NTT Technical Review
September 2012 Vol. 10 No. 9
View from the Top
Ettienne Reinecke
Chief Technology Officer, Dimension Data

Front-line Researchers
Shin’ya Nishida, Senior Distinguished Scientist,
NTT Communication Science Laboratories

Feature Articles: Quantum Computing
Research Frontier of Quantum Computers
Coherent Coupling between a Superconducting Qubit
and a Spin Ensemble
Theoretical and Experimental Study of
Quantum Nondemolition Measurement
Using an Electric Circuit
Electron Spin Rotation and Quantitative Determination of
Spin-Orbit Coefficients
Quantum Simulation with Ultracold Atomic Gases
in an Optical LatticeRegular Articles
Method of Handling Hard Disk Drive Failuresin Cloud Computing Environment

Global Standardization Activities
Establishment of New ITU-T Focus Group
Related to Disaster Relief

External Awards/Papers Published inTechnical Journals and Conference Proceedings
External Awards/Papers Published inTechnical Journals and Conference Proceedings

View from the Top

A Very Simple Strategy: Find Out
Exactly What the Client Needs and
How the Market Is Changing
Ettienne Reinecke
Chief Technology Officer
Dimension Data
Overview

Founded in 1983 in South Africa, Dimension Data is expanding worldwide as a comprehensive information technology (IT) solutions and services provider in the field of information and communications technology(ICT). Since joining forces with NTT in 2010, the company has been working to achieve an even higher level of performance. We sat down with Ettienne Reinecke, Chief Technology Officer, to learn about the growth strategy
and future aspirations of Dimension Data in a volatile, changing market.

Seeking a very simple but highly effective strategy—Mr. Reinecke, what has Dimension Data’s strategy
been up to now?
We have been moving forward by setting target
markets while establishing very close communication channels with our clients. In this process, wehave placed much importance on obtaining a thorough understanding of our clients’ businesses and the
problems they face and on developing our businessaccordingly.
The business environment has been undergoing
major changes for more than ten years. The information and communications technology (ICT) industry
has been maturing; clients have been seeking new
business models; and macroeconomic movements,
technical innovations, and business changes havebeen converging. These developments continue toimpact the markets that we pursue and the strategies
that we formulate. I believe that we too must change
in line with these developments and that we must
continue to evolve while keeping a watchful eye on


key trends.
At the risk of repeating myself, our mission is to
respond to our clients’ needs. The efforts that we havemade in creating maximum value have received highmarks in client satisfaction measured by establishedresearch methods such as Gartner’s Magic Quadrant.
—Your approach is to propose not what you want but
what the client needs. How did you conceive of thisapproach?
It is a very simple but extremely effective strategy.
Our clients—many of which are financial institutions—are involved in a wide range of businessactivities with diverse business models. However, allof them are deeply concerned about cost. They share
a common desire to expand business in the current
market using advanced ICT and to consciously control cost. Two key requirements to this end are a highlevel of technical competence and a reduction in
investment costs. Accordingly, if we cannot fully
grasp what the client needs and values, it will not beeasy to provide a solution that matches the client’s
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View from the Top

aspirations. For this reason, I believe that a sensational business can be developed if we can synchronize these key requirements with client goals.
What we want to provide are solutions that clients
who want to move their businesses forward havenever experienced. Our sincere desire to have our client experience such solutions underlies this way of
thinking. Though very simple in essence, this idea
has led to truly effective results and has, as a consequence, given birth to very amicable, long-term relationships with our clients.
Sincere, long-term relationships—It appears that building long-term relationships isvital to Dimension Data for developing a robust,
long-lasting business.
Let’s look back at the business path that we havefollowed over the last 30 years or so. We started out
as a very small-scale consulting company. At thattime, even our clients realized that they were in theearly days of applying information technology (IT) to
expand business, but it was an era in which theywould rack their brains trying to make sense of IT and
figure out how best to use it. The history of our business therefore began with making proposals to clients
who had a strong desire to use IT but did not understand how to use it to maximum effect. We thenpassed through various stages on our way to becoming a systems integrator. We began as a product integrator: in accordance with client needs, we proposeda set of appropriate products from diverse vendors.
We then grew into a technology integrator: we gathered and analyzed a variety of new technologies andmade proposals after assessing client needs. Now, asa systems integrator, our role is to understand andscrutinize ever-evolving ICT and provide our clientswith optimal systems in a comprehensive manner.
More recently, we have been presenting our clientswith proposals that focus on service-oriented business models. All of our clients suffer from a severelack of human resources and skills, as well as fromstrict demands placed on cost control. Needless tosay, our clients and we too are being pressured invarious ways by today’s worldwide, macroeconomic
conditions. Regardless, the secret to achieving arobust, long-lasting business even under difficult conditions such as these is to continue with our efforts inunderstanding each client’s state of affairs and determining what services we should be providing.

Vol. 10 No. 9 Sep. 2012

—What kind of insights have you had through yourrelationships with clients?
As you know, we set sail on our quest to expand
globally as a South African enterprise, and our first
venture was in emerging markets, where efforts must
begin with infrastructure building. I witnessed many
struggles in trying to figure out how to go aboutbuilding an infrastructure without adequate resources. But the times were changing with the coming of
globalization, and we were determined to expand ourbusiness globally. But above and beyond our struggles, our clients found that they were entering an era
in which they would be forced to globalize, and tosupport them in responding to that change, we ourselves had to change. That is, we had to find out what
we should be doing to support our clients to overcome their IT challenges. While expanding our business in line with clients’ needs, we continued to learn.
We started out by asking ourselves: “What does globalization mean in the first place?” We then reflected
on what lessons could be learned from our globalization efforts and what our outlook for the future shouldbe. Of course, we made many mistakes (bad choices),but by constantly revising our approach and learningfrom these mistakes, we arrived at our present state.
In the end, I feel that there’s more to our business thanjust knowing about technology. It also means learning about actual business conditions and working
on-site together with our clients in a truly participatory manner. In this way, we have been able to comeup with many effective solutions. There are times
when the market and industry find themselves at a
crossroads, which is typically associated with somekind of groundbreaking event. I have seen how sucha crossroads can stimulate innovation, and how theView from the Top

market and industry can undergo a sea change at sucha time. In fact, I feel that we are entering a period ofmassive transformation right now. We can see this incloud services: in the great changes that are taking
place in the way cloud services are used and the volume of their use. We can also see it in the amazing
advances that are taking place in mobile devices. A
wave of great change is truly enveloping us all.
ICT is not the prime objective—it’s a tool.
In the end, the focus should be on people.
—As the market becomes borderless and development accelerates with such technological advances,
how should we be making use of ICT?
Life today is forcing people to strive for a work-lifebalance to survive a rapidly changing environment
while coping with stress from work and other areas oflife. Under these conditions, it is not enough to focus
on only our own environment—we must also keep
abreast of how society and the world at large arechanging. That is, we must figure out how to ride thiswave of change. This is indeed a very tough challenge
for all of us. On the other hand, this wave helps topromote technical innovation, so figuring out how to
use new technologies to our benefit is one way ofsurviving a very stressful environment. A good exam-ple of this is learning how to make good use of thecloud, tablet computers, smartphones, and othermodern devices that are now becoming an integral
part of our lives. In short, it’s not just about technology—it is also important that we apply our own
knowledge or wisdom in coping with the situations
that we face in our lives. Surviving this society is also
very difficult for me, but I am making an effort tomake good use of ICT to enrich my life. Wheneverpossible, I opt for a home-office work style thatenables me to work and interact from home instead of
having to go to the office. I am also concerned about
managing my health and staying fit. For example,
I’ve been making an effort during this stay in Japan to
get into shape by running around the Imperial Palace
whenever I can.
I think it would be great if everyone could live a lifethat achieves such a balance, but there is also something that I want everyone to remember. Technologicaladvances have certainly made remote communicationeasier, but fundamentally speaking, communication
between remotely located people is based on trust
created through direct, face-to-face communication. I
often say to my colleagues and fellow employees that
communication is something that happens between
people while ICT is a tool for facilitating that process.
This may seem obvious, but misunderstandings inthis regard can easily occur.
—Has this way of thinking penetrated the company?
We communicate using all kinds of technologies.
Because we are expanding our business globally, we
make use of ICT to establish contact with any of ouroffices anywhere in the world. I have set up such
facilities in my home so that I can communicate with

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View from the Top

others in the company as needed. At the same time, I
fully understand the importance of face-to-face meetings, and I hold such meetings at least once every
quarter. Using ICT for work-related communications
is a given, but I also encourage others to use ICT todevelop close relationships with colleagues inside thecompany and even with friends outside the company
unrelated to work. Enjoying outside interests andcultural activities can help one to appreciate others
and cultivate even better human relationships. I
believe that this approach to life is essential to good
teamwork in the workplace.
—You talked about deepening one’s understanding
about cultural differences, but Dimension Data andNTT have major differences in corporate structure
and culture. Out of the many ICT-related companies
in the world, why did you think of tying up withNTT?
You might say that it’s in our DNA as a South
African company to give culture a high level ofimportance. As you can probably imagine, Africa isfilled with a great variety of distinctive cultures.
Perhaps it is because of this environment that South
Africans are open-minded about multiculturalism—
they respect cultural differences. During our negotiations with NTT, differences between our corporate
cultures did not present an obstacle.
It took a little more than three years before a final
decision was made, and during that time, we wereable to develop an appreciation for each other’s wayof thinking and build a relationship. This, I believe,
turned out very well. At that time, we had also
received offers from other companies, but none ofthem wanted to take the time to carefully develop
mutual understanding.
As you know, we are not a company that aims tocreate and provide something physical—what we dois provide services that our clients need. Accordingly,
people who create services are the foundation of ourbusiness. And if these people—our staff—are nothappy in their work, I think that it will be difficult toprovide solid and compelling services. This idea iscommon to both Dimension Data and NTT, and thisis why we joined forces.
—Looking forward, what do Dimension Data andVol. 10 No. 9 Sep. 2012

NTT aspire to achieve through this tie-up?
Our relationship with NTT has so far given us
access to R&D expertise and communication services
that we previously did not have. I am convinced that
we will be able to develop even stronger relationships
with our clients against a background of NTT technologies as our role in this tie-up. Up to now, I havebeen focusing my efforts on three main areas: further
expansion of cloud services, mobility, and playing arole in steering Dimension Data in the right direction.
On looking at recent world trends, I feel that we, as
an enlarged NTT Group, are well positioned to be
successful in the changing business world. I believethat we can learn a lot from past experiences, and I
would most certainly want to draw on these and do
my best to understand the present––and provide somelevel of guidance for the future, within Dimension
Data and the NTT Group.

Interviewee profile
Career highlights
Ettienne Reinecke received the BSc. degree inengineering from Stellenbosch University and theBSc. (Hons) and M.E. (Masters) degrees in engineering from the University of Pretoria, South
African Republic. He joined Dimension Data in
1991 and was appointed Chief Technology
Officer in 1998. He heads the Group’s new technology and research areas and provides leadership for its solution and services strategies. He is
also Dimension Data’s executive responsible forjoint development initiatives with the NTT
Group. In 1996, he was Managing Director of
Omnilink (a startup company and Dimension
Data subsidiary), which the Group merged with
Internet Solutions in 1999. In the late 1990s, he
was closely involved with the Group’s globalization efforts. From 1998 to 2002, he sat on the
Board of Dimension Data Holdings before joining the Group’s Executive Committee. In 2011,
he helped establish Dimension Data’s CloudBusiness Unit. For several years, he has played
the dual roles of Chief Technology Officer andGroup Executive for Network Integration.

Front-line Researchers

Exploring the Mechanism of Human
Perception: A Challenge Befitting NTT
Shin’ya Nishida
Senior Distinguished Scientist
NTT Communication Science
Laboratories
Shin’ya Nishida, an NTT Senior Distinguished Scientist, hasused detailed psychophysical methods to clarify that humanvisual perception is achieved through the mutual interaction ofprocessing modules in the brain and the integration of information
about an object’s movement, shape, color, and other attributes. We
asked him about the implication of research, and, with the hopethat he will continue to be involved in international activities,
about his future objectives.

Explaining vision through psychophysics—Dr. Nishida, please tell us about your main research
theme.
I research the mechanism behind the way that people look at things. When you think about vision, what
first comes to mind are words like eye and lens. Theeye plays the role of a camera (input device) whileneural circuits in the retina and brain do the processing. This scheme can extract useful information fromimages captured by the retina. The visual system
extending from the eye to the brain consists of a component that plays the role of a camera and a component that acts much like a processor in a digital camera. Thus, in simple terms, my research aims tounderstand these two functions and the underlying
mechanism, which I probe using psychophysicalmethods.
Psychophysics research has a long history. The
question of how the human brain processes and interprets visual information captured by the retina has
been examined by quantitatively analyzing judg

ments made by subjects on the basis of human perception.
The method of showing something to people andasking them how it looks has been used for about ahundred years. At one time, this was done using
simple stimuli such as a picture drawn on a piece of
paper. Recently, however, it has become possible topresent images that had previously been difficult toprepare—such as artificial materials created withcomputer graphics—and to elicit responses about thematerial quality, such as “It appears shiny.”
As a result of this research, it has gradually become
clear that human visual perception is achieved
through close coordination among cortical modules
that process different kinds of information such as an
object’s movement, shape, and color.
—It seems that your research can be described as an
attempt to determine and understand what people
feel when they see something.
This might sound a bit vague, but you can call it
research that investigates how we see an object itself
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Front-line Researchers

and its shape, texture, movement, etc. (Figs. 1 and2).
Let me compare human vision with machine vision.
We would like machines to be capable of recognizing
complex objects such as human faces. We humanobservers can immediately recognize our friends simply by observing their faces. However, trying to get amachine to recognize people in this way is difficult.
Up to now, there has been no popular alternative toinconvenient methods such as passwords for identifying people.
For a human being, recognizing who you are currently talking to is an everyday occurrence, but asking
a machine to do the same thing is far from easy.
Moreover, when people look at something, they cansense it to be glittering, feel it to be beautiful, or judge
it to have an interesting shape, for example. Needlessto say, it is difficult to have a machine judge an object
in the same way.
Thus, in a human being, there are a number ofvisual processing stages from sensing an input withthe eye to having an emotional response to the input
in the brain. If we don’t understand the mechanismof, say, how we judge the quality and material ofsomething that we are looking at, such as a metal orplastic object, we will not be able to understand how
any subsequent emotion arises from them. Strictly
speaking, we do not even know whether an emotional
response is due to a conscious process of judging acharacteristic like material or whether it is madethrough a separate, unconscious process. To fully
understand the mechanism of human perception, we
need to answer profound questions such as what is therole of conscious awareness.

Time

If the character “B” is viewed through fine slits,
it is unreadable if it is stationary but readable if it is moving.

Fig. 1. Mutual interaction between movement and shape.

