR extracted
from the original waveform; an HR transferred by theinterval encoding method; an HR reconstructed using
an event signal (the same as using only the first signal
in ET encoding); and an HR decoded from ET encod3

ing. When 5 bits of data were transferred from theunit, ET encoding gave an almost identical HR.
4. Future development
This article explained efficient encoding methods
that can be used in OCC with a low-frame-rate image
sensor. We introduced ET encoding, which sends therough timing of the HB in the first signal and refines
its location. Theoretically, ET encoding can send the
HR with better resolution than directly sending theinterval data in many cases.
We implemented ET encoding in a custom-madeelectrocardiogram sensor. The results showed that ET
encoding transferred HRs with higher resolution
when the same bit/beat was used. Concerns that thevisible light might be irritating to the human eye canbe allayed by using a near-infrared LED.
This system will be implemented in the future toobtain HR data and will be particularly useful forevaluating health risks. Since the system utilizes acommon camera, it can be combined with natural
image analysis. In addition, data transferrable by ET
encoding are not limited to HR. Other data such as
spikes in neuronal activity can also be transferred by
ET encoding. Further research will open up new possibilities of OCC.

NTT Technical Review

Regular Articles

2

ECG (mV)

1
0
−1
−2

0

30

60

90

(a)

Seating

Instant HR (bpm)80Standing

OriginalET
SignalInterval

70
60
50
40

0

30

60

90

Time (s)
(b)

ET

40

SignalError of HR interval (ms)Interval
20

0

−20

−40

0

30

60

90

Time (s)(c)
ECG: electrocardiogram

Fig. 4. (a) Waveform of electrocardiogram. (b) Results for HRs, extracted from original waveform; HRs reconstructed using
event signal (same as using only first signal in ET encoding); and HRs decoded from ET encoding at 5 bit/packet.
From the data, we can trace the activity of participants who stood up from a seated posture. (c) Error between HRs
observed from direct monitoring and the HR decoded from different methods. An almost identical HR was obtainedwith ET encoding.

Vol. 15 No. 7 July 20174

Regular Articles

References
[1]
[2]

[3]

M. Khalighi and M. Uysal, “Survey on Free Space OpticalCommunication: A Communication Theory Perspective,” IEEE
Commun. Surv. Tutor., Vol. 16, No. 4, pp. 2231–2258, 2014.
M. Kurihara, Y. Honji, J. Fujimori, Y. Oikawa, and Y. Yamasaki,“Examination of Sound Field Visualization-method Using Small
Visualization Devices Built with MEMS-Mic and LED,” Proc. of the
2nd Technical Committee Meeting on Acoustic Imaging, AcousticalSociety of Japan, 2009 (in Japanese).
X. Jouven, J. P. Empana, P. J. Schwartz, M. Desnos, D. Courbon, and
P. Ducimetière, “Heart-rate Profile during Exercise as a Predictor of
Sudden Death,” N. Eng. J. Med., Vol. 352, No. 19, pp. 1951–1958,
2005.

Takashi G. Sato

Research Scientist, Moriya Research Laboratory, NTT Communication Science Laboratories.
He received his B.S., M.S., and Ph.D. in information science and technology from the University of Tokyo in 2003, 2005, and 2008. His
research interests include human interfaces,
especially brain-machine, audio, and tactileinterfaces using psychophysiological measurements as feedback information. He is a memberof the Institute of Electrical and Electronics
Engineers (IEEE), the Institute of Electronics,
Information and Communication Engineers(IEICE), the Japan Neuroscience Society, and theSociety of Instrument and Control Engineers.

Yoshifumi Shiraki

Research Scientist, Moriya Research Laboratory, NTT Communication Science Laboratories.
He received a B.A. in natural science fromInternational Christian University in 2008, andan M.S. and Ph.D. in computational intelligenceand systems science from Tokyo Institute ofTechnology in 2010 and 2015. Since joining
NTT Communication Science Laboratories in2010, he has been studying signal processing,particularly distributed compressed sensing. He
is a member of IEICE and IEEE.

5

[4]
[5]

[6]

[7]

S. M. Pincus, T. R. Cummins, and G. G. Haddad, “Heart Rate Control
in Normal and Aborted-SIDS Infants,” American J. Physiol., Vol. 264,
No. 3, R638-R646, 1993.
L. Chevalier, S. Sahuguede, and A. Julien-Vergonjanne, “OpticalWireless Links as an Alternative to Radio-frequency for Medical
Body Area Networks,” IEEE J. Sel. Area. Commun., Vol. 33, No. 9,
pp. 2002–2010, 2015.
G. Pablo Nava, H. D. Nguyen, Y. Kamamoto, T. G. Sato, Y. Shiraki,
N. Harada, and T. Moriya, “A High Speed Camera-based Approach to
Massive Sound Sensing with Optical Wireless Acoustic Sensors,”
IEEE Trans. Computational Imaging, Vol. 1, No. 2, pp. 126–139,
2015.
T. G. Sato, Y. Shiraki, and T. Moriya, “Heart Rate Measurement Basedon Event Timing Coding Observed by Video Camera,” IEICE Trans.
Commun., Vol. E100.B, No. 6, pp. 926–931, 2017.

Takehiro Moriya

NTT Fellow, Head of Moriya Research Laboratory, NTT Communication Science Laboratories.
He received a B.S., M.S., and Ph.D. in mathematical engineering and instrumentation physics
from the University of Tokyo in 1978, 1980, and1989. Since joining the Nippon Telegraph andTelephone Public Corporation (now NTT) in
1980, he has been engaged in research on medium to low bitrate speech and audio coding. In
1989, he worked at AT&T Bell Laboratories, NJ,
USA, as a visiting researcher. Since 1990, he has
contributed to the standardization of coding
schemes for the Japanese Personal Digital Cellular system, ITU-T G.729, G.711.0, ISO/IEC
MPEG, MPEG-4 General Audio Coding,
MPEG-4 Audio Lossless Coding, and 3GPP
Enhanced Voice Services. He is an IEICE Fellow,
an IEEE Fellow, and a member of the Information Processing Society of Japan, the AudioEngineering Society, and the Acoustical Societyof Japan.

NTT Technical Review

Regular Articles

Path Loss Model to Evaluate
Interference for Small Cells between
Different Floors
Motoharu Sasaki, Minoru Inomata, Wataru Yamada,
Naoki Kita, Takeshi Onizawa, Masashi Nakatsugawa,
Koshiro Kitao, and Tetsuro Imai
Abstract

NTT Access Network Service Systems Laboratories and NTT DOCOMO have been carrying out joint
research and development (R&D) toward construction of the fifth-generation mobile communications
system (5G). This article presents a path loss model developed as one of our R&D activities.
Keywords: propagation, 5G, high frequency band

1. Introduction
NTT Access Network Service Systems Laboratories and NTT DOCOMO have been conducting a
joint study toward development of the fifth-generation mobile communications system (5G). The resultsof measurements carried out jointly made it possible
to clarify the path loss characteristics of the indoor
environment in the high frequency band. This article
presents the developed path loss model.
2. Path loss model
Frequency allocation of 5G, particularly the highfrequency bands above 6 GHz, will be discussed at
the World Radiocommunication Conference to beheld in 2019. To discuss the appropriate frequency
allocation, it is important to evaluate the interference
between 5G systems and other existing systems. In
the interference evaluation, it is necessary to understand the path loss (attenuation of the radio wave) inorder to understand the interference power.
Path loss varies depending on the frequency, so it isessential to construct a path loss model in the frequency band that will be used. It is assumed that the1high frequency band above 6 GHz will be used for
5G, and therefore, the standardization group 3rd Generation Partnership Project (3GPP) is standardizing
the path loss models of the high frequency band [1].
In 3GPP, office environments are assumed as theindoor small cell scenarios, but the path loss modelthat can be used in that environment is only for asingle floor (the same floor). In order to accurately
evaluate interference, an evaluation of inter-floor
interference [2] is also required (Fig. 1). NTT AccessNetwork Service Systems Laboratories has developed a path loss model for evaluating inter-floor
interference for a wide range of frequencies as part ofefforts to realize various wireless communication
systems.
3. Prediction results produced with developed
path loss model
To understand the inter-floor interference of indoor
small cells in an office environment, we measured
path loss using five frequency bands (0.8, 2.2, 4.7,
26.4, and 37.1 GHz), including the bands above 6
GHz in a multistory building of NTT DOCOMO(from the third to sixth floors). As a result, we clarified
NTT Technical Review

Regular Articles

Indoorsmall
cellsInterference
betweendifferent floors

Fig. 1. Interference between different floors in indoor small
cells.

6th floor

Tx

Window

Floor and ceiling
5th floor

Paths throughoutside4th floor
Paths throughinside
3rd floor
Rxthat the path loss characteristics between different
floors were represented by two dominant propagation
paths inside and outside the building in a wide frequency band (Fig. 2). Here, the propagation pathinside the building is the path transmitted through thefloors (and ceilings) between the transmitter (Tx) andthe receiver (Rx). The propagation path outside thebuilding is the path that is diffracted at the window
frame of the Tx floor, propagated outside the building, which then re-enters from the window of the Rx
floor. The propagation path inside the building attenuates due to transmission loss caused by ceilings andfloors, and the propagation path outside the building
attenuates because of the diffraction loss at the window.
The results of calculating the path loss using the
proposed model for these two dominant paths areshown in Fig. 3(a), and the results of measuring thepath loss between different floors are shown in
Fig. 3(b). The measurement results show the medianvalues of path loss at each floor. The value is normalized by the path loss when the Tx and Rx are set on thesame floor, and it therefore represents the floor penetration loss when the Tx and Rx are on different floors.
The calculation results in Fig. 3(a) show the pathloss of the path inside the building transmitted
through floors when varying the floor thickness w,
and that of the path outside the building that is diffracted at the window frame. The path losses are calculated from 0.1 GHz to 100 GHz. In the measurement environment, the thickness of the floor material
was 1.5 m per floor. It can be seen that the transmission loss increases rapidly as the floor thickness
increases (that is, as the distance between floors

Transmission pathinside building
w = 0.5 m
w = 1.5 m
w = 3.0 m

Path loss (dB)

0
20
40

Diffraction pathoutside building

60
80
100
0.1

1
10
Frequency (GHz)100(a) Calculation results using two dominant paths

Floor penetration loss (dB)

Fig. 2. Dominant paths between different floors.

30
1 floor2 floors
3 floors

40
50
60
70
0.1

1
10
Frequency (GHz)100(b) Measurement results between different floors

Fig. 3. Results of calculating and measuring path loss between different floors.

Vol. 15 No. 7 July 20172

Regular Articles

increases) or as the frequency increases.
In contrast, the diffraction loss of the path outside
the building increases gently with respect to the frequency increase. Therefore, when the frequency is
low or the distance between floors is small, the pathinside the building (transmission path) becomesdominant, and when the frequency is high or the distance between floors is large, the path outside thebuilding (diffraction path) becomes dominant.
The frequency dependence of the measurement
results in Fig. 3(b) can be explained by using the twodominant paths. The path loss for a distance consisting of one floor level increases sharply as the frequency increases up to 4.7 GHz, but over 4.7 GHz thepath loss increases gently. Also, in the case of two or
three floors, the path loss increases gently withrespect to the frequency increase. It is thought that thepath inside the building (transmission path) is dominant up to 4.7 GHz at a distance of one floor level(e.g., the distance between the fifth floor and the sixth
floor), and the path outside the building (diffractionpath) is dominant under the other conditions.
As discussed above, when the two dominant paths
are used, it is possible to express the path loss characteristics in a wide frequency band. From this result,for example, when base stations for indoor small cells
are installed near a window, it is necessary to pay
attention to the fact that interference between floors
becomes especially large.

3

4. Future overview
This article presented a developed path loss modelthat can represent the path loss characteristics
between different floors. The proposed model is suitable for evaluating inter-floor interference of indoor
small cells. We expect that the model will be utilized
for 5G, which is a key topic in the research, development, and standardization in the wireless communication field worldwide. A paper describing the proposed model won the Best Paper Award at the 2016
International Symposium on Antennas and Propagation (ISAP 2016). Please refer to the paper [3] formore detailed results.
In order to contribute to the construction of various
wireless communication systems, we intend to actively engage in the modeling of propagation phenomena
for a wider range of frequencies and environments.
References
[1]
[2]

[3]

3GPP TR 38.900 v14.1.0: “Study on Channel Model for Frequency
Spectrum above 6 GHz (Release 14),” Sept. 2016.
ITU-R Recommendation P.1238-8: “Propagation Data and Prediction
Methods for the Planning of Indoor Radiocommunication Systems
and Radio Local Area Networks in the Frequency Range 300 MHz to
100 GHz,” July 2015.
M. Sasaki, M. Inomata, W. Yamada, N. Kita, T. Onizawa, M.
Nakatsugawa, K. Kitao, and T. Imai, “Path Loss Characteristics
between Different Floors from 0.8 to 37 GHz in Indoor OfficeEnvironments,” ISAP 2016, Okinawa, Japan, Oct. 2016.

NTT Technical Review

Regular Articles

Motoharu Sasaki

Takeshi Onizawa

Research Engineer, NTT Access Network Service Systems Laboratories.
He received a B.E. in engineering and an M.E.
and Ph.D. in information science and electricalengineering from Kyushu University, Fukuoka,
in 2007, 2009, and 2015. In 2009, he joined NTT
Access Network Service Systems Laboratories,
where he has been engaged in researching propagation modeling of interference between mobile
terminals for spectrum sharing wireless accesssystems. He received the Young Researcher’sAward and the Best Paper Award from the Institute of Electronics, Information and Communication Engineers (IEICE) in 2013 and 2014,
respectively. He is a member of the Institute ofElectrical and Electronics Engineers (IEEE).

Senior Research Engineer, Supervisor, Group
Leader - Wireless Access Systems Project, NTT
Access Network Service Systems Laboratories.
He received a B.E., M.E., and Ph.D. from
Saitama University in 1993, 1995, and 2003.
Since joining NTT in 1995, he has been engaged
in the R&D of personal communication systems
and high data rate wireless local area network
systems. He is now in charge of strategies andpropagation of wireless communication systems.
He received a Best Paper Award, Young Investigators award, and Achievement Award fromIEICE in 2000, 2002, and 2006, respectively. He
also received the Maejima Award in 2008. He isa senior member of IEICE and a member ofIEEE.

Minoru Inomata

Masashi Nakatsugawa

Research Engineer, Wireless Access Systems
Project, NTT Access Network Service Systems
Laboratories.
He received a B.E. and M.E. in electrical engineering from Tokyo University of Science in2009 and 2011. In 2011, he joined NTT Wireless
Systems Innovation Laboratories. Since then, he
has been involved in R&D of radio propagationfor 5G systems and system design for unlicensed
wireless communication systems. He received
the Young Researcher’s Award from IEICE in
2016. He is a member of IEEE.

Wataru Yamada

Senior Research Engineer, Planning Section,
NTT Access Network Service Systems Laboratories.
He received a B.E., M.E., and Ph.D. from Hokkaido University in 2000, 2002, and 2010. Sincejoining NTT in 2002, he has been researchingpropagation characteristics for wide band accesssystems. From 2013 to 2014, he was a visiting
research associate at the Centre for Telecommunications Research at King’s College, London,
UK. He received the Young Researcher’s Award,
a Communications Society Best Paper Award,
and a Best Paper Award from IEICE in 2006,
2011, and 2014, respectively. He is a member ofIEEE.

Executive Manager, NTT Information Network Laboratory Group.
He received a B.E. and M.E. from Waseda
University, Tokyo, in 1987 and 1989, and an M.S.
from the California Institute of Technology,
USA, in 1999. He joined NTT Radio Communication Systems Laboratories in 1989. He has
researched MMIC circuit design, packaging
technologies, software-defined radio, and widearea wireless access systems. From 2010 to 2012,he was a senior manager in the Radio Division,
Technical Planning Department, NTT, where he
was involved in regulatory and standardization
activities for wireless systems. He is currently on
secondment at NTT Advanced Technology Corporation. He received the Young Researcher’sAward in 1996 and the Distinguished Contributions Award (Electronics Society) in 2017 fromIEICE, and the YRP Award from the YRP (Yokosuka Research Park) R&D Promotion Committee
in 2002. He is a member of IEEE, IEICE, and theJapan Society of Applied Physics.

Koshiro Kitao

Research Engineer, Research Laboratories,
NTT DOCOMO, INC.
He received a B.S., M.S., and Ph.D. from Tottori University in 1994, 1996, and 2009. He
joined NTT Wireless Systems Laboratories in
1996. Since then, he has been researching radio
propagation for mobile communications.