—How would you go about explaining that function?
I would, for example, have a subject look at amotion picture and ask him or her to make judgmentsthat would help me determine how human beings
process motion-related information. That is, insteadof measuring brain activity by some device, I would
want to find out what the subject is seeing—I would
listen to subjective information that only that person
could access. Although devising what to present to a
subject is, in a sense, a classic technique, we still
make use of it in our cutting-edge research.
There has been much research recently on brainmachine interfaces, as in methods of estimating what
is going on in the mind by capturing images of thebrain through techniques such as magnetic resonanceVol. 10 No. 9 Sep. 2012

With highlights, the object seems to have a glossy
appearance (upper image), but a simple manipulationof colors can eliminate the highlights and thereby
decrease this sense of gloss (lower image).

Fig. 2. Example of texture research.

imaging (MRI). However, it is still at an elementarystage of research—rather than reading the mind, it
has just begun to be possible to infer certain judgments in a piecemeal manner.
On the other hand, asking a subject directly, while
a classic technique, provides a much richer
Front-line Researchers

understanding of that person’s state of mind. It is noteasy to understand a person’s mind state from only
brain activity. Asking what a person senses and feels
is currently the best way to access that person’s stateof mind or emotional state, and I don’t believe that
this will change in the future.
Up to now, it has been believed that different typesof information such as that about movement, position, shape, and color have been processed separately
within the brain. For example, it has been thought that
information received about movement has not beenused in recognizing the shape or color of that moving
object. In accordance with this view, the brain region
responsible for motion-related processing is different
from the region responsible for processing shape orsome other characteristic. However, I have uncovered
a number of pieces of evidence for closely coordinated processing among these regions through mutually linked pathways, and I have shown that there are
misunderstandings in the conventional view of thisprocessing.
It all began by asking: “Why can humanbeings do things that machines cannot?”—Dr. Nishida, how did you become involved in thisresearch?
To begin with, I was a very argumentative student,
and I thought that the act of seeing was somewhat
mysterious. In high school, I was fascinated with theidea that the way in which something right in front of
my eyes appeared to exist was simply the way inwhich my brain recognized it.
Planning for college, I thought that I would want toenter a field of study focusing on humans, and thoughinitially unsure whether to pursue philosophy or psychology, I eventually entered the Faculty of Lettersand majored in psychology. I also aspired to psychophysics with which I could objectively analyze somewhat vague and subjective areas such as mind andspirit by a scientific approach.
I found it interesting that cortical mechanisms
could be clarified by simply analyzing what we perceive when looking at, for example, an optical illusion. Moreover, when I moved on to graduate school,
I encountered a number of attractive research topics
and somehow my path came to be set. I believe it all
started with my desire to understand why humanbeings have capabilities that machines do not.
—In this research field, when do you feel that youhave achieved something and obtained a good
response?
In research fields such as optical fibers and quantum computers, it should be easy to define results thatare clearly recognizable and there should be many
opportunities to receive concrete feedback. But inresearch targeting human beings, how to go about
understanding the obtained results is a problem in
itself, which makes it difficult to reach conclusions.
But it is exactly for this reason that one can become
addicted to such research and find it hard to extract
oneself. It’s like taking up the challenge of playing adifficult game that never ends. There is consequently
still a question as to what standpoint this researchshould be conducted from, and I continue to reflect
on this, though it makes me feel like a perpetual student.
However, science, being what it is, is a win-lose
proposition—it’s important to do good work, that is,compelling work, that can win the admiration ofpeople. As I described earlier, research on material
perception has expanded in recent years, and I believethat our results have contributed to this expansion. At
the same time, I have been working in this field formany years with many questions still unanswered,and this can be depressing and it sometimes prompts
me to question myself.
With regard to material perception, a pioneering
paper that I submitted in 1998 helped me to recognize
the value of my work, and from around 2000 on, my
research began to draw attention from those around
me. It was also reassuring to receive feedback indicating that my work had played a part in the growthof this research field.
I have also been recognized as a specialist in fields
other than material perception such as visual motion
and time perception, and I feel that I have been able
to have an impact on those fields (Fig. 3). But as there
are many things that are still not understood, I would
like to make more original proposals in the future,and to this end, I plan to put my nose to the grindstone
as a researcher with a worldview.
I worry every day about not obtaining convincing
answers in my research, but this worry seems to dissipate when I hold discussions with young researchers in NTT. It is also fortunate that NTT does notdemand short-term output. My aim is to exercise myideas freely in order to come up with something novelthat represents a leap from the present, and I can dothis because I am not expected to set detailed targets.
NTT Technical Review

Front-line Researchers

If a lag between light and sound continues to be experienced, the subject
will gradually stop feeling it. This illusion shows that the simultaneity of thevisual and auditory senses is automatically adjusted.

Fig. 3. Example of research on judging time.

—What with obtaining no answers and forever seeking answers, your work is really an extraordinary
endeavor. How do you keep motivated?
My research to date has not been very goal-oriented. Of course, there are various ideas about how myindividually obtained results might be applied. My
approach, however, is to feel my way along and break
down barriers a little at a time in my quest for a breakthrough (imagine the videogame Breakout). Whatbarrier to choose I leave to intuition. But once I
began, I want to break it down completely even if itmeans doing it one piece at a time.
I am also conscious of dropping an idea that I havebecome overly attached to and looking at the problem
from a different viewpoint. Nevertheless, it’s not thatI’m doing anything special to unleash new ideas. Callme obstinate or non-conformist, but this is just theway that I enjoy thinking.
I could not have reached where I am today without
the many things that I learned from overseas pioneering researchers in their journal papers. I have recently
had opportunities to talk with some of them at academic conferences, and they really enjoyed hearing this.
Perhaps they felt that I was continuing in their footsteps and carrying on their ideas. They made comments about their expectations of me and about ourcommon approaches, which I was very glad to hear.
Being a little different from others is fine.
Take joy in different ways of thinking!
—What advice would you give to young researchers?
Vol. 10 No. 9 Sep. 2012

Each and every researcher holds a completely different worldview, so there will always be some misunderstandings between them. Still, enjoying suchdifferences can be a meaningful thing.
Although it is a characteristic of Japanese to try toreach mutual understanding with others, I believe that
researchers are different—they are naturally argumentative. In research, it’s important to disagree. I
would like young researchers to enjoy the fact thatpeople can have different viewpoints and ways ofthinking.
I also believe that it is not good to overly study
other people’s work or listen excessively to what
other people say. You will become blind to your differences from other people unless you can listen
while having your own hypotheses and way of thinking. Isn’t it true that each person is born with something special? That, I feel, is the key to understanding
researchers. Of course, there are some occupations
that call for an ability to gather up and disseminate
information correctly, but in the case of researchers, I
think that they must first be able to develop a different
perspective from other people instead of trying toachieve a balance with those around them. There’s noneed to be a good boy or good girl. I never thoughtthat being different from other people was a badthing; indeed, I purposely set out to be a little different.
Researchers are often asked to write about andexplain their research in an easy-to-understand manner. However, for researchers who have unique perspectives on things, explaining their findings in a waythat anyone can understand on the basis of common
sense or accepted wisdom would appear to beFront-line Researchersdifficult. In today’s society, though, that excuse is
becoming more difficult to fall back on. The situation
is such that people without good social and communication skills may fail to win approval or praise.
Perhaps researchers will now be asked to have goodwriting skills and the social awareness of screenwriters and movie directors! I truly feel that this is necessary for people who specialize in certain fields likemyself in brain science. I would therefore like young
researchers to cultivate a sense of balance so that theirwritings will attract the interest of the general reader
while still being grounded firmly in science.
—Dr. Nishida, what objectives have you set for yourself looking forward?
I am still looking for answers, so it would be great
if I could put myself again in a situation in which I
could devote myself to round-the-clock research. I
want to go forward, but not in any particular direction. I want to open up new areas that have yet to be
fully explored, and I want to expand my endeavors
freely without being confined to a particular format. I
believe that new things are born when one engages
and clashes with new people. I want to continuously
challenge myself by examining my current situation
and asking myself whether that’s good enough.



Shin’ya Nishida
Senior Distinguished Scientist, Sensory Repre­
sentation Research Group, Human Information
Science Laboratory, NTT Communication
Science Laboratories.
He received the B.S., M.S., and Ph.D. degrees
in psychology from Kyoto University in 1985,
1987, and 1996, respectively. After a two-year
stay at ATR Auditory and Visual Perception
Laboratories in Kyoto as a research associate, hejoined NTT Basic Research Laboratories inTokyo (now Communication Science Laboratories) in 1992. He is a member of the Vision
Society of Japan, the Japanese Psychological
Association, the Japanese Psychonomic Society,and of Vision Sciences Society. He serves as an
editorial board member of the Journal of Visionand the Vision Research, as a visiting professor at
the Center for Multidisciplinary Brain Research,
National Institute for Physiological Sciences, andas a member (RENKEI KAIIN) of the Science
Council of Japan. He received the Second JSPS
(Japan Society for the Promotion of Science)
prize and the JPA (Japanese Psychological
Association) International Prize in 2006.

NTT Technical Review

Feature Articles: Quantum Computing

Research Frontier of
Quantum Computers
Hiroshi Yamaguchi
Abstract

This article introduces quantum computing, the topic of the Feature Articles in this issue. More thanten years have already passed since the study of devices for quantum computers began. During this time,
the technology for manipulating the quantum state of the basic element, the qubit, has greatly improved,
but fundamental problems have also been revealed. The Feature Articles in this issue review new attempts
underway in our laboratories to overcome such problems.

1. Development and applicationof quantum physics
The quantum computer has been attracting attention because of its potential ability to perform
extremely high-speed signal processing. More thanten years have passed since the start of competitive
studies of hardware for quantum computing. The
basic element for configuring a quantum computer iscalled a quantum bit, or qubit. Research towardachieving an ideal qubit has been actively pursued
using systems having various physical units.
The quantum computer operates according to theprinciples of quantum mechanics, a fundamental
theory of physics. Quantum mechanics describes
behavior in the natural world very differently from
Newton’s classical mechanics, although classicalmechanics is the source of our common sense. For
example, the statements “mechanical systems all
have the aspects of both waves and particles” and“dynamical systems can all be present at the sametime in multiple locations” arise as a consequence ofquantum mechanics. These ideas indicate that aphysical system has inherent multiplicity, referred toas quantum superposition, and this multiplicity ofphysical systems is the key mechanism for massively
parallel computation in a quantum computer.
Accepting these ideas also raises questions of aphilosophical nature: Are our bodies also like waves
and can they exist simultaneously in two places? Howabout the human mind? Can it also exist in two places
at the same time? And if so, then if it dies in one
place, does it also die in the other place? Since thebasic theoretical framework of quantum mechanics
was established nearly a century ago in the 1920s,
there have been ongoing discussions about questions
such as these. After serious debate over the course ofmany years, physicists have come to believe that thetheory is infallible. To date, they have found that all
experimental observations are successfully explainedby quantum mechanics.
From the viewpoint of engineering applications ofquantum mechanical phenomena, quantum mechanics has led to the creation of innovative technologies
such as semiconductor devices and lasers. For
instance, the band structure in a semiconductor could
not exist without quantum-mechanical effects, andlasers would not work without quantum mechanicalinteraction between light and matter. However, technology that uses an essential aspect of quantum
mechanics, such as the idea that an electron can exist
in two places at the same time, has been explored
only very recently in the form of quantum information technology, such as quantum computers andquantum cryptography.
We have been studying microfabrication and nanofabrication technologies for the development of electrical and optical semiconductor devices. As a new
application, the research of semiconductor and superconductor devices, where the quantum mechanicalnature appears, was initiated more than ten years ago(Fig. 1). Now, it is even becoming possible to control
the quantum mechanical multiplicity in these structures. Major technological advances using the essence
NTT Technical Review

Feature Articles

Manipulation of quantum states and application for quantum computation
Discovery of quantum
mechanics(early 20th century)

Technological development of
quantum manipulations(End of 20th century up to the present)

Creation of newscience andtechnology

NTT’s R&D activities

Quantum mechanics

Experimental tests offundamental principles

Artificial atoms and molecules(basic elements of qubits)

Revolution in
fundamental physics
together with Einstein’s
relativity

Proving multiplicity on
device platform

Superconductor qubit

Clarifying the multiplicityof microscopic systems

0.5 µmTest of quantum
mechanical observations
Innovative developments
in computer science

Quantum
computation

Semiconductor qubit
l1

Atomic structures

VD1
1st qubitl2

Spin qubit
1 µm

Utilizing quantum
mechanical multiplicity

VD2
2nd qubit

200 nmSchrödinger’s cat

High-speed calculations
Highly efficient searchenginesFig. 1. From the discovery of quantum mechanics to its application.

of quantum mechanics promise ultrafast and highly
energy efficient database searches and calculations.
They can be used in applications such as the discovery and design of new materials and pharmaceuticals.
2. Challenges of quantum computer research
As a result of various attempts over the past ten
years, a technique for precisely controlling the superposition of qubit states “0” and “1” has progressedsignificantly in individual studies. Simultaneously,
serious problems have been encountered as we proceed toward the development of more realistic quantum computers. One of the biggest problems is that it
is not easy to extend the present scheme to multibit
systems. For a classical computer, once you havecompleted the development of one element (such as
an OR gate or AND gate), joining elements to construct a multibit system is not difficult. The basic
elements linked by wiring perform the combination
Vol. 10 No. 9 Sep. 2012of their operations and there is no essential problem
in multibit operation. On the other hand, qubits facean essential problem concerning multibit operations.
That is, the quantum superposition cannot be maintained for a long enough time to perform multibit
operation. There is a limit to the amount of time forcoherent operation. With the current level of technology, it is not too difficult to manipulate the quantum
state once, but when we try to run it multiple times,
the quantum state is easily broken and the operation
cannot continue.
This problem had been regarded as important froman early stage and research has been seeking to find aqubit that maintains the quantum superposition state
for a longer time. Various different physical systemsof qubits have been devised and each has been foundto have both advantages and disadvantages. One system allows very fast computation but the quantum
state immediately becomes corrupted; another is notgood at bit operation but the quantum state can be
maintained for a long time. Moreover, some systems
Feature Articles

are well suited to the combination of multiqubit
gates, while others are not, although they have excellent performance in single-qubit operation. Therefore, constructing a hybrid system consisting of different types of physical systems with different advantages is regarded as the right direction, rather thanproceeding to force the construction of the best system with qubits of only a single type.
The Feature Articles in this issue introduce NTT’s
recent research related to a variety of different typesof qubits. These studies are at different stages ofresearch, ranging from confirming operation as aqubit to just exploring basic physical properties.
Below, I briefly describe the characteristics of eachquantum system.
3. Various physical systems studied for
configuring hybrid qubit systems
3.1 Superconducting circuits
Superconductivity is a phenomenon in which theelectrical resistance of materials vanishes at low temperature. Current flowing through a circuit loop madeof superconductors never stops because the material
has zero resistance. This current is very stable, andwe are attempting to use it to represent a quantum
state. The superconducting qubit is one of the mostsuccessful and advanced systems among solid-state
quantum devices.
The basic structure of the superconducting qubit isshown in Fig. 2. The parts for disturbing the superconducting current flow, called Josephson junctions,are formed on a looped circuit consisting of superconducting aluminium. These junctions mix the currents flowing clockwise and anticlockwise to achievea quantum superposition state, where the current
simultaneously circulates in two opposite directions.
The superconducting circuit offers very good quantum superposition control, but trying to configure
multibit operations only with superconducting qubits
has been a major challenge. Instead, we have recently
been attempting to integrate the circuit with quantum
memory implemented in other quantum systems.
More specifically, we are working to study the quantum spin states used as the memory with defect complexes called NV– centers in diamond. In addition, amajor issue is how to read out the qubit state without
destroying it because the signal induced by the current is very weak. This set of Feature Articles introduces studies addressing these two challenges.