Naoki Kita

Senior Research Engineer, Supervisor, Group
Leader, NTT Access Network Service Systems
Laboratories.
He received a B.E. from Tokyo Metropolitan
Institute of Technology in 1994, and an M.E. and
Ph.D. from Tokyo Institute of Technology in1996 and 2007. He joined NTT in 1996, where heresearched radio propagation characteristics andradio propagation modeling for wireless accesssystems. From 2009 to 2010, he was a visiting
scholar at Stanford University, CA, USA, andfrom 2012 to 2013, he was a visiting researchscholar at Waseda University, Tokyo. From 2013
to 2015, he was a senior manager in the planningdepartment of NTT Information Network Laboratory Group. He is currently responsible for the
R&D of future satellite communication systems
and radio propagation modeling for future wireless communication systems. He received theYoung Researcher’s Award, a Communications
Society Best Paper Award, and a Best Paper
Award from IEICE in 2002, 2011, and 2014,
respectively. He is a senior member of IEICE anda member of IEEE.

Vol. 15 No. 7 July 2017

Tetsuro Imai

Senior Research Engineer, 5G Laboratory,
NTT DOCOMO, INC.
He received a B.S. and Ph.D. from Tohoku
University, Miyagi, in 1991 and 2002. He joined
NTT Radio Communications Systems Laboratories in 1991. Since then, he has been engaged in
the R&D of radio propagation, antenna systems,
and system design for mobile communications.
He received the Young Researcher’s Award in
1998, Best Paper Awards in 2006 and 2014, andthe Communications Society Best Paper Awardin 2015 from IEICE. Dr. Imai is a member of
IEICE and IEEE.

4

Global Standardization Activities

Report on Progress of ITU-T Study
Group 3 at the World
Telecommunication Standardization
Assembly 2016 (WTSA-16)
Memiko Otsuki
Abstract

The World Telecommunication Standardization Assembly 2016 (WTSA-16) was held in Yasmine
Hammamet, Tunisia, from October 25 to November 3, 2016. The following is an outline of discussionson ITU-T (International Telecommunication Union - Telecommunication Standardization Sector) Study
Group 3 covered at WTSA-16.
Keywords: WTSA, ITU-T, SG3

1. Outline of ITU-T Study Group 3
The International Telecommunication Union Telecommunication Standardization Sector (ITU-T)
Study Group 3 (SG3) is a unique study group; it is the
only study group in ITU-T dealing with international
standardization of non-technical aspects such as tariff
settlement and economic policies. Traditionally,
SG3’s main role was to establish standards for tariffs
for telecommunications, such as coming up with theprinciples of tariff settlement or making recommendations aimed at reducing international roaming
rates. In the past few years, however, more and more
proposals have been made to study themes that go
beyond the framework of telecommunications, orthemes concerning domestic regulations and policies.
These proposals have mainly been coming frommembers from developing countries, and the discussions on such topics have been heated at the meetings.
The discussions at the World Telecommunication
Standardization Assembly 2016 (WTSA-16) [1],which is the supreme decision-making body of ITUT, were no exception, and many of the developed
countries opposed many of the SG3-related proposals
presented by developing countries. The discussions
1

and results are introduced in the following section,
along with points to be noted in future SG3 activities.
2. Discussions at WTSA-16 relevant to SG3
The discussions relevant to SG3 covered a range ofissues from specific technical topics to the SG3 study
mandates. They are briefly described here.
2.1 Change of mandate
The term for the domain of study of each SG is amandate. As mentioned, in the past few years, developing countries have been increasingly vocal in
expressing their desire for ITU-T to make recommendations in order to justify domestic regulations or
policies. At WTSA-16 also, Russia and the Arabstates proposed adding the study of regulatory issues.
However, the United States, western European countries, and Japan opposed the addition of this study
theme, saying that regulatory issues are solely
domestic issues. As a result, the mandate of SG3 wasdefined as International telecommunication/ICT(information and communication technology) policy
and economic issues and tariff and accounting matters(including costing principles and methodologies).
NTT Technical Review

Global Standardization Activities

Table 1. Approved recommendations.
Recommendation no.

Subject ofrecommendation

Outline

D.52
[New]

Establishing and connecting
regional Internet exchange
points to reduce costs ofinternational Internet
connectivity

Recommendation ITU-T D.52 “Establishing and connecting regional Internet exchange
points (IXPs) to reduce costs of international Internet connectivity” guides regional
collaboration to establish central hubs or IXPs that enable local Internet traffic to be routed
locally, saving international bandwidth and reducing the costs of international Internet
connectivity.

D.53
[New]

International aspects ofuniversal service

Recommendation ITU-T D.53 “International aspects of universal service,” while
recognizing the sovereign right of Member States to define and regulate their universal
service/access policies, proposes general outlines to guide governments and regulators intheir tasks and management functions regarding universal service funds in a globalized
digital environment.

D.97
[New]

Methodological principles fordetermining international
mobile roaming rates

Recommendation ITU-T D.97 “Methodological principles for determining international
mobile roaming rates” proposes possible approaches to the reduction of excessive
roaming rates, highlighting the need to encourage competition in the roaming market,
educate consumers and consider appropriate regulatory actions such as the introductionof caps on roaming rates.

D.261
[New]

Regulatory principles formarket definition andidentification of operatorswith significant market
power - SMP

Recommendation ITU-T D.261 “Regulatory principles for market definition andidentification of operators with significant market power” proposes principles andguidelines to assist countries in defining and identifying significant market power in thetelecommunications sector.

D.271[Modified]

Charging and accounting
principles for NGN

ITU-T Recommendation D.271 sets out the general principles and conditions applicable byadministrations for the capability to transport Internet protocol (IP) packets over IP-based
networks between standards-based interfaces and the services that they support.
*Revision adds/deletes part of description on charging parameters.

NGN: Next Generation Network

The US and western European countries further
insisted that study themes be limited to international
issues, and the title of SG3 was also agreed to be
ITU-T Study Group 3: Tariff and Accounting Principles and International Telecommunication/ICT Economic and Policy Issues, and by adding the wordinternational, it was mandated that SG3 is not to discuss solely domestic issues [2].
2.2 Approval of five recommendations
Five recommendation proposals (four new and one
revision) were submitted to WTSA-16 and were allapproved (Table 1). However, the US, Australia, Germany, Canada, and others pointed out that D.52,
D.53, and D.261 were domestic issues, and the recommendations were adopted with reservations fromthe opposing countries [3].
2.3 Compiling the new resolutions
At WTSA-16, the RCC (Russian regional group)
proposed a resolution stating that the ITU-T should
get involved in studying all manners of policies for
consumer protection in telecommunications/ICT,
including fair competition, business models, quality
assurance, establishment of reliability, and the assurVol. 15 No. 7 July 2017

Source: ITU Journal, Vol. 47, No. 1, 2017

ance of security. The developing countries supported
this, but the developed countries, including Japan,
opposed it, pointing out the broadness of the scope of
consumer protection, and that many of the issues suchas quality assurance and security would be redundantwith the existing work of ITU-T. As a result, the content was amended to read “to resolve that ITU-T
Study Group 3……within their mandates, should
carry out studies, including on standards for the protection of consumers and users of telecommunication/ICT services,” and it was newly approved asStudies concerning the protection of users of telecommunication/information and communication technology services (New Resolution 84) [4].
2.4 International mobile roaming (Resolution
88)
Ever since the bill shock issue, which refers toreceiving unexpectedly high mobile phone bills,
began to surface, SG3 has been considering establishing a recommendation for lowering international
mobile roaming rates. However, some members
voiced their concern that the existing recommendation was not being fully utilized, and at WTSA-16,
the CITEL (American regional group) proposed a2