Josephson junctions:
Aluminium & aluminium oxide
3 µm

“0”

“1”

(superconductivity: T < 1.2 K)
The current simultaneously flows
in opposite directions.

Fig. 2. Superconducting qubit.

3.2 Electron spin
Spin is a quantum mechanical property of elementary particles such as electrons and protons (Fig. 3).
It is a kind of angular momentum that the particle
possesses in itself, so spin is often regarded as rotation about an axis. Quantum theory combined with
Einstein’s theory of relativity requires an electron toeternally exhibit angular momentum, i.e., the rotation
is sustained forever. This is similar to a superconducting current and can represent a quantum state depending on whether the clockwise or anti-clockwise direction of rotation is sustained.
Electron spin is used in several applications, themost well-known of which is magnets. The propertiesof a magnet could not be generated without electron
spins. We can intuitively understand this by considering electron rotation to induce a kind of rotating current that generates a magnetic field like a coil orsolenoid.
The possibility of obtaining long-lived quantum
states for spin qubits has been pointed out. For example, the spin state in an NV– center in diamond isreported to be maintained for a time on the order ofmilliseconds. This is longer by two or three orders ofmagnitude than that in solid-state qubit systems, so
its use for quantum memory is highly promising. An
article in this issue describes a study on controllingelectron spins by applying a voltage to semiconductor
devices and one that covers quantum memory using
NV– centers.

NTT Technical Review

Feature Articles

Anti-clockwise→ |0>

Clockwise→ |1>• Eternal rotation (relativistic effect)• Fixed angular momentum• Clockwise and anti-clockwise rotations
correspond to “1” and “0” states, respectively.

Superposition statea|0> + b|1>

Fig. 3. Schematic explanation of electron spin.

Laser light
Lattice spacing: d=λ /2 (λ: laser wavelength)~several hundred
nanometersAtomtr lighLasetr lighLase

Laser light
Periodic traps are formed by interference among laser lights injectedfrom four different directions.

Fig. 4 Schematic illustration of optical lattice.

3.3 Cold atoms
Until recently, cold atoms had been considered to
be unsuitable for developing solid-state quantum
computers because they are not available in solid
form, although they can sustain quantum states for avery long time. However, in recent years, they havebecome strong candidates for solid-state qubits following successes in integrating cooled atoms on apiece of solid material, called an atom chip.
The atom is one of the physical systems where
quantum phenomena were confirmed at a very earlystage in the history of quantum mechanics. This isbecause atoms in gas are isolated from each other,which can maintain the quantum mechanical nature.
Vol. 10 No. 9 Sep. 2012

However, if you try to control the quantum state, theatoms move too rapidly to detect and control the individual quantum states.
The solution to this problem is to cool the atoms.
The rapid motion is caused by thermal energy, solowering the temperature can confine the atoms in alimited space. In practice, it is not easy to cool atoms
down to temperatures where atomic motion is sufficiently suppressed, but a technique called laser cooling is promising for this purpose. This issue introduces a method for applying laser cooling in an
attempt to construct a regular lattice of cooled atoms(Fig. 4).

Feature Articles

4. Diversity and future of quantum technologies
Einstein’s famous theory of general relativity, proposed in 1915, provided many extraordinary predictions, such as that time passes at different speeds onthe ground and in space, and it completely changed
our world view. Although the theory has had a great
impact on the scientific community, people believed
until recently that it would not be of practical use intechnologies relevant to our daily life because its
influence is extremely small. However, the global
positioning system (GPS) that we currently use
would be inaccurate if its design did not take into
account the theory of general relativity. Advances intechnology are surprisingly fast, and technologies
that were initially thought to be impossible spreadvery quickly once they have been developed.
As mentioned already, nearly a century has alreadypassed since the discovery of quantum mechanics but

the study of how to use its essence in practical technology has begun only very recently. In that sense,
the study of the quantum computer is still just beginning and it will certainly take a long time to come tofruition. Research scientists love to try to achieve
major breakthroughs, and the research activities presented in the Feature Articles in this issue are examples of such attempts. By combining the techniques
covered here, we may be able to create quantum computers with excellent performance in the near future.
Alternatively, some researchers expect big developments in applications of quantum phenomena completely different from the quantum computer. Quantum mechanics is very different from classicalmechanics, which governs the basic principles of current technology, and it has such a wide diversity thatthe quantum computer may well be only one example
of its applications.

Hiroshi Yamaguchi

Senior Distinguished Researcher and Group
Leader of the Nanostructure Technology
Research Group, NTT Basic Research Laboratories.
He received the B.E. and M.S. degrees in physics and the Ph.D. degree in engineering from
Osaka University in 1984, 1986, and 1993,
respectively. He joined NTT Basic ResearchLaboratories in 1986. His current interests aremicro/nanomechanical devices using semiconductor heterostructures. He has been a guest
professor at Tohoku University since 2006. He isa fellow of the Institute of Physics and a memberof the Japan Society of Applied Physics, thePhysical Society of Japan, and IEEE.

NTT Technical Review

Feature Articles: Quantum Computing

Coherent Coupling between a
Superconducting Qubit anda Spin Ensemble
Shiro Saito, Xiaobo Zhu, William John Munro,
and Kouichi Semba
Abstract

This article reviews an experiment that demonstrated the principle of quantum memory operation in asuperconductor-diamond hybrid system. Superconducting qubits have excellent controllability and scalability but short memory (coherence) time because they are artificial structures; on the other hand, electron spins in diamond are natural structures and have the opposite characteristics: they show extremely
long memory times (seconds or greater) but are hard to control and manipulate. A superconductor-diamond hybrid system could offer the best properties of these systems without the associated disadvantages.

1. Introduction
The coherent control of quantum two-level systems, which are applicable to quantum bits (qubits),
has attracted significant interest in the context ofquantum computing and quantum information processing. Among the various candidates for quantum
bits, superconducting qubits based on Josephson
junctions have gained in importance because of theircontrollability and scalability. Their coherence time
has reached a few tens of microseconds [1], compared with several nanoseconds at first in 1999 [2]. A
few qubit gates have been demonstrated in superconducting circuits [3]. To execute larger and more useful quantum algorithms in the future though, it will benecessary to achieve a long coherence time. However,
we are unsure whether this can be achieved solely
with superconducting circuits.
The key components of a conventional computer
are the central processing unit (CPU) and the random
access memory (RAM) (Fig. 1). Each component
plays its own unique role. The CPU processes theinformation while the RAM stores it. On the other
hand, superconducting qubits have played the roles ofboth a processor and memory. In view of the above

mentioned issues, NTT has aimed to create a hybridof superconducting qubits and another suitable system that can store quantum information for longer
times [4] (Fig. 2).
2. NV- centers in diamond
Nitrogen-vacancy (NV-) centers in diamond haveattracted significant attention as a potential candidate
for a quantum memory for superconducting qubits.
The NV- center is a complex defect in a diamond
crystal, formed of (1) a nitrogen (N) atom substituting for a carbon atom and (2) an adjacent vacancy (V)(Fig. 3(a)). As shown in Fig. 3(b), its electron state isspin 1 with a zero field splitting of 2.88 GHz between
the ms = 0 and ms = ±1 states at zero magnetic field,
where ms is the magnetic quantum number. The electron spin is known for its long room-temperature
coherence time of 2 ms [5], so it seems to be suitable
for a quantum memory. We can perform a memory
operation under low magnetic fields because the zero
field splitting of 2.88 GHz is compatible with thetransition frequency of superconducting qubits. Furthermore, there is an optical transition in addition to
the microwave one, so we may be able to create a
NTT Technical Review

Feature Articles

Conventional computer
CPU: processor (semiconductor device)

RAM: memory (semiconductor device)

Quantum computer using superconductor
Quantum bit: processor + memory (superconductor device)

Fig. 1. Components of a computer.

Controllability, scalability
Superconducting qubits Good (various circuit designs)
NV- centers in diamond

Memory time (coherence time)
Poor (coupling to noise environment)

Poor (weak coupling between spins) Good (isolated from external noise)

Write

Quantum processor



5 µm

〉 –

ms = ±1〉〉

Tunable ∆


〉 +

[001]
z

2.88 GHzNvyms = 0〉〉x
Diamond NV- center(electron spin)

Superconducting flux
qubit (supercurrent)∆: energy gap

Quantum memory

Read

Fig. 2. Superconductor-diamond hybrid system.

quantum transducer––a microwave-to-optical frequency converter. This would be a key technology forquantum repeaters, quantum networks, and quantum
computers.
3. Gap-tunable superconducting flux qubit
A superconducting flux qubit consists of a superconducting aluminium loop with three Josephson
junctions. One of them is α ~ 0.8 times smaller thanthe other two, which makes its Josephson energy proportionally smaller, i.e., equal to α × EJ (see Fig. 4(a)).
Vol. 10 No. 9 Sep. 2012

The system works as a quantum two-level system,that is, a qubit, when we apply an external magnetic
field Φex ~ (m+1/2) Φ0 through the qubit loop (here,
m is an integer and Φ0 is the magnetic flux quantum).
Figure 4(a) shows the lowest two energy levels of thesystem when Φex ~ 1.5 Φ0. The clockwise circulating
current state |R> and the anticlockwise one |L> arethe energy eigenstates when Φex is far away from 1.5
Φ0. These states are composed of persistent currents
Ip ~ 300 nA. The equal superposition states between|L> and |R> are realized at the degeneracy point
where Φex = 1.5 Φ0 because of tunneling due to theFeature Articles

[001]
Z

E

3

PL: 638 nm
Optical wavelength(quantum communication)NV

ms = ± 1y2.88 GHzA

3

Microwave(compatible withsuperconducting qubit)

ms = 0X(a) Structure(b) Energy levels
PL: photoluminescenceFig. 3. NV- center in diamond.

3 µm

5 µm

Qubit

R〉

EJ

L〉

α EJ

EJ

Φex
EJ

Josephson
junction

EJ
Optimal point

R〉

Energy levels

Energy levels∆: fixedL〉+R〉
L〉

Φqb

α
2 EJ

Φα

α
2 EJ

L〉

L〉–R〉

1.496

Qubit

∆ is tunable
by Φα

∆

R〉
1.498

1.500

1.502

1.504

1.496

1.498

1.500

1.502

1.504

Flux through the qubit Φex (Φ0)

Flux through the qubit Φex = Φqb + Φα /2(Φ0)(a) Superconducting flux qubit(b) Gap-tunable superconducting flux qubit

Fig. 4. Superconducting flux qubits and their energy levels.

charging energy of the junctions. The tunnelingcauses energy anticrossing between |L> and |R>, so
the transition energy between the ground state and theexcited state of the qubit is a minimum at this degeneracy point, which is called the optimal point because

it is where the qubit is immune to external flux noiseand exhibits its longest coherence time.
We can control the energy gap ∆ of the standard
flux qubit at this optimal point by an appropriate
design of the junction area ratio α; however, we
NTT Technical Review

Feature Articles

1 mm
Diamond crystal

Gap-tunablesuperconducting
flux qubit

Sapphire substrate

Fig. 5. Superconductor-diamond hybrid system.

cannot change it after sample fabrication. For our
purposes, we need a gap-tunable superconducting
flux qubit (Fig. 4(b)) in order to tune our qubit inresonance with other quantum systems, for example,
an NV- spin ensemble, while keeping it at the optimal
point. We therefore need to replace the smallest junction (α junction) by a dc-superconducting quantum
interference device (SQUID), whose EJ can be controlled by the external flux through it. The energy gap∆ can then be tuned in-situ by several gigahertz
within a few nanoseconds [6]. The fact that a typicalgap is between 1 GHz and 10 GHz thus indicatesexcellent compatibility with an NV- spin ensemble indiamond.
4. Superconductor-diamondhybrid quantum system
The coupling strength between two physical systems is the key factor necessary for quantum memory
operation in hybrid systems because the time taken totransfer information between the systems is inversely
proportional to the coupling strength, so faster operation within its coherence time leads to high-fidelity
operation. In the case of the coupling between a flux
qubit and the NV- spin ensemble, reducing the spatial
distance between them increases the couplingstrength because the spin ensemble couples to thequbit through the magnetic field induced by thequbit’s persistent current Ip. Moreover, increasing thedensity of the NV- spin leads to a stronger coupling
because the coupling is enhanced by a factor N√owing to a collective effect when the qubit couples to
N electron spins. Therefore, it is important to preparea diamond crystal that contains a high density of NVcenters.
Vol. 10 No. 9 Sep. 2012

To achieve this, we injected carbon ions into a highpressure high-temperature (HPHT) synthesized diamond crystal and annealed it in vacuum. We estimated the density of NV- centers to be N ~ 1.1×
1018 cm-3 by comparing the photoluminescence spectroscopy from a single center with that of our ensemble.
We fabricated our superconducting flux qubit on asapphire substrate by using electron beam lithography and a shadow evaporation technique. The diamond crystal was then glued on top of it, achieving aspatial separation of less than 1 µm. The hybrid
sample after mounting of the diamond crystal isshown in Fig. 5. We estimated the coupling gi between
a single NV- spin and the flux qubit to be approximately 8.8 kHz from the geometrical configuration.
This gave the estimated collective coupling gens(estimated)
as about 50 MHz for the NV- center density.
5. Spectroscopy
We performed microwave spectroscopy measurements at 12 mK using a dilution refrigerator. The
spectrum of the flux qubit with the diamond crystal
shows a clear energy anticrossing at 2.88 GHz(Fig. 6(a)) corresponding to the transition frequency
of NV- spins. On the other hand, the spectrum taken
before the diamond was mounted (Fig. 6(b)) does notshow an anticrossing. From the above results, we canconclude that the splitting at 2.88 GHz comes fromthe strong coupling gens between the flux qubit andthe spin ensemble. The coupling gens of 70 MHz is ingood agreement with the coupling gens(estimated) estimated using the NV- center density.

Feature Articles(b) Without a diamond crystal

3.3

0.16
0.14
Switching probability

Frequency (GHz)

3.2
3.1
3
2.9

0.12
0.1
0.08
0.06
0.04

2.8

Vacuum Rabi
splitting of 70 MHz

2.7
−1

−0.5

(a) With a diamond crystal

00.5
Φex −1.5Φ0 (mΦ0)

1

0.02
0

Fig. 6. Spectra of a superconducting flux qubit.

(a) Excitation of a qubit(b) Shift to the resonance

1〉qb0〉ens
0〉qb1〉ens

0〉qb0〉ens(c) Oscillation(e) Readout andinitialization(d) Shift to the readout point

Fig. 7. Measurement sequence for observing vacuum Rabi oscillation.

6. Time domain measurement
We performed time domain measurements according to the sequence depicted in Fig. 7 to demonstratememory operations, that is writing to the spin ensemble and reading from it. First, we excited only the flux
qubit by applying a microwave π-pulse to it when thequbit and the spin ensemble were well detuned (Fig.
7(a)). The qubit was quickly tuned by Φex into reso

nance with the spin ensemble (Fig. 7(b)). This tuning(much faster than the coupling strength gens) causes a
non-adiabatic transition, and the flux qubit can thenstart coherently exchanging a single quantum ofenergy with the spin ensemble as follows (Fig. 7(c)):|1〉qb|0〉ens ⇔ |0〉qb|1〉ens ,where qb and ens indicate the flux qubit and the spin
ensemble, respectively. The 0 (1) represents theground (excited) state of each part of the hybrid
NTT Technical Review

Feature Articles

0.25Superconducting
qubit

Switching probability

0.2

0.15

Superconducting
qubit

0

0.05

0

0

10

20

30

40

50

60

Time (ns)SpinensembleSpinensembleFig. 8. Vacuum Rabi oscillations between a flux qubit and an NV- spin ensemble.

system. The resulting oscillation with frequency gensis called a vacuum Rabi oscillation. Finally, to finish
the memory operation, we detuned the qubit nonadiabatically away from the spin ensemble (Fig. 7(d))
and read out the qubit state with a dc-SQUID (Fig.
7(e)). The vacuum Rabi oscillations that we observed
are shown in Fig. 8. The blue (red) curve represents
experimental data (simulation). These results indicatethat quantum information in the flux qubit was transferred to and then retrieved from the spin ensemble.
7. Concluding remarks
The vacuum Rabi oscillations shown in Fig. 8 continued for only 40 ns, which is much shorter than the
decay times of the flux qubit (T2,qb ~ 150 ns) and theNV- spin ensemble (T1,NV >> 10 µs). This indicates an
unexpectedly short coherence time for the spin
ensemble. There are several likely sources of thisdephasing. The most probable is a large electron spin
– ½ bath derived from the 100 ppm (parts per million)
P1 centers (a nitrogen atom substituting for a carbon
atom) present in our diamond crystal. The degeneracyof the ms = ±1 states could also be a source of decoherence. Hence, the future challenges are to apply an
in-plane magnetic field to remove this degeneracy
and to prepare a diamond sample with fewer P1 cenVol. 10 No. 9 Sep. 2012ters.
In summary, we have experimentally demonstrated
strong coherent coupling between a flux qubit and aspin ensemble in a diamond crystal. Furthermore, wehave observed the coherent exchange (transfer) of asingle quantum of energy between them. This is thefirst step towards a long-lived quantum memory for a
superconducting qubit. It could also lead to an interface between the microwave and optical domains.
References
[1]

[2]
[3]

[4]

[5]

H. Paik, D. I. Schuster, L. S. Bishop, G. Kirchmair, G. Catelani, A. P.
Sears, B. R. Johnson, M. J. Reagor, L. Frunzio, L. Glazman, S. M.
Girvin, M. H. Devoret, and R. J. Schoelkopf, “Observation of High
Coherence in Josephson Junction Qubits Measured in a Three-Dimensional Circuit QED Architecture,” Phys. Rev. Lett., Vol. 107, No. 24,240501, 2011.
Y. Nakamura, Y. A. Pashkin, and J. S. Tsai, “Coherent Control of
Macroscopic Quantum States in a Single-Cooper-pair box,” Nature,
Vol. 398, No. 6730, pp. 786–788, 1999.
M. D. Reed, L. DiCarlo, S. E. Nigg, L. Sun, L. Frunzio, S. M. Girvin,
and R. J. Schoelkopf, “Realization of Three-qubit Quantum Error
Correction with Superconducting Circuits,” Nature, Vol. 482, No.
7385, pp. 382–385, 2012.
X. Zhu, S. Saito, A. Kemp, K. Kakuyanagi, S. Karimoto, H. Nakano,W. J. Munro, Y. Tokura, M. S. Everitt, K. Nemoto, M. Kasu, N.
Mizuochi, and K. Semba, “Coherent Coupling of a Superconducting
Flux Qubit to an Electron Spin Ensemble in Diamond,” Nature, Vol.
478, No. 7368, pp. 221–224, 2011.
G. Balasubramanian, P. Neumann, D. Twitchen, M. Markham, R.
Kolesov, N. Mizuochi, J. Isoya, J. Achard, J. Beck, J. Tissler, V.

Feature Articles

Jacques, P. R. Hemmer, F. Jelezko, and J. Wrachtrup, “Ultralong Spin
Coherence Time in Isotopically Engineered Diamond,” Nature Mater.,Vol. 8, No. 5, pp. 383–387, 2009.

Shiro Saito

Distinguished Researcher, Senior Research
Scientist, Superconducting Quantum Physics
Research Group, Physical Science Laboratory,
NTT Basic Research Laboratories.
He received the B.S., M.S., and Dr.Eng.
degrees in applied physics from the University ofTokyo in 1995, 1997, and 2000, respectively.
Since joining NTT Basic Research Laboratories
in 2000, he has been engaged in quantum information processing research using superconducting circuits. He was a guest researcher at Delft
University of Technology, the Netherlands, during 2005–2006. He received the Young Scientist
Award from the Japan Society of Applied Physics(JSAP) at the spring meeting in 2004. He is amember of JSAP and the Physical Society ofJapan.

Xiaobo Zhu

Senior Research Associate, Superconducting
Quantum Physics Research Group, Physical Science Laboratory, NTT Basic Research Laboratories.
He received the Ph.D. degree from the Instituteof Physics at the Chinese Academy of Sciences in
2003. During 2003–2008, he worked as an assistant/associate professor in the Institute of Physics
at the Chinese Academy of Sciences. He moved
to NTT Basic Research Laboratories in Japan in
2008. His research interests include superconducting qubits and circuits and their applicationin hybrid systems.

[6]

X. Zhu, A. Kemp, S. Saito, and K. Semba, “Coherent Operation of a
Gap-tunable Flux Qubit,” Appl. Phys. Lett., Vol. 97, No. 10, 102503,
2010.

William John Munro

Research Specialist Quantum Optical State
Control Research Group, Optical Science Laboratory, NTT Basic Research Laboratories.
He received the B.Sc. degree in chemistry andphysics in 1989, M.Sc. degree in physics in 1991,
and D.Phil. degree in quantum optics in 1994
from the University of Waikato, New Zealand.
He entered the computer industry in 1995, where
he worked on various projects. In July 1997, he
accepted an Australian Research Council Fellowship at the Department of Physics in the University of Queensland. During this fellowship, hedeveloped an interest in entanglement, methods
to characterize it, and its practical use in quantum
information processing. In 2000, he became asenior researcher in the Australian Special Centre
for Quantum Information Processing). In late2000, he joined HP Labs as a research scientist.
In early 2010, he moved to NTT Basic ResearchLaboratories as a research specialist, where he isworking on hybrid-system approaches to bothquantum computation and communication. He iscurrently a visiting professor at the National
Institute of Informatics in Japan, the Universityof Queensland in Australia, and Leeds University
in the UK.

Kouichi Semba

Research Professor, Research Support Coordinator, Global Research Center for Quantum
Information Science, National Institute of Informatics.
He received the B.S., M.S., and Dr.Eng.
degrees from the University of Tokyo in 1983,
1985, and 2002, respectively. He joined the NTT
Electrical Communications Laboratories in 1985
and moved to NTT Basic Research Laboratories
in 1987. From 2003 to 2012, including the period
when the research described in this article wasconducted, he was the leader of the superconducting quantum physics research group. He wasa visiting researcher in the Department of Nanoscience at Delft University of Technology, theNetherlands, during 2002–2003. He was a lecturer at Waseda University in 2005 and a guest
professor at Tokyo University of Science during
2008–2012; he has been a guest professor at the
National Institute of Informatics since 2010. His
achievements include being the first to characterize the electronic state of Penrose’s non-periodic
tiling as an ideal 2D-quasicrystal; the first to
reveal the relevant elemental composition of thehigh-Tc cuprate superconductor YBCO (Tc = 90K); the first to demonstrate entanglement in asolid device, namely vacuum Rabi oscillation ina macroscopic superconducting Josephson-qubit
LC-resonator system; and the first to demonstrate
the coherent coupling of a superconducting flux
qubit and NV spins in diamond.

NTT Technical Review

Feature Articles: Quantum Computing

Theoretical and Experimental Study of
Quantum Nondemolition Measurement
Using an Electric Circuit
Kosuke Kakuyanagi, Shiro Saito, Hayato Nakano,
and Kouichi Semba
Abstract

We are investigating, from both the theoretical and experimental standpoints, how to perform measurements on quantum systems by using an electric circuit that has quantum characteristics. A superconducting flux qubit, which is constructed with a superconducting loop circuit, has discrete energy, so it can be
regarded as an artificial atom.

1. Superposition state and its measurement
In the quantum world, we can achieve the superposition of several states simultaneously. It is possible
to carry out multiple calculations by using a superposition state; therefore, we expect to be able to use it tosolve certain types of problems that are difficult tosolve with a conventional calculator. An example of asuperposition state is shown in Fig. 1(a). A single
electron that passes through a double slit exhibits aninterference pattern although only one electron reaches the screen because what passes through slits 1 and
2 is a superposition state. If we install an electron
detector behind slit 2 to detect which slit the electron
passes through, the superposition state disappears
and the interference pattern also disappears because
the state is decided (Fig. 1(b)).
Similar to this phenomenon, a superposition state isstochastically reduced to a state called an eigenstate
by measurement (projection). If the electron is notabsorbed by the detector and retains its eigenstate
after measurement, this measurement is called aquantum nondemolition measurement. How do wedeal with a quantum state measurement that appears
strange? We use a superconducting flux qubit as an
artificial atom and a Josephson bifurcation amplifier(JBA) readout*1 system, which achieves quantum
nondemolition measurement and we study how toperform quantum system measurements both theoretically and experimentally. This study is related tothe basic problem of quantum mechanics, namely
quantum state measurement. Our goal is to achieve
more efficient quantum measurement in the future
through an understanding of the mechanism. From apractical point of view, it is important to approachquantum state measurement correctly. Because of theinteraction between a qubit and the environment, thequbit coherence, which contains quantum information, gradually decreases. We need quantum error
correction to recover this quantum information. We
prepare a logical qubit, including an ancillary qubit
and a control qubit state that depends on the ancillary
qubit’s measured state. In the error correction scheme,
our control of the quantum state depends on the resultof the ancillary qubit measurement. To make quantum algorithms such as error corrections, it is very
important to understand the mechanism of quantum
measurement.
2. Superconducting flux qubit
In a superconducting loop, the phase of a*1	 Josephson bifurcation readout method: A method for detecting
small displacements by using a bistable state caused by the nonlinearity of a Josephson junction.

NTT Technical Review

Feature Articles

1

e−

e−

2

1
2

Measurement(a) Single-electron interference(b) No interference: disappearance causedby measurement.

Fig. 1. Quantum superposition state and measurement.

Φ/Φ0=0.5γ−

LR(a) Superconducting flux qubit

62
60

50
γ+

SQUID

60

E/EC

Flux
qubit

E/EC

70

Φ0: fluxoid quantum
Φ: external magnetic flux(b) Double well potential in phase space

58

40
0.3

56
0.48 0.49 0.50 0.51 0.52
Φ/Φ0

0.4

0.5
Φ/Φ0

0.6

0.7

(c) Energy diagram of flux qubit

Fig. 2. Superconducting flux qubit as an artificial atom.

superconducting wave function must be continuous,
so only discrete magnetic flux values are allowed. If
a Josephson junction*2 is inserted into the loop, asmall phase gap is allowed at the junction, so thesuperconductor phases have time evolution. Theenergy of a Josephson junction is changed by thejunction’s phase. Therefore, the energy of the three
Josephson junction loops shown in Fig. 2(a) have adouble well potential in the phase space shown in Fig.
2(b). By using the phases at these junctions γ1, γ2, andγ3, owing to the periodic boundary condition of thephases, we can approximately describe γ+ and γ- by
γ+=γ1+γ2, γ-=γ3. Because of the charge energy of ajunction, phases trapped at two wells denoted L and*2 Josephson junction: A junction that exhibits the Josephson effect,
which is the phenomenon whereby a supercurrent flows without
voltage in a three-layer structure of superconductor, thin insulator, and superconductor.

Vol. 10 No. 9 Sep. 2012

R can jump between the wells quantum mechanically.
This situation is very similar to that of an electron
trapped in the nucleus of an atom. As a result, theenergy of a superconducting loop also has discreteenergy, as shown in Fig. 2(c), and behaves as an artificial atom. A superconducting flux qubit is a twolevel system formed of two of the lowest discreteenergy levels of a superconducting loop.
3. Qubit state measurement
Two minimum energy regions in phase space correspond to the clockwise and anti-clockwise supercurrent states of a superconducting loop. Therefore,
the supercurrent direction is different for the qubit
ground state and the excited state. A quantum superposition state is achieved in a qubit, so we can achieve
the superposition of clockwise and anti-clockwiseFeature Articles

0〉ψ 〉 = a0〉 + b1〉
a 2+b 2=1

Measurement
1〉0〉: anti-clockwise current state
1〉: clockwise current state(a) Measurement of superposition flux qubit state and state reduction(b) State detection by SQUID

Fig. 3. State measurement of flux qubit.

current states. However, this superposition state isstochastically reduced to an eigenstate by measurement, so the detected state is either the clockwise oranti-clockwise current state (Fig. 3(a)). The supercurrent of a superconducting loop generates a small
magnetic field. The direction of this magnetic fielddepends on the superconducting qubit’s quantum
state; therefore, by detecting this small magnetic
field, we can measure the qubit state. A superconducting quantum interference device (SQUID) isused as a device for measuring small magnetic fields.
A small current flows in a SQUID without any voltage (superconducting state); however, voltage does
appear when the current exceeds the critical current.
The critical current of a SQUID is very sensitive to amagnetic field, so we can detect small changes inmagnetic field by detecting the voltage of the current
flowing in a SQUID near the critical current(Fig. 3(b)). This method is easy to use, but the superconducting state is destroyed when the SQUID enters
the voltage state, so the qubit state is also destroyed.
Therefore, we can detect the qubit state, but one moremeasurement does not reproduce the qubit state. Thismeans that we have no information about thedestroyed qubit state. Therefore, we need to develop
a new measurement technique that does not destroy
the quantum information.