Global Standardization Activities

new resolution to execute D.98 (Charging in international mobile roaming service). D.98 was approved in
2012; it recommended increased transparency ofrates, provision of alternate plans, and appropriate
regulatory measures. The new Resolution D.97 on
international roaming is listed in Table 1; Resolution
88 encourages each member state to execute D.98,
which had been compiled previously.
2.5 Mobile financial service (Resolution 89)
There are reportedly 2 billion unbanked people in
the world. This means they remain outside of thebanking system. Providing access to financial services to these people is referred to as financial inclusion. In ITU-T also, a focus group to discuss digitalfinancial services was established in June 2014, andits target was to expand access to financial services
through the use of mobile money. In SG3, somemembers expressed the desire to take leadership in
studying this issue, and work is being done to compile recommendations that encourage countries tointroduce competition principles and regulations offees for service transactions.
However, more proposals were put forth fromdeveloping countries to include, for example, theissue of granting a license to financial service providers in SG3 recommendations, which is clearly thejurisdiction of financial authorities, and this has
become a contentious issue. In WTSA-16, the Araband African countries proposed a new resolution on“Promoting the use of information and communication technologies to bridge the financial inclusiongap” and resolved to gain the cooperation of telecomand financial authorities. At the same time, it wasagreed to avoid redundancy with the work of otherstandardization bodies and to have the study carriedout within the scope of the ITU mandate dealing with
telecommunications. The progress in the study on
mobile financial services is to be reported at theannual Board meeting and to WTSA in 2020.
2.6 Over the top (OTT) players and services
The activities of OTT players who are deliveringcontent such as video and music and providing voiceor messaging services have been gaining more prominence recently. Some members of SG3 concernedwith the economic impact of these OTT players on
network operators proposed making recommendations on the definition of OTT and the principles ofregulation, and a rapporteur group was created to
discuss specifics.
At WTSA-16 also, a new resolution was proposed3by the African countries that seeks to resolve the
OTT’s impact in an appropriate way. In the meeting,there were severely conflicting opinions between
developed countries—primarily the United States—
and the African and Arab countries on the scope ofservice the resolution covers, and the ensuing discussion was difficult. The developed countries of theWest wanted to avoid subjecting a wide range of services to the resolution and also wanted to limit its
scope to voice service and messaging service operators that use public numbering resources (E.164). The
African countries that wanted to include more general OTT services strongly opposed this, and consensus was not reached by the end of the session. Multiple official and unofficial meetings were held, and as
a result, while a resolution on OTT services wasdeferred, as a compromise, additions were made to
the mandates of SG2 and SG3 of Resolution 2, adding “Study of the economic and regulatory impact of
the Internet” to the mandate of SG3.
3. Future development
In addition to the topics above, study questions ofeach SG in the next study period were approved at
WSTA-16. The questions for SG3 are listed inTable 2. The field of study covers a wide range, andas time changes, one can see it is no longer an SG
dealing with telecommunications tariffs. However, it
should be cautioned that this shift of the agenda has
been driven by the developing countries. With SG3 inparticular, the profile of attendees has changed dramatically, with over 70% now from developing countries, and a large number of them are from regulatory
agencies. Most of the study agendas were proposedby developing countries, which have unique issues
such as the lack of competition among private companies. There is also less accumulation of know-how
to set regulations. For this reason, rather than solving
international issues through standardization, there isan increasing tendency to attempt to resolve their own
domestic issues through international agreements.
With this type of agenda proposal, the developed
countries expressed the view that these issues havealready been resolved through regulatory/policy
frameworks or agreements among operators, and thatissues unique to developing countries should be dealt
with by ITU-D*, not ITU-T. The standard response*

ITU-D: ITU Telecommunication Development Sector; its role isto promote utilization and development of ICT in developing
countries.

NTT Technical Review

Global Standardization Activities

Table 2. SG3 questions.
Questions

Title

Q1/3

Development of charging and accounting/settlement mechanisms for international telecommunications services using
next-generation networks (NGNs), future networks, and any possible future development, including adaptation of existing
D-series Recommendations to the evolving user needs

Q2/3

Development of charging and accounting/settlement mechanisms for international telecommunications services, other thanthose studied in Question 1/3, including adaptation of existing D-series Recommendations to the evolving user needs

Q3/3

Study of economic and policy factors relevant to the efficient provision of international telecommunication services

Q4/3

Regional studies for the development of cost models together with related economic and policy issues

Q5/3

Terms and definitions for Recommendations dealing with tariff and accounting principles together with related economic andpolicy issues

Q6/3

International Internet connectivity including relevant aspects of Internet protocol (IP) peering, regional traffic exchange points,
cost of provision of services and impact of transition from Internet protocol version 4 (IPv4) to Internet protocol version 6 (IPv6)

Q7/3

International mobile roaming issues (including charging, accounting and settlement mechanisms and roaming at border areas)

Q8/3

Alternative calling procedures and misappropriation and misuse of facilities and services including calling line identification(CLI), calling party number delivery (CPND) and origin identification (OI)

Q9/3

Economic and regulatory impact of the Internet, convergence (services or infrastructure) and new services, such as over the
top (OTT), on international telecommunication services and networks

Q10/3

Definition of relevant markets, competition policy and identification of operators with significant market power (SMP) as it
relates to the economic aspects of international telecommunications services and networks

Q11/3

Economic and policy aspects of big data and digital identity in international telecommunications services and networks
Source: https://www.itu.int/en/ITU-T/studygroups/2017-2020/03/Pages/questions.aspx

Table 3. SG3 Chairman and Vice-chairmen.
SG3: Tariff and accounting principles and international telecommunication/ICT
economic and policy issues
ChairmanSeiichi TSUGAWA

Japan

Vice-chairmen

Joséphine ADOU BIENDJUICôte d'Ivoire

Mohammad Ahmad ALMOMANI Jordan
Abraao BALBINO E SILVA

Brazil

Liliana Nora BEIN

Argentina

Alexey BORODIN

Russian Federation

Adel DARWISH

BahrainAminata DRAME

SenegalMuneer ELMAKI

Sudan (Republic of the)

Byoung Nam LEE

Korea (Rep. of)

Karima MAHMOUDI

Tunisia

Raynold MFUNGAHEMA

Tanzania

Ahmed SAID

Egypt

Dominique WURGESFranceSource: http://www.itu.int/net4/ITU-T/lists/mgmt.aspx?Group=3

from participants from developing countries was that
the ITU-D cannot make recommendations. Consequently, another issue that emerged at WTSA-16 wasdetermining how ITU should deal with issues thatdeveloping countries are trying to resolve throughcompilation of international recommendations, andhow they should form a consensus. In SG3, althoughVol. 15 No. 7 July 2017

their mandate has been limited to international issues
through revision of Resolution 2, many of the vicechairs are from developing countries (Table 3), so
this tendency may manifest more strongly. Even
broader development of the discussions is envisaged
going forward, including the economic aspects oftelecommunications and how regulations should be
4Global Standardization Activities

established. Therefore, I think we need to be clearly
conscious of what the role of our company and ofJapan should be in our endeavors.
References[1]

Y. Goto, K. Takaya, N. Araki, and H. Iwata, “Report on WTSA-16(World Telecommunication Standardization Assembly 2016),” NTT

[2]
[3]
[4]

Technical Review, Vol. 15, No. 3, 2017.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201703gls.html
ITU‑T Study Group 3, Mandate and lead roles, http://www.itu.int/en/
ITU-T/studygroups/2017-2020/03/Pages/mandate.aspx
ITU-T Recommendations under Study Group 3 responsibility,
http://www.itu.int/ITU-T/recommendations/index_sg.aspx?sg=3
ITU-T Resolutions and Opinions,
http://www.itu.int/pub/T-RES

Memiko Otsuki

Assistant Manager, Carrier & Regulatory
Affairs Office, Corporate Strategy & Planning
Department, NTT DOCOMO, INC.
She received a B.A. in liberal arts from International Christian University in 2000 and an M.A.
in public policy from Australian National University in 2007. She has served as an expert oninternational telecommunications regulations
and has engaged in standardization work for tariffs and telecommunications policy in ITU andother organizations. She participated in WCIT-12
as a member of the U.S. delegation when she was
the manager of the Washington, D.C. Office ofNTT DOCOMO USA in 2012. In 2016, she
contributed as the drafting group chairman indrafting a resolution for WTSA-16. She received
the ITU-AJ award from the ITU Association ofJapan in 2017.

5

NTT Technical Review

Short Reports

Report on NTT Group Exhibits at
CeBIT 2017—Showcasing
Cutting-edge Use of ICT at theWorld’s Largest Information
Technology Trade Fair
Kei Karasawa
Abstract

The NTT Group exhibited at CeBIT 2017, which ran for five days (March 20–24, 2017) in Hanover,Germany. In its first participation in 12 years, NTT showcased examples of its cutting-edge use of information and communication technology in a variety of industrial fields. The exhibits were presented inan easily comprehensible manner and were highly appreciated by visitors. This article introduces themain exhibits and visitors’ responses to them.
Keywords: ICT, Internet of Things, artificial intelligence1. Objective of exhibiting at CeBIT 2017
The NTT Group had a booth at CeBIT 2017 for the
first time in 12 years in order to introduce digitaltransformation solutions offered by NTT Group companies, which have been expanding their global business. NTT sought to make the exhibits easy for anyone to understand. For example, it offered interactive
demonstrations that enabled visitors to directly
appreciate the new value that digitization offers, andpresented information and communication technology (ICT) use cases in videos in three languages (English, German, and Japanese) to help visitors understand the advantages of selecting the NTT Group as abusiness partner. The result was an uninterrupted
flow of visitors up to the last day, totaling more than
5000 people.
Under the theme “d!conomy – no limits,” which
had been adopted since 2015, CeBIT 2017 exhibitedthe latest ICT technologies such as digital transformation, the Internet of Things (IoT), and artificial
intelligence (AI). It is noteworthy that Japan became1a partner country in 2017, when German Chancellor
Angela Merkel and Japanese Prime Minister Shinzo
Abe jointly announced the Hanover Declaration,
under which the two countries agreed to jointly formulate international standards and undertake researchand development (R&D) in cutting-edge technologies such as IoT and AI. The declaration acknowledged Japan’s open stance regarding the use of technology as one area in which Germany considers
Japan to be outstanding. NTT, for its part, exhibited anumber of trailblazing uses of ICT in areas where
ICT had been slow to penetrate, such as sports andculture. The company believes that through its CeBIT
exhibits, it was able to demonstrate the group’s ability to provide a wide range of technologies, not limited to telecommunications.
2. Overview of NTT Group exhibits at
CeBIT 2017
The NTT Group exhibited at several sites, including the business security zone and partner booths.
NTT Technical Review

Short Reports

The largest site was the NTT Group booth inside theJapan Pavilion. Exhibited there were new work- andlife-styles made possible through the use of ICT,
including path-forging activities in areas that are
attractive to many people, such as sports and culture,and broad-ranging activities that cut across a variety
of industries supporting Japan’s economy [1]. This
article introduces the NTT Group’s major exhibits.
2.1 Sports zone
The NTT Group is pursuing digital transformation,even in fields where the human senses play importantroles such as sports.
(1) Application of IoT to sports
For the last two years, Dimension Data, one of the
NTT Group integrators, has been providing a service
for participants in bicycle races. The service offers
various kinds of data based on the position of theuser’s bicycle, which are obtained from a GPS (global positioning system) device installed under thebicycle saddle. For example, it can provide information about the cyclist such as his/her speed, altitude,and exact position on a map, as well as strategic data
for team management such as distances between
competitors. The provision of such information during live coverage of bicycle races has enhanced therealistic sensations experienced by viewers watching
the television (TV) coverage of the event. Dimension
Data is considering expanding the service to include
transmission of the physical conditions of cyclists,
for example, the degree of fatigue, in real time using
sensors installed in their clothing (Photo 1).
This will be made possible through the use of thewearable sensing fabric “hitoe,” which is developed
jointly by NTT and Toray Industries, Inc. The fabriccan be woven into a uniform so that accurate cardiogram and electromyogram data can be obtained without causing any discomfort to the wearer. Thus, thephysical condition of a cyclist can be measured evenduring a race. We believe that, if TV watchers canunderstand the physical status of cyclists while
watching live race coverage, they can develop a senseof unity with individual racers and become more
emotionally engaged with the athletes.
At CeBIT 2017, the presenter himself wore a shirt
made of “hitoe” and demonstrated that, while cycling,
he was able to check his heart rate and degree of
fatigue on a bicycle-mounted smartphone. WhenNTT President Hiroo Unoura introduced this bicycle
to Chancellor Merkel and Prime Minister Abe, whoare both sports enthusiasts, they showed keen interest(Photo 2).
Vol. 15 No. 7 July 2017

Photo 1. R eal-time sensing of cyclists’ physical conditions.

Photo 2. N
 TT President Unoura (right) introduces “hitoe”
technology to Chancellor Merkel and Prime
Minister Abe.

Shirts made of “hitoe” have also been worn by drivers participating in car races such as the Indy 500.
The drivers have highly praised the shirts, noting thatthey allowed the drivers to determine what kind ofdriving practice they needed to do, because they wereable to easily and objectively monitor their physicalstates. (Photo 3)(2) Smart Stadium
NTT also exhibited a Smart Stadium, in which ICT
provides new value for spectators at sports events. In
this exhibit, a VIP (very important person) room at a
stadium was recreated, where the user could order ameal or replay a scene using a tablet connected to theInternet via Wi-Fi*1. This enabled the user to watch agame with the sensation of being in a real stadium*1 Wi-Fi is a registered trademark of Wi-Fi Alliance.

2

Short Reports

Photo 3. Indy 500 race driver Tony Kanaan praises his“hitoe” shirt.

Photo 4. Smart Stadium solution enables spectators towatch a game with the sensation of being in areal stadium.

while feeling as relaxed as he/she would be when
watching TV at home (Photo 4). In addition, in cooperation with Panasonic Corporation, NTT exhibited
augmented reality (AR), which was achieved by presenting translucent information on the glass windowsof the VIP room. This enables spectators of a game tosee information overlaid on the real field that is conventionally only available on TV, such as detailed
information about their favorite players, team formations, or the off-side line (Photo 5). Visitors commented that this would make their watching of agame at a stadium even more enjoyable.
(3) Sports VR
Virtual reality (VR) technology enables the user totake on the perspective of a player on the field. At the
NTT booth, visitors were able to experience the sensation of a goalkeeper fielding the ball from a penalty
kick by a J-League (Japan Professional Football
3

Photo 5. S mart Stadium solution enables spectators in a
VIP room to see information on the glass
windows overlaid on the real field.

Photo 6. V isitors experienced the sensation of agoalkeeper enabled by VR technology.

League) player. They had a high sense of immersion
in this experience because not only did they havevisual experience through head-mounted displays,
but they also wore vibrating devices on their hands
and abdomens, which generated vibrations when thevirtual ball was received. This proved to be a very
popular exhibit. A Euronews correspondent reported
enthusiastically on this technology (Photo 6).
2.2 Culture (Human Heritage) zone
The NTT Group is also working towards digitaltransformation through ICT in the area of culture.
(1) Digital archive
In cooperation with the Vatican Apostolic Library,
NTT DATA is digitizing the library’s valuable manuscripts for preservation. At CeBIT 2017, NTT displayed
a huge bible borrowed from the library and its digitally archived version in parallel. Visitors were able to
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Short Reports

experience a situation in which they could read thebible on a digital display at any time while looking at
the precious real bible alongside. A video showing
the painstaking process of archiving was also presented so that visitors could appreciate the significance of this activity. The activity was highly appreciated by those concerned with libraries.
(2) Cultural experience
NTT is experimenting with providing new experiences by fusing kabuki, one of the Japanese performing arts, and ICT. It conducted a demonstration inwhich ICT was used to enable visitors to experiencekumadori, an unusual style of stage makeup used inkabuki, in which vividly colored makeup is applied toan actor’s face. In the demonstration, makeup was notapplied to the visitor’s face with a real brush. Instead,NTT’s angle-free object search technology was used
to apply kumadori on an image of his/her face displayed on the screen so that visitors could experience
the world of kabuki without discomfort. Specifically,
the angle-free object search technology automaticallydetects the type of mask chosen regardless of themask’s angle. It then uses AR to overlay kumadori on
the person’s face shown on a display like a mirror, insuch a way that the makeup appears to adhere to theface, even when the subject changes his/her facial
expression or direction, giving the realistic impression that the cosmetics are actually applied. Those
visitors who were well versed in kabuki enjoyed making defiant expressions or gestures with kumadori on
their faces, just as if they were actors (Photo 7).
(3) Proposal for new culture
At CeBIT 2017, NTT collaborated with Tokyo
National University of Fine Arts and Music in a demonstration of the use of a moving shade to provide an
impression of movement of a self-portrait of Vincent
van Gogh. Light projection technology known as
HenGenTou (Deformation Lamp) created an illusionthat the famous painter was eating something. This
was an unusual display that could be understood
intuitively without the need for explanation, andmany visitors stopped and stared in surprise. The
Deformation Lamp demonstration attracted a lot ofinterest, especially from those concerned with art
museums as a way to give fresh, dramatic impact topaintings on display.
2.3 Cross Industry Collaborations zone
While manufacturing has traditionally played acentral role in Japan’s economic development, NTT
believes that Japan’s high-quality agricultural products and infrastructure services such as its wellVol. 15 No. 7 July 2017