4. Nonlinear bifurcation
A Josephson junction works as an inductor in electric circuits, as shown in Fig. 4(a). Unlike a conventional inductor, the Josephson junction exhibits nonlinear characteristics as the applied current isincreased. When we drive a nonlinear resonator that
includes a Josephson junction, the junction’s inductance increases with increasing driving current. Consequently, the resonance frequency shifts to a lower
frequency. When we drive the resonator at a frequency slightly lower than the base resonance frequency,
a bistable state appears. One stable state is a lowamplitude state and the other is a high-amplitudestate, which is frequency-shifted owing to nonlinearity (Fig. 4(b)). If an external driving current isapplied, the resonance state will converge to either
the low- or high-amplitude state over time (Fig. 4(c)).
The stable state that appears is sensitively dependent
on the resonator parameters.
By driving near the bistable state’s boundary, we
can detect a small difference in a system that is coupled to a resonator as a difference in the resonator’s
resonance state. When we establish a magnetic coupling between a resonator and a superconducting
qubit and drive in the optimum microwave condition,
we can detect the qubit state by checking whether the
JBA resonator is in the high- or low-amplitude state.
NTT Technical Review

Feature Articles

V=

Low-amplitude state

Φ0 dφ
2π dt

High-amplitude state
Superconductor
I =IC sinφSuperconductorω

2.0×10−8

Nonlinear resonator

Current (A)Insulator

LJ =3.0×10−8Φ02πIC 1−(I / IC)2

IC: Josephson critical current
LJ : Josephson inductanceφ : phase at junction(a) Josephson junction as anonlinear inductance

1.0×10−8
0.0−1.0×10−8−2.0×10−8ω−3.0×10−8

Linear resonator

High
Low
0

10

20

30

Time (ns)(b) Resonance curves of linear andnonlinear resonators(c) Driven nonlinear resonatorFig. 4. Characteristics of nonlinear resonator.

In this case, the superconducting circuit remains inthe superconducting state, so the qubit quantum stateis more difficult to break when this readout method isused than when the switching readout method is used.
This method is called the JBA readout method.
5. Strength of measurement andprojection to eigenstateWith the JBA readout method, we drive a nonlinear
resonator with a readout microwave and read thequantum state. How does the projection from a superposition state to an eigenstate occur? We studied
when the projection occurred by changing the readout pulse height.
First, we prepared a qubit superposition state andapplied a short readout pulse. By using a gate operation, we can reproduce the previous state in the caseof a superposition state. However, a mixed state after
projection generally does not change to the previous
state as the result of a gate operation. So we read thequbit state after a gate operation: from the readout
pulse, we can know whether or not projection has
occurred. To modify the measurement strength, wechange the amplitude of the readout pulse and measure it. As a result, the α value, which indicates projection to the eigenstate, suddenly becomes 0 as thereadout pulse increases (Fig. 5). This result meansthat projection does not occur (| α |≠0) when the measurement is weak, and sudden projection (| α |=0)
Vol. 10 No. 9 Sep. 2012

occurs if the measurement strength reaches a criticalvalue.
6. Theoretical analysis of measurement
It is easy to construct a theoretical model of the JBA
measurement system for comparison with other systems because a JBA measurement system consists of
only a nonlinear resonator and a coupled qubit.
We analyzed the JBA measurement system theoretically on the basis of this model. As a result, we
confirmed that the superposition state of a qubit interacts with a JBA resonator through an applied readout
pulse, and the qubit and JBA resonator form a quantum-entangled state as a result of time evolution. Thisquantum entangled state achieves the superpositionof (1) a qubit ground state and a JBA low-amplitudestate and (2) a qubit excited state and a JBA highamplitude state. Owing to the time evolution of thisquantum system, the entangled state is suddenly
destroyed by decoherence and is stochastically convergent to one of the two states. These are our predictions for measurement based on theoretical analysis.
This measurement analysis corresponds to the sudden
projection into the eigenstate that we observed in our
experiment when we increased the measurement
amplitude, so this phenomenon strongly supports thevalidity of our theoretical analysis of measurement in
a JBA system.

Feature Articles

Readout probability

1.0

0.5

0.6

0.7

0.8

1.0

1.1

1.2

0.0
1.0

1.0

α 

VisibleNo projection

0.5

Projection indicator α 1.3

π pulseWithout pulse
Visibility

0.5

0.9

0.5

0.0

0.0
Projection−0.5

Demolition −0.5

−1.0

−1.0
0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

Readout pulse height

Projection indicator α  , α⊥Pulse height h
0.7

0.8

0.9

0.4

1.0

1.1

Sudden projection
1.2

1.3

α 

0.3
α 

0.6

0.2
α⊥
0.1
−0.2

−0.1 0.0
0.0

0.1

0.2 0.3

0.4 0.5

0.6

0.4

−0.1
−0.2

0.2

−0.3
−0.4

0.0
0.8

1.0

1.2

Pulse height hFig. 5. Measurement strength and projection.

7. Future prospects
Our understanding of how to measure a superconducting flux qubit with the JBA measurement process
has progressed. By using this knowledge, we aim toachieve fast measurement and control of quantumstates with high accuracy toward the achievement of
quantum error correction, which will be essential for
quantum computing in the future.

NTT Technical Review

Feature Articles

Kosuke Kakuyanagi

Researcher, Superconducting Quantum Physics Research Group, NTT Basic Research Laboratories.
He received the B.S., M.S., and Ph.D. degrees
in science from Hokkaido University in 2000,
2002, and 2005, respectively. He joined NTT
Basic Research Laboratories in 2005 and has
been studying superconducting qubits. He is currently engaged in an experimental study of a
Josephson bifurcation readout method. He is amember of the Physical Society of Japan (JPS)
and the Japan Society of Applied Physics(JSAP).

Shiro Saito

Distinguished Researcher, Senior Research
Scientist, Superconducting Quantum Physics
Research Group, Physical Science Laboratory,
NTT Basic Research Laboratories.
He received the B.S., M.S., and Dr.Eng.
degrees in applied physics from the University ofTokyo in 1995, 1997, and 2000, respectively.
Since joining NTT Basic Research Laboratories
in 2000, he has been engaged in quantum information processing research using superconducting circuits. He was a guest researcher at Delft
University of Technology, the Netherlands, during 2005–2006. He received the Young Scientist
Award from JSAP at the spring meeting in 2004.
He is a member of JPS and JSAP.

Vol. 10 No. 9 Sep. 2012

Hayato Nakano

Research Scientist, Superconducting Quantum
Physics Research Group, NTT Basic ResearchLaboratories.
He received the B.E. and M.E. degrees in communication engineering from Osaka University
in 1983 and 1985, respectively and the Ph.D.
degree in applied physics from Hokkaido University in 1997. He joined NTT Ibaraki ElectricalCommunications Laboratories in 1985 and studied fiber optics. From 1987, he was engaged insolid-state quantum physics in NTT Basic
Research Laboratories. In 2004, he was a guest
researcher at Scuola Normale Superiore in Pisa,
Italy, studying geometrical quantum computations. During 2007–2010, he was a guest professor of quantum information at Nara Advanced
Institute of Science and Technology (NAIST).
He is a member of the Institute of Electronics,
Information and Communication Engineers(IEICE), JPS, and the European Physical Society.

Kouichi Semba

Research Professor, Research Support Coordinator, Global Research Center for Quantum
Information Science, National Institute of Informatics.
He received the B.S., M.S., and Dr.Eng.
degrees from the University of Tokyo in 1983,
1985, and 2002, respectively. He joined the NTT
Electrical Communications Laboratories in 1985
and moved to NTT Basic Research Laboratories
in 1987. From 2003 to 2012, including the period
when the research described in this article wasconducted, he was the leader of the superconducting quantum physics research group. He wasa visiting researcher in the Department of Nanoscience at Delft University of Technology, theNetherlands, during 2002–2003. He was a lecturer at Waseda University in 2005 and a guest
professor at Tokyo University of Science during
2008–2012; he has been a guest professor at the
National Institute of Informatics since 2010. His
achievements include being the first to characterize the electronic state of Penrose’s non-periodic
tiling as an ideal 2D-quasicrystal; the first to
reveal the relevant elemental composition of thehigh-Tc cuprate superconductor YBCO (Tc = 90K); the first to demonstrate entanglement in asolid device, namely vacuum Rabi oscillation ina macroscopic superconducting Josephson-qubit
LC-resonator system; and the first to demonstrate
the coherent coupling of a superconducting flux
qubit and NV spins in diamond.

Feature Articles: Quantum Computing

Electron Spin Rotation and
Quantitative Determination of
Spin-Orbit Coefficients
Takaaki Koga and Yoshiaki Sekine
Abstract

In recent research, there have been proposals to realize quantum bits (qubits) with electron spins,
where these qubits compose a quantum computer. In this article, we explain the mechanism of electron
spin precession that is caused by the Rashba spin-orbit interaction and introduce the latest research
results on quantitative determination of the Rashba and Dresselhaus spin-orbit coefficients. These findings represent a step toward the utilization of electron spins as qubits of a quantum computer.

1. Background
1.1 Spin in conventional electronic devices
An electron has not only a charge degree of freedom but also a spin degree of freedom associatedwith the spin angular momentum. If a charged particle has angular momentum, magnetic moment isgenerated. Therefore, each electron, having a spin
angular momentum, acts as a micromagnet. The spin
degree of freedom can be interpreted, in one way, as
the freedom in the orientation of the micromagnet. In
other words, spin is not a conserved quantity. Thus, it
has not been easy to use them in conventional devices. Recently, research on electron spin in semiconductors has become increasingly important because
future technologies that will transcend existing concepts, such as quantum computers based on spin
qubits, will be based on current basic research such as
the fundamental research studies described in thisarticle.
1.2 Spin and magnetic moment
While spin is naively interpreted as a micromagnet,
its quantum mechanical properties are complicatedand the spin sometimes behaves contrary to everyday
experience. For example, one would associate theword spin with the spin of a tennis ball, where themagnitude of the ball’s angular momentum can bevaried continuously by applying a torque. However,

the spin of an electron cannot be varied continuously.
To describe such peculiar properties of electron spin,
we dare to provide a quantum mechanical explanation of electron spin in this article.
1.3 The Stern-Gerlach experiment
The fact that the magnitude of electron spin is notcontinuously variable but takes only discrete values
was confirmed by the Stern-Gerlach experiment(Fig. 1) [1]. While tiny particles of silver were used in
the actual experiment, the experimental results reflect
the properties of electron spin. When electrons passthrough space containing a magnetic field whose
magnitude is graded in the perpendicular direction,
their spins are decoupled into two states with different magnetic moments, as if they have been sorted(taking a bar magnet as an example, these are an upspin state with the south (S) magnetic pole facing
upward and a down-spin state with the S pole facing
downward). A straightforward interpretation of themeasured results leads to the conclusion that the magnetic moment of electron spins can take only twovalues: either the value corresponding to the upward
direction or the value corresponding to the downward
direction. Then, might there be electrons with a spinstate that is neither upward nor downward vertically,such as those whose S pole is pointing horizontally
among the electrons that have passed through thedetector? To give the conclusion first: yes, there are
NTT Technical Review

Feature Articles

Classical prediction
Experimental result

S

Gradient of magnetic fieldNnote that the up/down direction is the direction of themagnetic field (z direction) within the experimental
apparatus (detector).
Since we have freedom in the direction of the magnetic field to be set in the detector of the experimental
apparatus, we can also select any arbitrary direction
for the observation of a magnetic moment. We would
like to emphasize that the phenomena we are explaining here are not related to the torque that the magnetic field in the detector may apply to the electron
spin to change its orientation. We are saying that theorientation of the spin is varied as a result of the
reduction of the wave packet.
To understand the above description, one needs toknow that any state (wave function) of electron spinais described by a wave function a|↑〉z + b|↓〉z= ,b
1 |which is a superposition of the up-spin≡ ↑〉z and
00
down-spin≡ |↓〉z wave functions. One needs to1
accept the quantum mechanical conclusion that theprobabilities of observing the up- and down-spinastates upon measurement of spin state a|↑〉z + b|↓〉z=b
are given by |a|2 and |b|2, respectively, where a and b
are complex numbers such that |a|2 + |b|2 = 1. Moreover, if we perform the measurements on the sameaspin state a|↑〉z + b|↓〉z= while varying the directionbof the magnetic field in the detector, we will find acertain direction for which observation of only upspin electrons will result (up-spin with respect to thechosen magnetic field direction). This direction iscalled the direction (or orientation) of the electron
spin. Generally, electron spin orientations differ fromone electron to another. However, the conclusion ofquantum mechanics is that any electron spin can be
expressed as a|↑〉z + b|↓〉z by finding appropriate complex numbers a and b, whichever direction it is pointing in.
From the above discussion, we can conclude thefollowing. If there are two spin states |↑〉x and |↓〉x thatare orientated in the +x and -x directions, respectively,
we should be able to write them as a superposition of|↑〉z and |↓〉z, which ultimately leads to |↑〉x= 1 |↑〉z +√2|↓〉z and |↓〉x= 1 |↑〉z – |↓〉z . If we interpret this
√2probabilistically, then the probability that a spin-up or
spin-down state is detected (with respect to the z
direction) is 50% upon the measurement of these()Fig. 1. Stern-Gerlach experiment.