Photo 7. V isitors enjoyed having kabuki makeup on theirface shown on the screen.

developed public transportation systems are playing alarger role in this regard and have become globallycompetitive in recent years.
In Europe, there are many people running largescale farms. Visitors from the agricultural field studied NTT’s exhibit that showed solutions to several
important agricultural issues such as the optimum
timing of sowing. In the field of manufacturing, NTTshowed a video and exhibited robots from FANUC
Corporation. Visitors showed great interest in the
precise movements of the robots and real-time analysis of manufacturing conditions made possible byedge computing. In the field of transportation, NTT’s
exhibit on the use of “hitoe” to support safe driving
attracted a lot of interest (Photo 8).
Thus, the NTT Group is working to create newvalue in collaboration with customers by providing
ICT across different industrial fields. Videos of theuse cases presented at CeBIT 2017 have been posted
on YouTube*2 [2].
2.4 Responses within the NTT GroupThe activities introduced above garnered appreciation from employees of the NTT Group’s overseas
subsidiaries:“I think that the Smart Stadium solution provided
by NTT R&D was amazing and aligned very nicely
with the story of the Tour de France and our sponsorship of Team Dimension Data for Qhubeka. Sport isclearly an interesting attraction for visitors to CeBIT
and it brought more people into the stand and allowed
us to engage with them. I don’t think many visitors*2 YouTube is a registered trademark of Google Inc.

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Short Reports

Photo 8. The use of “hitoe” to support safe driving.

understood that the NTT Group was made up of somany powerful brands (NTT DATA, NTT DOCOMO, NTT Communications, and Dimension Data),
and so a visit to the stand certainly provided us witha stage to be able to communicate that message to themany visitors.”—Digital Practice, Dimension Data
Group.
“The CeBIT was a fantastic opportunity to show thepower of collaboration and solution capacities wehave in the NTT Group and to make understandable
for so many highly interested visitors what NTT, withall of its powerful brands, can deliver in the era ofdigitization to help transform society, sports, andindustry, as well as creating value for business transformation. From a German perspective of Dimension
Data, it absolutely helps to strengthen our brand andawareness in the market, which will also support ourbusiness.”—Marketing/PR, Dimension Data Germany.
“The NTT Communications part of the NTT Group
presence at CeBIT was dominated by one item—areplica of the 2017 McLaren Honda Formula 1 racing
car which gained lots of interest from visitors to theexhibition and helped improve our brand awarenessin Germany, especially those who were not familiar
with NTT Communications. The Technology Partnership with McLaren Honda began in July 2016, so
the exhibition gave us a chance to explain to people
how the use of data is transforming F1 and how NTT
Communications is playing a part in that transformation by supplying cloud, network, and cloud management functions. Our network connects the McLaren
backbone and supports all of their businesses. Our
Enterprise Cloud provides increased capacity andreliability for their apps and services which, forexample, helps them simulate all kinds of racing con5

ditions. These technologies support McLaren Honda’s efforts to improve performance at each race.
Development is continual, and more and more components are being added to the overall solution as we
progress. For example, we are developing a race day
dashboard for the cloud management platform thatwill help McLaren’s team to optimize performance.
We were also able to discuss our R&D activities that
can help improve the experience of fans and the F1team alike. In one instance, an editor from WIRED
magazine was fascinated by the possibility of using“hitoe” for monitoring the pit crew and improving
their performance, and he especially enjoyed the
Smart Stadium demonstration and the saké that wasdelivered to him one minute after ordering it!”—
Senior Director Marketing, Europe, NTT Communications.
“I was in charge of the exhibition of the communication robot Sota*3 that introduced the NTT Groupbooth. Many people stopped at the Sota robots, listening to their talks and taking pictures of them. I also
got several questions for actual business cases andintroduced the solution proposals and proof-of-concepts for elderly people support and bank reception
support. Also, supported languages were another frequent question, which indicates customers from various countries are interested in Sota.”—EMEA Innovation Labs, NTT DATA EMEA Ltd.
“For DOCOMO, CeBIT provided a great opportunity to showcase our current and future IoT solutionsin the fields of agriculture, manufacturing, and transport. Unlike many mobile-specific events, CeBIT
visitors come from a wide range of backgrounds
across many industries. This not only enabled us toleverage our exposure across industries, but also gave
us an opportunity to gain access to creative new ideas
on ways our technology can be applied to tackle realworld issues. It was particularly rewarding to be able
to work alongside our sibling companies under thesame NTT banner, learning about each other’s business activities and building a mutual sense of camaraderie. I feel grateful for the new friendships I developed, and have no doubt that opportunities like thiswill create a synergy that can make us even more
efficient and dynamic as a group going forward.”—Public Relations Department, NTT DOCOMO, INC.
In addition, we have posted visitor interviews on
the official NTT channel on YouTube [2].

*3 Sota is a registered trademark of Vstone Co., Ltd.

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Short Reports

3. Conclusion
The NTT Group is proposing a B2B2X (businessto-business-to-X) model for creating new services incollaboration with its customer companies. In particular, with 2020 in mind, the group is seeking tocontinue to use ICT to create new value in such fields
as sports and culture. With a wide range of ICT developed by the group centering on NTT laboratories, theNTT Group is endeavoring to provide new value forits customers and to broaden its business horizons.
References[1][2]

Website of NTT Group CeBIT 2017,
http://www.ntt.co.jp/activity/cebit/en/
Videos of NTT Group CeBIT 2017,https://www.youtube.com/channel/UCXPR5vkTnK8G497WNdq4Zgg/

Kei Karasawa

Senior Manager, Research and Development
Planning Department, NTT.
He received a B.E., M.E., and Ph.D. from the
Department of Information Systems Engineering, Faculty of Engineering, Osaka University in
1994, 1996, and 1999. Since joining NTT in
1999, he has been engaged in research on IPv6
networking, network security, and cloud computing. From 2005 to 2006, he was a visiting scholar
in the Applied Cryptography Group at Stanford
University, California, USA. He is a member of
the Institute of Electronics, Information andCommunication Engineers and the Information
Processing Society of Japan.

Vol. 15 No. 7 July 20176

Short Reports

NTT Communications to Deploy
400-Gbit/s Optical Transmission
System for Datacenter Network
Connections

1. Introduction
NTT Communications Corporation (NTT Com),the information and communication technology solutions and international communications businesswithin the NTT Group, began deploying an ultralarge-capacity, space-saving, and energy-saving
400-Gbit/s (400G) optical transmission system in
datacenters for network connection on April 14,
2017. The system raised the transmission capacity of
NTT Com’s core network above 19 Tbit/s per opticalfiber, or more than double the existing capacity. The
system utilizes advanced digital-signal processing
technology, which is the result of NTT’s research anddevelopment (R&D).
2. Background
NTT Com achieved its existing 100-Gbit/s-perchannel optical transmission system through the early
adoption of digital coherent optical transmission
technology in response to rapidly increasing traffic
on its core optical fiber network, as well as expanding
video data and cloud services. NTT Com has now
taken practical steps to upgrade transmission technology to 400G due to ever-increasing network demands
for higher speeds and more data, including for 4K/8K
and other high-resolution video data, the full-fledged
expansion of the Internet of Things, and big-data processing.
3. Features and advantages of 400G system
The main features and advantages of the 400G system, as depicted in Fig. 1, are described here.
(1) Top-level energy and space saving (Fig. 2)
1

The new system reduces energy consumption per
bit by 75% and space requirements by 80% comparedto the existing system. The savings were achieved
through a synergistic combination of advanced digital-signal processing technology and 16-nm CMOS
(complementary metal-oxide semiconductor) technology. Energy-saving integrated circuits can beconstructed rapidly, enabling systems to be quickly
deployed even in confined spaces at datacenters.
(2) Higher transmission capacity
The new system achieves more than double thetransmission capacity per optical fiber compared toits predecessor system. To generate 400G signals, thesystem utilizes NTT’s research result: 16-quadratureamplitude modulation (QAM) for both phase andamplitude, and sub-carrier multiple transmission.
(3)	New optical transport network (OTN) and
400-Gbit/s Ethernet (400GbE) interfaces
In connection with the deployment, NTT Com willoffer new optional network services for enterprises,
including the sequential launch of OTN interfaces(optical-channel transport unit (OTU)2, OTU2e, andOTU4) primarily for wholesalers and datacenter
users, and a 400GbE interface incorporating framing
technology compatible with OTUCn, which is theresult of NTT’s R&D.
(4) Disaggregation architecture
To deliver new services and functions with evengreater speed and flexibility, NTT Com will combine
software-defined networking (SDN) technologies
and disaggregation architecture, which can be redeployed as required according to function or module,
replacing existing high-function, all-in-one dedicatedequipment.
Going forward, NTT Com and NTT will jointly
investigate further possibilities, including the proof
NTT Technical Review

Short Reports

SDN
controller(4) Disaggregation architecture

New 400G system
Mux/ Optical
Transponders demux amplifier

Optical Mux/ Transamplifier demux ponders…

…

OTN (OTU4, OTU2, and OTU2e)
400GbE(compatibility
planned)(3) New interfaces
100GbE,
10GbE, etc.

Transmit 400G
(16-QAM x 2ch)

Clientinterfaces

Functional components

DSP

Och

E/O

Optical modules(1) & (2) Digital-signal
processing technology
E/O: electrical-to-optical conversion
Mux/demux: multiplexer and demultiplexer
Och: optical channel

Fig. 1. Main features and advantages of 400G system.

(a) Energy consumption per bit(b) Space per bit

1

1

75%

0.25

80%

0.2

Existing system

New system

Existing system

New system

Fig. 2. Energy and space saving effect.

of concept deployment to deliver high-speed Ethernet
signals, including 400GbE. Mindful of IEEE (Institute of Electrical and Electronics Engineers) standardization trends, they will pursue increased flexiVol. 15 No. 7 July 2017

bility and agility for transmission networks by leveraging disaggregated equipment and SDN technologies.

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Short Reports

For Inquiries
Innovation Division, Customer Services Department,
NTT Communications Corporation
Email: aline-si@ntt.com
Public Relations Section, Planning Department,
NTT Information Network Laboratory Group
Email: inlg-pr@lab.ntt.co.jp
http://www.ntt.co.jp/news2017/1704e/170407a.html

3

NTT Technical Review

External Awards
Communications Society: Distinguished Contributions
Award
Winner: Hayato Fukuzono, NTT Access Network Service Systems
Laboratories
Date: September 21, 2016
Organization: The Institute of Electronics, Information and Communication Engineers (IEICE)
For his contributions as a peer review member for papers submitted
to IEICE Transactions on Communications.
JSAI 2016 Annual Conference Award
Winner: Koh Mitsuda and Ryuichiro Higashinaka, NTT MediaIntelligence Laboratories; Toshirou Makino, NTT Software Corporation; and Yoshihiro Matsuo, NTT Media Intelligence Laboratories
Date: March 1, 2017
Organization: The Japanese Society for Artificial Intelligence(JSAI)For “Collection and Analysis of Implicit Information in Chat-oriented Dialogue.”
Published as: K. Mitsuda, R. Higashinaka, T. Makino, and Y. Matsuo, “Collection and Analysis of Implicit Information in Chat-oriented Dialogue,” JSAI 2016 (The 30th Annual Conference of theJapanese Society for Artificial Intelligence), Fukuoka, Japan, June
2016.
Kambayashi Young Researcher Award
Winner: Kyosuke Nishida, NTT Media Intelligence Laboratories
Date: March 8, 2017
Organization: The Database Society of JapanFor his achievements in authoring published papers such as“Understanding the Semantic Structures of Tables with a Hybrid
Deep Neural Network Architecture,” “Probabilistic Identiﬁcation ofVisited Point-of-Interest for Personalized Automatic Check-in,” and“Improving Tweet Stream Classification by Detecting Changes in
Word Probability.”
Published as: K. Nishida, K. Sadamitsu, R. Higashinaka, and Y.
Matsuo, “Understanding the Semantic Structures of Tables with a
Hybrid Deep Neural Network Architecture,” Proc. of the Thirty-FirstAAAI Conference on Artificial Intelligence, San Francisco, CA,
USA, Feb. 2017.
K. Nishida, H. Toda, T. Kurashima, and Y. Suhara, “Probabilistic
Identiﬁcation of Visited Point-of-Interest for Personalized Automatic
Check-in,” Proc. of the 2014 ACM International Joint Conference on
Pervasive and Ubiquitous Computing, pp. 631–642, Seattle, WA,
USA, Sept. 2014.
K. Nishida, T. Hoshide, and K. Fujimura, “Improving Tweet Stream
Classification by Detecting Changes in Word Probability,” Proc. ofthe 35th international ACM SIGIR Conference on Research andDevelopment in Information Retrieval, pp. 971–980, Portland, OR,
USA, Aug. 2012.
Excellent Interactive Award
Winner: Kyosuke Nishida, Kugatsu Sadamitsu, Ryuichiro Higashinaka, and Yoshihiro Matsuo, NTT Media Intelligence Laboratories
Date: March 8, 2017
Organization: The 9th Forum on Data Engineering and Information
Management (DEIM2017)1

For “TabNet: A Hybrid Deep Neural Network that Understands the
Semantic Structures of Tables.”Published as: K. Nishida, K. Sadamitsu, R. Higashinaka, and Y.
Matsuo, “TabNet: A Hybrid Deep Neural Network that Understands
the Semantic Structures of Tables,” Proc. of DEIM2017, B6-4,
Takayama, Gifu, Japan, Mar. 2017 (in Japanese).
SIGGN-101 Best Presentation Award
Winner: Tomohiro Kokogawa, NTT Secure Platform Laboratories
Date: March 11, 2017
Organization: Information Processing Society of Japan (IPSJ) Special Interesting Group on Groupware and Network Services (SIGGN)
For “Visualization and Study of Incident Response Capability ofOrganizations Based on ISO 22320.”
Published as: T. Kokogawa, Y. Maeda, A. Amano, and Y. Kohno,“Visualization and Study of Incident Response Capability of Organizations Based on ISO 22320,” IPSJ SIG Technical Report, Vol. 2017GN-101, No. 23, Mar. 2017 (in Japanese).
The Awaya Prize Young Researcher Award
Winner: Yuma Koizumi, NTT Media Intelligence Laboratories
Date: March 16, 2017
Organization: The Acoustical Society of Japan (ASJ)
For “A Study of Reinforcement Learning for Sound Source
Enhancement to Maximize Perceptual Score.”
Published as: Y. Koizumi, K. Niwa, K. Kobayashi, and Y. Haneda,“A Study of Reinforcement Learning for Sound Source Enhancement
to Maximize Perceptual Score,” Proc. of the 2016 Autumn Meetingof ASJ, Toyama, Japan, Sept. 2016.
Young Scientist Award of the Physical Society of Japan
Winner: Yuichiro Matsuzaki, NTT Basic Research Laboratories
Date: March 18, 2017
Organization: The Physical Society of JapanFor his research on robust magnetic field sensing beyond the standard quantum limit.
Published as: T. Tanaka, P. Knott, Y. Matsuzaki, S. Dooley, H. Yamaguchi, W. J. Munro, and S. Saito, “Proposed Robust Entanglementbased Magnetic Field Sensor beyond the Standard Quantum Limit,”
Phys. Rev. Lett., Vol. 115, 170801, Oct. 2015.
Y. Matsuzaki, X. Zhu, K. Kakuyanagi, H. Toida, T. Shimo-Oka, N.
Mizuochi, K. Nemoto, K. Semba, W. J. Munro, H. Yamaguchi, and S.
Saito, “Improving the Coherence Time of a Quantum System via a
Coupling to a Short-lived System,” Phys. Rev. Lett., Vol. 114,
120501, Mar. 2015.
Y. Matsuzaki, S. C. Benjamin, and J. Fitzsimons, “Magnetic Field
Sensing beyond the Standard Quantum Limit under the Effect ofDecoherence,” Phys. Rev. A, Vol. 84, 012103, July 2011.
The Young Scientists’ Prize, The Commendation for Science and Technology by the Minister of Education, Culture, Sports, Science and Technology
Winner: Yasuhiro Fujiwara, NTT Software Innovation Center
Date: April 19, 2017
Organization: The Ministry of Education, Culture, Sports, Scienceand Technology (MEXT)

NTT Technical Review

External Awards

For his research on fast graph mining algorithms that support artificial intelligence.
Prize for Science and Technology, Development Category,
The Commendation for Science and Technology by the
Minister of Education, Culture, Sports, Science and Technology
Winner: Kazuhide Nakajima, NTT Access Network Service Systems Laboratories; Izumi Sankawa, NTT Electronics; and Shigeru
Tomita, NTT Advanced Technology
Date: April 19, 2017
Organization: MEXT