()()

such electrons. To understand why and to make correct predictions about spin-related phenomena insemiconductor devices, we need an accurate understanding of quantum mechanics.
1.4 P
 robability interpretation of the wave function and reduction of the wave packet
Two well-known concepts in quantum mechanics
are probability interpretation (of the wave function)and reduction of the wave packet. Considering a onedimensional world for simplicity, let us express thewave function of an electron by Ψ(x).
Probability interpretation means the following.
When we try to measure the location of an electron
described by Ψ(x), the probability that the electron isfound at a position between x and x+Δx is given by|Ψ(x)|2Δx. Reduction of the wave packet is related toprobability interpretation. It is a term that describes
how an electron in a state with a wide probabilistic
distribution |Ψ(x)|2 (wave packet) before measurement collapses to a point at a certain position x as aconsequence of the measurement.
1.5 Spin states other than up and down
Besides an electron’s position, its spin state is also
more correctly described by a wave function. If weapply the concept of reduction of the wave packet,
then the Stern-Gerlach experiment is interpreted as
the collapse of the wave function of electron spin toeither an up state or a down state by measurement. We
Vol. 10 No. 9 Sep. 2012

()

()

)

(

)

(



Feature Articles

spins. Conversely, the state a|↑〉z + b|↓〉z can also be
(a+b)
(a–b)
|↑〉x +|↓〉x using |↑〉x and |↓〉x.
written as

√2

√2
Therefore, if the direction of the magnetic field in thedetector is in the x direction, the up- and down- spin|a+b|2states will be observed with probabilities
and2|a–b|2, respectively.
2
We finally ask which direction is the electron spina|↑〉z + b|↓〉z pointing in? We provide only the conclusion here. The three-dimensional vector for the orientation of this spin is (x, y, z) = (2Re(a*b), 2Im(a*b),|a|2 – |b|2) , where Re(a*b) and Im(a*b) are the realand imaginary parts of the complex number a*b,
respectively.
1.6 Spin splitting due to the Rashba effectIn our research, we use a spin-orbit interaction
called the Rashba effect [2] in controlling electron
spins. A Hamiltonian*1 of the spin-orbit interaction
generally takes the form (∇V×p)•σ. Using the confinement potential V(z) of the quantum well for V,
i.e., ∇V(z) = (0, 0, -eEz), and hk for p (momentum),
we obtain the Hamiltonian for the Rashba effect as
HR = aso 〈Ez〉 (kyσx – kxσy). Here, aso is a materialdependent intrinsic constant for the Rashba effect,〈Ez〉 is the expectation value of Ez, and σx and σy are
the Pauli spin matrices.*2 If we use a plane wave forthe orbital part of the wave function for simplicity
with a two-dimensional wave vector k≡(kx, ky)=(k, 0),
the wave functions (eigenfunctions) including theeikx ispin part are given bywith respective eige+1

√2 −
h2k2
nenergies± aso 〈Ez〉k. Thus, the energy splitting
2m*eikx ibetween the two states
at a constant wave
+1

√2 −
vector, which is caused by a spin-orbit interaction
associated with the confinement potential of semiconductor heterostructures, is called the Rashba splitting*3 (Fig. 2).

( )

( )

*1	 Hamiltonian: Operator that provides a physical quantity corresponding to energy in quantum physics.
*2	 Pauli spin matrices: Three matrices σx = 0 1 , σy = 0 − i , andi 0
10σz = 1 0 used to describe the spin angular momentum in
0−1
quantum mechanics, and a vector form of them, i.e., σ= (σx, σy,σz).
*3	 Rashba splitting: Spin splitting in the energy dispersion relationof an electron that is caused by the Rashba spin-orbit coupling
effect.

(



)

( )

(

)

e ikx i2 –1

e ikx i2 1

E

aso 〈Ez〉kEF

aso 〈Ez〉k

∆k ∆k

kx

k

Fig. 2. E nergy dispersion of two-dimensional electron
system with the Rashba splitting.

1.7 Spin rotationi(k−∆k)x
Consider a superpositioned wave function e2i(k+∆k)x i
cos (∆kx)
i
e
ikx
=ie
−1 +
sin (∆kx) that is com1
2
posed of two wave functions associated with two
points on the Fermi surface*4 (k±Δk, 0), where Δk =aso 〈Ez〉 m*/h2, in a system having the Rashba splitting. The spin part of this wave function at x = 0 is
1
, where the spin is orientated in the z direction. The
0electron spin states for x ≠ 0 are  cos (∆kx) = sin (∆kx) cos(∆kx) 1 +sin(∆kx) 0 , the spin orientation of
 0
 1
which is (sin(2Δkx), 0, cos(2Δkx)) in a three-dimensional vector. Thus, the spin rotates in the x-z plane asa function of the electron’s position x. An electron ina semiconductor can be regarded as a wave packet
having a central wave number. Since the propagation
speed of a wave packet is given by its group velocityhk(Fermi velocity) m* , an electron in uniform linear
motion changes its position according to its Fermi
velocity. Thus, the spin orientation will rotate accordingly. The angular velocity of the electron spin

( )

()

(

)

()

*4 Fermi surface: Curved surface in the wave number space (k-space)defined by E(k) = EF, where E(k) is the band energy dispersionof an electron in a solid and EF is the Fermi energy. Regions in
k-space that have energies lower than EF are filled with electrons.
The electron transport properties of a solid are usually closely related to the properties of the Fermi surface including its shapeand how it interacts with phonons and impurities.

NTT Technical Review

Feature Articles(a) If 〈Ez〉 = 0

Time(b) If 〈Ez〉 > 0Time(c) If 〈Ez〉 < 0TimeFig. 3. Examples of spin precession.

rotation is given as ω = 2aso 〈Ez〉 k/h in this case. The
tunability of 〈Ez〉 by an external gate or a specificdesign of the heterostructure enables us to control theangular velocity of spin rotation, which is one of thegreatest attractions of the Rashba effect. Some examples of spin rotation for an electron moving at the
Fermi velocity are shown in Fig. 3, where aso acts as
a parameter indicating how easy it is to control theelectron spin precession by an electric field 〈Ez〉.
In research on the Rashba effect up to now, it has
been important to obtain a value called the Rashba
coefficient α, which corresponds to aso 〈Ez〉. Pieces ofcircumstantial evidence showing that the α values arecontrollable by a gate had been accumulated, though
nobody had been able to show a quantitatively linear
relationship between α and 〈Ez〉. The factors that havedelayed the progress of this research include (1) alack of experimental methods that enable one tomeasure the value of 〈Ez〉 within a quantum welldirectly and (2) uncertainty about the size of the Dresselhaus spin splitting (the spin splitting arising fromthe bulk inversion asymmetry), which coexists with
the Rashba spin splitting. In collaborative work
between Hokkaido University and NTT Basic
Research Laboratories, we have demonstrated thelinear relationship α = aso 〈Ez〉 quantitatively by using
In0.53Ga0.47As semiconductor quantum wells (Fig. 4),
which are lattice-matched to (001) InP; this led to theVol. 10 No. 9 Sep. 2012

successful quantitative determination of the value of
aso [3]. A key to our success was the coincidental
discovery of an epitaxial wafer that has the property
that the value of 〈Ez〉 can seemingly be varied frompositive to negative values by means of gating. Careful experiments with this epitaxial wafer and a seriesof similar wafers resulted in our quantitative determination of aso.
2. Latest results
2.1 Experimental procedure for determining aso
The testpiece used in our experiment was a 10-nmthick In0.53Ga0.47As semiconductor quantum wellgrown on the (001) plane of an InP substrate (withbarrier layers of In0.52A10.48As). We controlled theelectron density and the perpendicular electric field〈Ez〉 within the quantum well by means of a surface
gate in a field-effect-transistor structure (Fig. 4). We
used a dilution refrigerator to cool the sample down
to ~100 mK (electron temperature), in order to minimize the effect of thermal disturbance and increase
the measurement sensitivity, and measured the electric resistance as a function of the perpendicular magnetic field (magneto-resistance) for various gate voltages. In these measurements, a distinctive phenomenon called the weak antilocalization appeared in thevicinity of zero magnetic field, owing to theFeature Articles

SurfaceCarrier
Lay Supp
er
ly

Qu
an
Wa tum
ll
Ca

rrie
r
Lay Supperly(5 nm)
i-In0.52Al0.48As
nm)
.5
(1
s
lA
i-A
(20 nm)i-In0.52Al0.48As−3
18
5×10 cm(6 nm) 2.
0.48AsAl
0.52
Inn(6 nm)i-In0.52Al0.48As)(1
As 0 nm
i-In0.53Ga0.47)nm(6i-In0.52Al0.48As−3
18
1.5×10 cm
)
nm
(6
As
n-In0.52Al0.48(200 nm)
i-In0.52Al0.48As
bstrateSu(001) InP

Gate
electrode

Vapor-deposited
film of gold

Sourceelectrode

Drain electrode

Electric field
i: intrinsic
n: negative

Fig. 4. Experimental sample and FET structure.

Electric resistance

Weak localization

Incident electron
wave

Weak antilocalization
Magnetic field B(a) Generation mechanism(b) Magneto-resistance effectFig. 5. Weak localization and weak antilocalization effects.

interference of the electron wave functions. We wereable to determine the value of the Rashba coefficient
α by theoretically analyzing this experimentallyobserved phenomenon [4].
2.2 W
 eak localization and weak antilocalization
effects
Weak localization is a phenomenon that occurs
when an electron is scattered sequentially in a loop by
impurities distributed randomly in the conductor,
whereby the electrons are weakly localized in theloop as a result of constructive interference of the
wave functions. For example, if an electron happens
to follow the closed path indicated by blue arrows in
Fig. 5(a) after the injection of the electron as indicated by the black arrow, then the red path, which is

the time-reversed path of the blue one, should also bepresent quantum mechanically with the same probability as the blue path. Since the lengths of the twopaths are exactly equal, constructive interference
always occurs at the endpoint of the two paths (theposition of the green impurity in Fig. 5(a)), if theelectron propagation is not accompanied by spin rotation. This results in enhancement of the backscattering probability. This is the mechanism of weak localization.
The weak localization effect is partially destroyed
if a magnetic field is applied perpendicular to thequantum well. As a result, a negative magneto-resistance appears in the vicinity of the zero magnetic
field, as shown by the black curve in Fig. 5(b). The
weak localization effect observed in the vicinity of
NTT Technical Review

120

The states of spin rotation in the regions (a), (b),and (c) in these figures correspond to those of (a),(b), and (c) in Fig. 3, respectively.

Vg (V)
0.25
90

0.19

(c)

(b)

0.13
0
60

(a)

−0.12

−0.24

30

(b)

−0.30
0
−0.2

−0.1

0.0

0.1

Negative

0.2

B (10−3 T)(a) Experimental results

(a)

(c)

Rashba spin-orbit coefficient

Quantum correction to the electric conductivity (10−6Ω−1)Feature Articles

0

Positive

Gate voltage Vg (V)(b) Gate voltage dependence of the Rashba coefficient α.

Fig. 6. Measurement of weak localization and weak antilocalization.

zero magnetic field is also suppressed by the rotationof spins, which is called weak antilocalization. In a
more precise description, the mechanism of the weak
antilocalization is related to the non-commutativityof spin rotation operators (AB ≠ BA for arbitrarily
chosen A and B). This results in a positive magnetoresistance if the weak antilocalization is occurring(the red curve in Fig. 5(b)).
The results of our experiment are shown in Fig. 6(a),where the electric conductivity of a quantum well
sample is plotted (on the vertical axis) as a functionof the magnetic field (horizontal axis) for various
values of the gate voltage Vg. It is necessary to invert
the + and - signs on the vertical axis (or invert up anddown) when comparing the results with the schematic weak localization/antilocalization effects
shown in Fig. 5(b) because the data plotted here are
the electric conductances, not the resistances.
Because Vg is a relative quantity, its value is set to 0
V when the Rashba splitting is minimized in thisanalysis. This experimental result shows that the
weak antilocalization effect becomes stronger as theabsolute value of Vg increases. Then, the value of the
Rashba coefficient α can be extracted from the valuesof the fitting parameters that fit the experimental
result best assuming a recently developed theoreticalmodel. The gate voltage dependence of the Rashba
spin-orbit interaction coefficient α of this sample

Vol. 10 No. 9 Sep. 2012

obtained from such analyses is shown in Fig. 6(b).
We note that the angular velocity of the electron spinrotation rises faster as α increases. The fact that we
successfully made accurate predictions of the valuesof α as a function of the gate voltage Vg as in Fig. 6(b)indirectly means that we succeeded in controlling theangular velocity of the electron spin rotation freely bymeans of the gate of a transistor. For example, we canrotate the spins of electrons within a semiconductor
about some specific axis ((b) in Fig. 3), halt the rotation ((a) in Fig. 3), and rotate them in reverse ((c) in
Fig. 3) all by means of a gate. As a result, we expectthat the knowledge acquired in this research willbring us one step closer to being able to utilize electron spins in future electronic devices such as quantum computers and ultra-low-power logic devices.
References
[1]
[2]
[3]
[4]

W. Gerlach and O. Stern, “Das Magnetische Moment des Silberatomes,” Zeitschrift für Physik, Vol. 9, No. 1, pp. 353–355, 1922.
Yu. A. Bychkov and E. I. Rashba, “Oscillatory Effects and the Magnetic Susceptibility of Carriers in Inversion Layers,” J. Phys. C: Solid
State Phys., Vol. 17, No. 33, pp. 6039–6045, 1984.
S. Faniel, T. Matsuura, S. Mineshige, Y. Sekine, and T. Koga, “Determination of Spin-Orbit Coefficients in Semiconductor Quantum
Wells,” Phys. Rev. B, Vol. 83, No. 11, 115309, 2011.
T. Koga, J. Nitta, T. Akazaki, and H. Takayanagi, “Rashba Spin-Orbit
Coupling Probed by the Weak Antilocalization Analysis in InAIAs/
InGaAs/InAIAs Quantum Wells as a Function of Quantum Well
Asymmetry,” Phys. Rev. Lett., Vol. 89, No. 4, 046801, 2002.



Feature Articles

Takaaki Koga

Associate Professor, Graduate School of Information Science and Technology, Hokkaido University.
He received the B.S. degree in engineeringfrom the University of Tokyo in 1992 and the
M.S. and Ph.D. degrees in applied physics from
Harvard University, USA, in 1995 and 2000,
respectively. He joined NTT Basic ResearchLaboratories as a research associate in 2000 andstudied semiconductor spintronics. In 2004, hemoved to Hokkaido University as an associate
professor. He is currently engaged in an experimental study of spin-orbit interaction in semiconductors. He is a member of the American
Physical Society, the Physical Society of Japan(JPS), and the Japan Society of Applied Physics(JSPA).



Yoshiaki Sekine

Researcher, Spintronics Research Group, NTT
Basic Research Laboratories.
He received the B.S. degree in engineeringfrom Hokkaido University in 1994 and the M.S.
and Ph.D. degrees in science from the Universityof Tokyo in 1996 and 1999, respectively. He
joined NTT Basic Research Laboratories in 1999
and has been studying semiconductor spintronics. He is currently engaged in an experimental
study of spin-orbit interaction in semiconductors.
He is a member of JPS and JSPA.

NTT Technical Review

Feature Articles: Quantum Computing

Quantum Simulation with Ultracold
Atomic Gases in an Optical Lattice
Makoto Yamashita and Kensuke Inaba
Abstract

In this article, we review quantum simulation for exploring various properties of matter via ultracold
atomic gases at nanokelvin temperatures confined in an optical lattice, which is an artificial crystal created by laser lights. We also explain our recent research conducted in collaboration with Kyoto University. Novel quantum states of matter have been successfully achieved for the first time by using ultracold
ytterbium atoms in optical lattices.