For “Experimental Demonstration of a Flow Mining Based Proactive Network Control Technology on a Wide Area SDN Testbed.”
Published as: Y. Takahashi, A. Suzuki, M. Tsujino, N. Kamiyama, K.
Ishibashi, K. Shiomoto, T. Otoshi, Y. Ohsita, and M. Murata, “Experimental Demonstration of a Flow Mining Based Proactive Network
Control Technology on a Wide Area SDN Testbed,” The 18th NV
Symposium, Tokyo, Japan, Apr. 2016.
ITU-AJ Accomplishment Award
Winner: Kazuhide Nakajima, NTT Access Network Service Systems Laboratories
Date: May 17, 2017
Organization: The ITU Association of Japan (ITU-AJ)

For the development of low bending loss optical fibers.
Prize for Science and Technology, Research Category,
The Commendation for Science and Technology by the
Minister of Education, Culture, Sports, Science and Technology
Winner: Akira Fujiwara, NTT Basic Research Laboratories
Date: April 19, 2017
Organization: MEXT
For his research on the ultimate control of electrons in nanostructures, and device applications.
Tsujii-Shigeo Security Paper Award
Winner: Mehdi Tibouchi, NTT Secure Platform Laboratories
Date: April 20, 2017
Organization: Japan Society of Security Management
For “Zeroizing Attacks on Indistinguishability Obfuscation over
CLT13.”
Published as: J.-S. Coron, M. S. Lee, T. Lepoint, and M. Tibouchi,“Zeroizing Attacks on Indistinguishability Obfuscation over CLT13,”
Public-Key Cryptography—PKC 2017—the 20th IACR International Conference on the Practice and Theory in Public-Key Cryptography, Amsterdam, The Netherlands, Mar. 2017, pp. 41–58, Springer.
Best Paper
Winner: Kosuke Nakamura, Masahiro Suzuki, Hideyuki Torii,
Kazutake Uehira, Kanagawa Institute of Technology; Youichi
Takashima, NTT Service Evolution Laboratories
Date: May 9, 2017
Organization: IARIA (International Academy, Research, and Industry Association)
For “Embedding and Detecting Patterns in a 3D Printed Object.”Published as: K. Nakamura, M. Suzuki, H. Torii, K. Uehira, and Y.
Takashima, “Embedding and Detecting Patterns in a 3D Printed
Object,” Proc. of PATTERNS 2017 (The Ninth International Conferences on Pervasive Patterns and Applications), Athens, Greece, Feb.
2017.
IEICE-CS NV Best Presentation Award
Winner: Yosuke Takahashi, Akito Suzuki, and Masayuki Tsujino,
NTT Network Technology Laboratories; Noriaki Kamiyama, Fukuoka University; Keisuke Ishibashi, NTT Network Technology Laboratories; Kohei Shiomoto, Tokyo City University; Tatsuya Otoshi,
Yuichi Ohsita, and Masayuki Murata, Osaka University
Date: May 11, 2017
Organization: The Technical Committee on Network Virtualization(NV), IEICE Communications Society (CS)Vol. 15 No. 7 July 2017For his commitment in international standardization activities
concerning optical fibers in International Telecommunication
Union - Telecommunication Standardization Sector (ITU-T) Study
Group 15.
Best Paper Award
Winner: Daiki Chiba, Takeshi Yagi, Mitsuaki Akiyama, Kazufumi
Aoki, Takeo Hariu, NTT Secure Platform Laboratories; Shigeki
Goto, Waseda University
Date: May 19, 2017
Organization: IEICE Communications Society
For “BotProfiler: Detecting Malware-infected Hosts by Profiling
Variability of Malicious Infrastructure.”
Published as: D. Chiba, T. Yagi, M. Akiyama, K. Aoki, T. Hariu, and
S. Goto, “BotProfiler: Detecting Malware-infected Hosts by Profiling Variability of Malicious Infrastructure,” IEICE Trans. Commun.,Vol. E99-B, No. 5, pp. 1012–1023, May 2016. DOI:10.1587/
transcom.2015AMP0001.
Best Paper Award
Winner: Riichi Kudo, B. A. Hirantha Sithira Abeysekera, Yusuke
Asai, Takeo Ichikawa, Yasushi Takatori, and Masato Mizoguchi,
NTT Access Network Service Systems Laboratories
Date: May 19, 2017
Organization: IEICE
For “User Equipment Centric Downlink Access in Unlicensed
Spectrum for Heterogeneous Mobile Network.”
Published as: R. Kudo, B. A. H. S. Abeysekera, Y. Asai, T. Ichikawa,Y. Takatori, and M. Mizoguchi, “User Equipment Centric Downlink
Access in Unlicensed Spectrum for Heterogeneous Mobile Network,” IEICE Trans. Commun., Vol. E98-B, No. 10, pp. 1969–1977,
Oct. 2016.
KIYASU-Zen’iti Award
Winner: Tatsuaki Okamoto, NTT Secure Platform Laboratories, andKatsuyuki Takashima, Information Technology R&D Center, Mitsu­
bishi Electric Corporation
Date: June 1, 2017
Organization: IEICE
For “Adaptively Attribute-Hiding (Hierarchical) Inner ProductEncryption.”
Published as: T. Okamoto and K. Takashima, “Adaptively AttributeHiding (Hierarchical) Inner Product Encryption,” IEICE Trans. Fundamentals., Vol. E99-A, No. 1, pp. 92–117, Jan. 2016.
Outstanding Paper Award 2016
Winner: Hiroaki Kikuchi, Meiji University, and Katsumi Takahashi,

2

Papers Published in Technical Journals and Conference Proceedings

NTT Secure Platform Laboratories
Date: June 2, 2017
Organization: IPSJ

cation from Trajectory Data.”
Published as: H. Kikuchi and K. Takahashi, “Zipf Distribution
Model for Quantifying Risk of Re-identification from Trajectory
Data,” J. Info. Process, Vol. 24, No. 5, pp. 816–823, Sept. 2016.

For “Zipf Distribution Model for Quantifying Risk of Re-identifi-

Papers Published in Technical Journals andConference Proceedings
Visual Wetness Perception Based on Image Color Statistics
M. Sawayama, E. H. Adelson, and S. Nishida
Journal of Vision, Vol. 17, No. 5, May 2017.
Color vision provides humans and animals with the abilities todiscriminate colors based on the wavelength composition of light andto determine the location and identity of objects of interest in cluttered scenes (e.g., ripe fruit among foliage). However, we argue thatcolor vision can inform us about much more than color alone. Sincea trichromatic image carries more information about the optical properties of a scene than a monochromatic image does, color can help us
recognize complex material qualities. Here we show that humanvision uses color statistics of an image for the perception of an ecologically important surface condition, i.e., wetness. Psychophysicalexperiments showed that overall enhancement of chromatic saturation, combined with a luminance tone change that increases the darkness and glossiness of the image, tended to make dry scenes lookwetter. Theoretical analysis along with image analysis of real objects
indicated that our image transformation, which we call the wetness
enhancing transformation (WET), is consistent with actual opticalchanges produced by surface wetting. Furthermore, we found that the
WET operator was more effective for the images with many colors(large hue entropy) than for those with few colors (small hue entropy).
The hue entropy may be used to separate surface wetness from3other surface states having similar optical properties. While surface
wetness and surface color might seem to be independent, there arehigher order color statistics that can influence wetness judgments, inaccord with the ecological statistics. The present findings indicatethat the visual system uses color image statistics in an elegant way tohelp estimate the complex physical status of a scene.

Metal-organic Vapor-phase Epitaxy of InP-based HEMTStructures with InAs/In0.8Ga0.2As/In0.53Ga0.47As Composite
ChannelH. Sugiyama, T. Hoshi, A. El Moutaouakil, and H. Matsuzaki
The 29th International Conference on Indium Phosphide andRelated Materials (IPRM 2017), Berlin, Germany, May 2017.
InP-based high electron mobility transistor (HEMT) structureswith an InAs/In0.8Ga0.2As/In0.53Ga0.47As composite channel weresuccessfully fabricated by metal-organic vapor-phase epitaxy(MOVPE). We managed to grow highly strained channels at a lowtemperature of 440ºC. In practical ultra-high-speed HEMT structureswith 9-nm barrier-spacer layers, high mobility of around 17,000 cm2/
Vs with a sheet carrier concentration of around 2.5x1012 cm−2 was
obtained at room temperature.

NTT Technical Review