1. Introduction
In order to explore various properties of matter, weare studying quantum simulation by using ultracold
atomic gases at nanokelvin temperatures confined in
an optical lattice, which is an artificial crystal createdby laser lights. We can introduce our research using
the photograph shown in Fig. 1. The green spot at thecenter indicated by the arrow corresponds to a coldgas of ytterbium atoms emitting fluorescence duringlaser cooling in the ultrahigh vacuum chamber. Thisatomic gas is about 1 mm in diameter and contains
about ten million ytterbium atoms; the temperature isof the order of 10 μK. An advanced experimental
technique based on evaporative cooling has succeeded in further lowering the gas temperature by fourorders of magnitude and achieving ultracold atomic
gases at temperatures of a few nanokelvins. Suchgases are considered to be the coldest matter on earth
and they obey laws of physics that are contrary to thecommon sense that we use in our daily lives, namely
they obey quantum mechanics.
A new approach for studying fundamental problems in quantum mechanics via ultracold atomic
gases has recently been attracting many researchers’interest. One can simulate complicated quantum
mechanical phenomena in a well-controlled manner
by utilizing the highly controllable atoms available atextremely low temperatures. This approach is therefore referred to as quantum simulation [1].
We know that quantum mechanical effects cause

Atoms: ~107
Temperature: ~10 µK
Laser wavelength: 556 nm
Size of atomic gas: ~1 mm
Photograph by Dr. Seiji Sugawa, Kyoto University

Fig. 1. C old atomic gas of ytterbium confined in an ultrahigh vacuum chamber.

various phenomena found in condensed matter, e.g.,
the metallic, insulating, magnetic, and superconducting properties of materials. Some of them, however,are highly complicated, so it is still very difficult toobtain a full theoretical understanding of their properties even using supercomputers. A typical example
is that the mechanism of high-temperature superconductivity has not yet been completely clarified thirty
years after its discovery. We can expect these unresolved problems in condensed matter physics to besolved in the near future through quantum simulation
using ultracold atomic gases.
NTT Technical Review

Feature Articles

Optical lattice

Atom

Laser lights
Ultracold atomic gas

An optical lattice is an artificial crystal
created by laser lights.

Fig. 2. Schematics of an optical lattice.

On the other hand, experimental techniques formeasuring and manipulating single atoms are showing rapid progress, providing us with promising
future applications of atomic gases in quantum technologies. One of the final targets is to construct aquantum computer in which each single atom works
as a quantum bit. NTT Basic Research Laboratories
has been exploring the enormous potential ability ofultracold atoms both theoretically and experimentally[2].
In this article, we review quantum simulation withultracold atomic gases and then explain our recent
research that has successfully demonstrated the novel
quantum state of matter using ytterbium atoms in collaboration with Professor Yoshiro Takahashi’s groupat Kyoto University [3].
2. Optical lattice: artificial crystal
created by laser lights
An optical lattice is one of the key experimental
techniques for quantum simulation based on ultracold
atomic gases. We create an optical lattice by applying
counterpropagating laser lights to the ultracold cold
atomic gas, as schematically depicted in Fig. 2. Here,the laser light wavelength is greatly detuned and is far
from the resonance wavelength of the atoms. In thissituation, the atoms do not absorb laser photons;
hence, heating of the atoms does not occur. The counterpropagating laser lights form a standing wave so as
to produce a lattice-like potential that changes periodically with spatial position. An optical lattice corresponds to this periodic potential with atoms confined inside. When laser lights illuminate an atomic
gas from six directions, as shown in Fig. 2, the latticeVol. 10 No. 9 Sep. 2012

structure becomes a simple cubic one and the lattice
distance is exactly equal to half the laser wavelength.
The right diagram in Fig. 2 shows a view of atoms
confined on a certain plane in the cubic optical lattice.
Atoms move around in the periodic lattice potential
by means of quantum tunneling and they interact witheach other when multiple atoms coexist at the samelattice site. From this diagram, we understand that an
optical lattice acts as an artificial crystal created bylaser lights, and confined atoms behave just like electrons in solids. If highly stabilized lasers are used, this
optical lattice becomes an ideal crystal without any
influence from lattice vibrations, lattice defects, or
impurities. Furthermore, the potential depth of an
optical lattice changes depending on the laser intensity, which enables the dynamical motion of atoms inthis artificial crystal to be controlled via the lasers.
Through the development of optical lattices, researchon ultracold atomic gases has now become closely
related to condensed matter physics.
3. Bosons and fermions
In quantum mechanics, all particles are classified
into two kinds of fundamental particles according totheir quantum statistics: bosons (bosonic particles)
and fermions (fermionic particles). Their energy distributions at the temperature of absolute zero areshown in Fig. 3. Bosons have the property that any
number of them can occupy a single energy level.
Familiar examples of bosons are photons and helium
atoms (mass number: 4). Moreover, superfluidity,
which is exhibited by liquid helium at low temperatures, is attributed to the bosonic nature of heliumEnergy levels

Feature Articles

BosonsFermions

Fig. 3. Two different types of quantum-mechanical particle.

(a) Superfluid state(b) Mott-insulator state

Atoms freely move around in an
optical lattice.

Atoms localize at lattice sites because ofinter-atomic interactions.

Fig. 4. Simulation of quantum phase transition from a superfluid to a Mott-insulator.

atoms. On the other hand, fermions have a completely different property from bosons: multiple fermionscannot occupy a single energy level. Typical examples of fermions are electrons, protons, and neutrons.
Textbooks on condensed matter physics explain
many properties of metals and semiconductors on thebasis of the fermionic nature of electrons. Furthermore, superconductivity arises from the fact that fermionic electrons form an ensemble of electron pairs
and each electron pair behaves as a boson.
In condensed matter physics, we generally explore
the universal features of matter by studying the widest possible range of materials. Quantum simulation
via ultracold atomic gases in an optical lattice provides us with a different approach: we investigate
universal features directly via artificial model systems created by highly controllable atoms and laser
lights that mimic materials. It should be noted herethat isotopes play an important role in such quantum
simulation. Most atomic species have several stable
isotopes containing different numbers of neutrons

inside the nucleus. For example, ytterbium has sevenstable isotopes: five of them are bosons while theother two are fermions. By using this rich range ofisotopes, we can freely choose bosons and/or fermions to investigate. This indicates the great advantageof quantum simulation. Even a single atomic species
like ytterbium allows us to investigate various aspectsof matter such as superfluids, metals, insulators,
superconductors, and magnets.
4. Simulation of quantum phase transitions
Next, we briefly introduce the pioneering work
done by the research group at the Max Planck Institute of Quantum Optics in Germany [4], which demonstrated quantum simulation using optical lattices
for the first time.
In their experiments, bosonic atoms of rubidium(mass number: 87) were cooled to ultralow temperatures on the nanokelvin order and confined in an optical lattice. As depicted in Fig. 4(a), bosonic atoms
NTT Technical Review

Feature Articles

stay in the superfluid state at such a low temperatureand they can move around freely over all of the latticesites. However, this situation is limited to the casewhere the intensity of lattice lasers is low enough andthe depth of the optical lattice potential is correspondingly shallow. When the intensity of the lattice lasers
is stronger and the optical lattice potential becomesdeeper, the interaction between atoms plays a significant role in the quantum states of atoms. In particular,
in the case of repulsive interactions where atoms
repel each other, bosonic atoms cannot move freely
because of the interactions and they are finally localized at lattice sites, as shown in Fig. 4(b). Such localization of bosonic atoms reflects their insulating
properties. This insulator state caused by interactions
is called a Mott insulator after its theoretical proposer, Sir Nevill Mott.
A quantum phase transition (QPT) drasticallychanges the characteristics of a substance. This phenomenon is caused by many quantum mechanicalparticles included in the substance such as electrons.
QPTs are therefore one of the main research topics incondensed matter physics as a typical example ofcomplicated many-body problems. Figure 4 indicates
that, surprisingly, we can simulate the QPT from asuperfluid to a Mott-insulator in condensed matterphysics by means of ultracold atomic gases confinedin optical lattices. Note that the Mott-insulator state isstrongly related to the mechanism of high-temperature superconductivity. Similar studies using ultracold fermionic atoms have already been performed
mimicking electrons in high-temperature superconductors. On the other hand, as clearly seen in Fig.
4(b), all of the atoms in the Mott-insulator state arestrictly ordered in alignment with lattice sites. Thissheds light on another promising application of ultracold atoms other than quantum simulation. We shouldbe able to build a scalable quantum computer utilizing every single atom in a lattice as an ideal quantumbit.
5. Recent results: Diverse quantum statesachieved in mixtures of bosons and fermions
The diverse properties of substances are closely
related to the different characteristics of bosons andfermions, as mentioned above. Perhaps you wonder
what kind of quantum states will be achieved by mixing these two fundamental particles—bosons andfermions—in an optical lattice. Of course, such a
mixture does not exist in nature. Kyoto University
and NTT worked in collaboration to elucidate thisVol. 10 No. 9 Sep. 2012

unresolved problem [3].
In this collaboration, the research group at Kyoto
University performed the experimental studies. Twokinds of atomic gas consisting of bosonic and fermionic ytterbium isotopes were cooled to temperaturesof a few nanokelvins and mixed in the optical lattice.
All of the experiments were conducted under the condition that the optical lattice was sufficiently deep;
hence, both ytterbium gases were in the Mott-insulator regime. This significantly simplified the characteristics of mixtures and made it possible to focus on
the influence of interactions between bosons and fermions. The research group at NTT theoretically analyzed the experimental data measured by the Kyoto
University group. We used our own calculation
method, which corresponds to an extension of the
Gutzwiller approximation* to finite temperatures. We
precisely compared the experiments and calculations
and clarified that the ytterbium isotopes form thenovel quantum states in which bosons and fermions
are highly entangled. These states are far from thesimple Mott-insulators shown in Fig. 4.
The quantum states of boson-fermion mixtures elucidated by our research collaboration are summarized
in Fig. 5. We have studied two cases classified bywhether the interaction between bosons and fermionsis repulsive or attractive. First, when the interaction isrepulsive and the numbers of both particle types arecomparable, bosons and fermions repel each otherand mix randomly, leading to a random arrangementof single bosons and single fermions at the opticallattice sites, as shown in Fig. 5(a). This state is a completely new quantum state of matter that has been
confirmed for the first time in the world; it is referred
to as a mixed Mott-insulator. However, when thenumber of fermions greatly exceeds the number ofbosons, both particles are distributed among the optical lattice sites in a spatially separated manner, as
shown in Fig. 5(b). We can understand that such aquantum state (i.e., phase separated state) reflects thedifference between bosons and fermions depicted inFig. 3. Bosons tend to occupy lattice sites with multiple bosons at each site, while fermions tend to
exhibit single occupancy.
Next, when the interaction between bosons andfermions is attractive, the quantum states of the mixtures are completely different to those in the*

The Gutzwiller approximation is an important theoretical methodof approximate calculations in condensed matter physics formulated by Martin C. Gutzwiller. This method enables us to efficiently calculate the effects of atomic interactions in an opticallattice.

Feature Articles(a) Mixed Mott-insulator state(b) Phase separated state

Boson

Fermion(c) Composite particle stateFig. 5. Diverse quantum states observed in mixtures of bosons and fermions.

above-mentioned case of repulsive interaction. We
have revealed that multiple bosons and fermionscombine at lattice sites and form composite particle
states, as shown in Fig. 5(c). Interestingly, this composite particle state has different components depending on the numbers of bosons and fermions includedin the mixture.
In summary, we have observed diverse quantum
states in mixtures of bosons and fermions confined in
an optical lattice as a result of the interplay between
the characteristics and the interactions of these twodifferent fundamental particles. These quantum simulation results provide us with the important basic
concept for one possible origin of diverse phenomenathat we generally find in condensed matter systems.
6. Future prospects
Quantum simulation by means of ultracold atomic
gases in optical lattices has been making rapid progress in recent years, involving various research fields
such as condensed matter physics, quantum information, and quantum chemistry. We are sure that a wide

range of research activities seeking both basic studies
and future applications is now very important. NTT
Basic Research Laboratories is developing quantum
technologies for ultracold atomic gases through closemutual cooperation between theoretical and experimental studies. The final aim is to construct a quantum computer based on an optical lattice utilizing
single atoms as quantum bits.
References
[1]
[2]

[3]

[4]

I. Bloch, “Ultracold Quantum Gases in Optical Lattices,” Nature
Physics, Vol. 1, No. 1, pp. 23–30, 2005.
T. Mukai, “Making a Quantum Computer Using Neutral Atoms,”
NTT Technical Review, Vol. 6, No. 1, 2008.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr2008
01sp7.html
S. Sugawa, K. Inaba, S. Taie, R. Yamazaki, M. Yamashita, and Y.
Takahashi, “Interaction and Filling-induced Quantum Phases of Dual
Mott Insulators of Bosons and Fermions,” Nature Physics, Vol. 7, No.
8, pp. 642–648, 2011.
M. Greiner, O. Mandel, T. Esslinger, T. W. Hänsch, and I. Bloch,“Quantum Phase Transition from a Superfluid to a Mott Insulator in a
Gas of Ultracold Atoms,” Nature, Vol. 415, No. 6867, pp. 39–44,
2002.

NTT Technical Review

Feature Articles

Makoto Yamashita

Senior Research Scientist, Optical Science
Laboratory, Quantum Optical State Control
Research Group, NTT Basic Research Laboratories.
He received the B.E., M.E., and Dr.Eng.
degrees in applied physics from Osaka University in 1987, 1989, and 1992, respectively. Sincejoining NTT Basic Research Laboratories in
1992, he has been experimentally studying quantum noise in semiconductor lasers and theoretically studying the quantum dynamics of ultracold atoms. His current research interests arequantum phase transitions and quantum dynamics of ultracold atoms in optical lattices. Since
2008, he has been collaborating with Professor
Yoshiro Takahashi at Kyoto University as a member of the CREST project of the Japan Scienceand Technology Agency (JST). He is a memberof the Physical Society of Japan (JPS), the Japan
Society of Applied Physics, and the American
Physical Society.

Vol. 10 No. 9 Sep. 2012

Kensuke Inaba

Optical Science Laboratory, Quantum Optical
State Control Research Group, NTT Basic
Research Laboratories.
He received the B.E., M.E., and Dr.Eng.
degrees from the Department of Applied Physics,
Osaka University, in 2003, 2005, and 2007,
respectively. He joined NTT Basic ResearchLaboratories 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 andtheir application to a quantum simulator. Since
2008, he has been collaborating with Professor
Yoshiro Takahashi at Kyoto University as a member of the CREST project of JST. He is a memberof JPS.

Regular Articles

Method of Handling Hard Disk Drive
Failures in Cloud Computing
Environment
Jin Hiwatashi and Sotetsu Iwamura
Abstract

In this article, we present a method of handling failures of hard disk drives (HDDs) in a cloudcomputing environment deployed for research and development purposes. We identified two problems
associated with HDD failures: (1) how to ascertain the failure frequency of each HDD and (2) how toascertain the lifetime of a specific HDD. For the first problem, we devised a method for estimating thenecessary number of replacement HDDs by using an HDD lifetime distribution. For the second problem,
we devised a method for estimating the remaining life of an HDD by using HDD failure physics andreplacing all HDDs on a preventative basis during regular maintenance.

1. Introduction
NTT Software Innovation Center is researching anddeveloping an R&D (research and development)
cloud in order to establish technologies for developing cloud computing services. Because this R&D
cloud consists of a large number of diverse devices,
one important issue for its operation is how to handle
frequent device failures. This issue can be divided
into two main problems.
The first problem is how to ascertain the failure
frequency of each type of device. Failed devices areusually replaced during maintenance. To ensure thatenough replacement devices are kept on hand, it isnecessary to forecast the number of device failures ina given time period by considering the failure frequency of each type of device. To solve this problem,
a failure rate model that uses a statistical lifetime
distribution*1 independent of the target device iseffective.
The second problem is how to ascertain the lifetimeof a specific device. Devices expected to fail are usually identified and replaced as a preventative measureduring regular maintenance. Determining the likelihood of failure requires forecasting the lifetime ofeach device. While it is possible to apply the failure
rate model with a statistical lifetime distribution asdescribed above for this problem, its forecasting
accuracy is low, a few percent at best, so it cannot be
used without modification. To solve this problem, afailure rate model with improved forecasting accuracy achieved through active use of knowledge (failure
physics*2) about the particular device is effective.
In this article, we report examining hard disk drives(HDDs), which fail relatively frequently, and developing an HDD failure rate model for a cloud computing environment used to implement an R&D cloud.
We analyzed the past failure statistics of an HDD
cluster that had a particularly high rate of HDD failures in the R&D cloud and devised two methods of
estimating HDD failure rates (Sections 2 and 3,
respectively). Section 2 describes the application of a
failure rate model using statistical lifetime distributions for HDD failures. As a specific application ofthis model, we present a method for estimating thenumber of HDDs required over a specific time period
in order to have a sufficient number of replacement
HDDs on hand. Section 3 describes the application ofa failure rate model using failure physics for HDD
failures. As a specific application of this model, we*1	 Lifetime distribution: A distribution that follows the time until an
item fails.
*2 Failure physics: Research into failures from the perspective of thephysical and chemical processes that lead to failure or damage.

NTT Technical Review

Regular Articles

present a method for discovering HDDs likely to failand replacing them preventatively during regular
maintenance. Finally, Section 4 concludes with asummary.

we established a method of estimating the necessary
number of replacement HDDs by applying a failure
model based on a statistical lifetime distribution for
HDD failures.
Before constructing a failure rate model based on alifetime distribution, it is necessary to investigate
which distribution model the lifetime distribution
adheres to. In the following sections, we find the lifetime distribution from past failure statistics of HDDs
in Cluster A and attempt to fit three lifetime distribution models. Finally, we evaluate the three lifetime
distributions to determine which has the best fit to thelifetime distribution.

2. Method for ascertaining
HDD failure frequencies
Replacement HDDs were stockpiled prior to thestartup of the R&D cloud’s high-failure-rate cluster(Cluster A). The number intended to be sufficient fora year was calculated assuming, on the basis of existing research [1], [2], the annual failure rate*3 to be
3%. After Cluster A went into operation, however,
HDD failures began occurring at an annual failure
rate of over 10% and the stock of replacement HDDs
was exhausted in three months. From this experience,

2.1 Determining past failure statistics
A log was kept noting the time and date of each
HDD replacement. The failure statistics are summarized in Table 1. The HDD lifetime distribution in
Fig. 1 was derived from the failure statistics given in
Table 1. The horizontal axis in Fig. 1 represents thenumber of days elapsed from the reference date andthe vertical axis represents the reliability of the HDDs
in use on the reference date. The sudden drop in reliability at around the 120-day mark is explained by thediscovery and replacement of failed or malfunctioning

Table 1. Summary of failure statistics.
TargetCluster A

PeriodJanuary 1 to October 4, 2010

No. of servers

285

No. of HDDs

1140 (112 failed)

HDD specifications SATA 1 TB (consumer-grade devices)*3 Annual failure rate: The percentage of devices that will fail overa period of one year out of the total number of devices.

SATA: serial AT attachment (AT derives from IBM PC/AT personal
computers)

1
0.99
0.98

Reliability R(t)

0.97
0.96
0.95
0.94

Failed/malfunctioning HDDsdiscovered and replaced during
maintenance operations

0.93
0.92
0.91
0.9

0

50

100

150

200

250

300

Lifetime t (in days)
Actual results

Fig. 1. HDD reliability calculated from failure statistics.

Vol. 10 No. 9 Sep. 2012



Regular Articles

HDDs during maintenance conducted between April
30 and May 6, 2010.

µ
R(t)=Φ  Le -1n t  σLe 2.2 F
 itting past failure statistics to lifetime distribution models
Next, we selected three lifetime distribution models—exponential distribution, Weibull distribution,and log-normal distribution—as candidates for fitting
the past failure statistics shown in Fig. 1.
The exponential distribution expresses the reliability R(t) with Equation 1, where the failure rate λ(t) isconstant.

2uwhere Φ(u)= 1 ⌠ exp - u  du
 2 √2π ⌡−∞

R(t)=exp(−λ0 • t)

(1)

(3)

We estimated parameters to fit the failure statistics
in Fig. 1 to these three distribution models. The
results are listed in Table 2, and the values predictedusing the lifetime distribution models are given in
Fig. 2.
2.3 Evaluating the goodness of fit
It is not possible to tell from Fig. 2 which of thethree models is the best fit. Therefore, we used the

The Weibull distribution expresses the reliability
R(t) with Equation 2, where the failure rate varies
according to shape parameter α and scale parameter β.
αR(t)=exp −  t  (α > 0, β > 0)
(2)
β 
The log-normal distribution expresses the reliability R(t) and failure rate λ(t) with Equation 3, where thelog of lifetime t follows a normal distribution.

Table 2. E stimated parameters and AIC results forlifetime distribution models.
Lifetime distribution

Estimated parameters

Exponential distribution λ0 = 0.0003

AIC−475.9Weibull distribution

α = 1.508, β = 1315

−591.7

Log-normal distributionµLe = 7.433, σLe = 1.301−567.0

1
0.99
0.98

Reliability R(t)

0.97
0.96
0.95
0.94
0.93
0.92
0.91
0.9

0

50

100

150

200

250

300

Lifetime t (in days)
Values predicted using the
exponential distribution model
Actual results

Predicted values with the
Weibull distribution model
Predicted values with the
log-normal distribution modelFig. 2. HDD reliability calculated with lifetime distribution models.



NTT Technical Review

Regular Articles

Akaike information criterion (AIC) *4, a measuredeveloped by Hirotsugu Akaike, to evaluate eachmodel’s goodness of fit. The AIC value for eachmodel was found using Equation 4.





S
AIC = n × 1n 2π e  +1 +2(p+2)
n 



(4)

In this equation, n is the number of samples, Se is theresidual sum of squares, and p is the number of predictor variables. For the exponential distribution
model, p = 1 was used, whereas for the Weibull distribution and log-normal distribution models, p = 2
was used. The AIC results for the lifetime distribution
models are also listed in Table 2. We can see that the
Weibull distribution is the best fit for the past failure
statistics and the log-normal distribution is the next
best fit. This result is also consistent with existing
research [2]. Therefore, it is reasonable to fit the
Weibull distribution to the HDD lifetime distribu*4 Akaike information criterion (AIC): An indicator of the relative
goodness of a statistical model. It judges models using a balance
between the complexity of the model (number of parameters) andthe model’s goodness of fit by using real data.

tion.
2.4 M
 ethod for estimating the necessary numberof replacement HDDs
As a final step, we devised a method for estimating
the necessary number of replacement HDDs. To dothis, we first estimated the parameters of the failure
rate model (Weibull distribution) for each HDD product using failure statistics over the most recent
six-month to one-year period. For example, it is possible, using the failure rate model (Weibull distribution) shown in Fig. 3, to estimate the number of failures over a certain period by multiplying the total
number of HDDs by the difference in reliabilities
between the start date (day 100) and the final date(day 200). The necessary number of replacement
HDDs can be easily worked out from the number of
failures.
3. Method for ascertaining HDD lifetimes
The R&D cloud requires maintenance each time an
HDD fails in order to replace or repair the HDD. User
convenience can be improved if HDDs with little
remaining life can be discovered and replaced pre-

1
0.99
0.98
No. of failures

Reliability R(t)

0.97
0.96
0.95
0.94
0.93
0.92
Prediction period

0.91
0.9

0

50

100

150

200

Start date

250

300

Final date

Lifetime t (in days)
Values predicted using the
exponential distribution model
Actual results

Predicted values with the
Weibull distribution model
Predicted values with the
log-normal distribution modelFig. 3. Estimation of the necessary number of replacement HDDs.

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ventatively during regular maintenance. Since theforecasting accuracy of failure rate models based on
lifetime distributions is too low, as mentioned in Section 1, we applied a failure rate model that uses failure physics to improve the forecasting accuracy for
HDD failures.
3.1 Fitting results to a rate process model
The rate process model is widely used in accelerated tests for electronic components including HDDs.
It expresses the lifetime t with Equation 5. Therefore,
we continue the argument in this article on theassumption that HDD failure physics conforms to therate process model.
Ut = BS −n • exp  
 kT 

(5)

In this equation, S is the load (physical stresses andvoltage), U is the activation energy, k is Boltzmann’sconstant, T is the absolute temperature, and B and n
are constants.
The absolute temperature can be considered to beconstant since the R&D cloud’s computing environment is climate controlled to ensure uniform temperature and humidity levels. Assuming constanttemperature means that B • exp(U/kT ) is also constantbecause U, k, and B are constants. Replacing thisconstant in Equation 5 with C gives Equation 6.
t = CS −n 

(6)Taking the log on both sides of Equation 6 gives
Equation 7.
1n t = −n • 1n S + 1n C
(7)Equation 7 can be applied as a failure rate model(lifetime model) based on HDD failure physics under
fixed temperature conditions. However, it is necessary to estimate parameters C and n for each HDD
product in order to forecast the lifetime t by using
Equation 7’s failure rate model.
Estimating C and n requires measuring the load S(physical stresses and voltage) applied to the HDD
and the resulting lifetime t. Because of the difficultyin making these measurements, we made the following two assumptions for the purposes of this article.
3.1.1 Assumptions about load S
The load S (physical stresses and voltage) applied
to an HDD consists of vibrations and impacts (physical stresses) and excess voltages. For example, applying vibrations or impacts to an HDD will cause media
scratches and media failures. If spare sectors areassignable, the RAID (redundant array of independent
disks) card will record the number of media failures,but if spare sectors are not available, the HDD will
fail. Thus, applying load may cause a failure. If
recovery from the failure is possible, this is recordedin the HDD information, but if not, the HDD will
fail.
Since the load can be approximated from HDD
information that is measureable via the RAID controller, we assumed a linear relationship exists
between the load and some HDD information.
3.1.2 Assumptions about lifetime t
While it is possible to measure the lifetime t of an
HDD to which a load S has been applied once all
HDDs in a cluster have failed, lifetime estimation ismeaningless at this point. Instead, we collected
HDDs where the applied load fell in a certain region
using statistical techniques and assumed that the
average lifetime µ found from the reliability R(t) of
HDDs in the collection is equivalent to the lifetime t[3].
The average lifetime µ is expressed with Equation
8 in the failure rate model (Weibull distribution)
described in Section 2. Furthermore, α can beassumed to be constant within the range of ordinary
usage, although α does change when larger loads areapplied.
1µ = β • Γ  +1 
α 

(8)

3.2 E
 xamining the correlation between HDD
information and average lifetimes
With the above two assumptions, it is possible toestimate parameters C and n from the correlation
between HDD information and average lifetimes. In
this section, we provide an example of estimatingthem for Cluster A, as used before in Section 2.
3.2.1 Measuring HDD information
The measurement conditions and parameters used
to measure HDD information are given in Tables 3
and 4, respectively. After measuring the HDD information, we found the frequency distribution of HDDs
where HDD information values fell in certain
segments (i.e., the number of HDDs) and the frequency distribution of failed HDDs in each segment(i.e., the number of failures). The segments selected
were 0 to 1 and 2n–1 to 2n – 1 (where n is an integer
larger than 1). The segments were selected using theknowledge that the equations are logarithmic.
From the number of HDDs and the number of
failures, we found the reliability R(t) in each segment
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lifetime. Of the HDD information measurement
parameters in Table 4, some degree of correlation was
found for the number of aborted commands, the number of media failures, and the number of connection
failures. Parameters C and n were estimated using
this correlation.
The correlation between aborted commands A.C.
and average lifetime µ is shown in Fig. 4. We performed a linear regression analysis with the expectation of a negative linear relationship between thelogarithm of aborted commands and the logarithm of
average lifetime. Equation 9 was obtained as an
approximation curve.

Table 3. HDD information measurement conditions.
Targeted cluster

Cluster A

Targeted HDDs

HDDs operating on the reference date
(1140 devices in total, 83 failed devices)

Measurement date

September 1, 2010

Measurement values

Directly recorded values for eachmeasurement parameter (see Table 4)

Table 4. HDD information measurement parameters.
Parameter

Description

Aborted commands

Number of times commands issued by
the RAID controller were aborted

Media failures

Number of times spare sectors wereassigned because of HDD media failures

Connection failures

Number of times the RAID controller
failed to connect to the HDD

Parity errors

Number of parity errors

Hardware errors

Number of hardware errors

1n µ =−0.1410×1n A.C.+7.169

(9)

Note that the Pearson product-moment correlation
coefficient is R = -0.7555.
The correlation between media failures M.E. andaverage lifetime µ is shown in Fig. 5. We performed
a linear regression analysis again, and Equation 10
was obtained as an approximation curve.

S.M.A.R.T. warnings Number of S.M.A.R.T. parameter warnings
S.M.A.R.T: self-monitoring, analysis, and reporting technology

1n µ =−0.6649×1n M.E.+6.744
and found β with Equation 2 and the average lifetimeµ with Equation 8. Because α was assumed to beconstant, the value found in Section 2 (α = 1.508) wasapplied.
3.2.2 Identifying HDD information needed forcorrelation with average lifetime
Using the method described above, we found a correlation between HDD information and the average(10)

Note that the Pearson product-moment correlation
coefficient in this case is R = -0.9616.
The correlation between connection failures L.F.
and average lifetime is shown in Fig. 6. We performed
a linear regression analysis again, and Equation 11
was obtained as an approximation curve.
1n µ =−0.2776×1n L.F.+6.535

(11)

Average lifetime µ (in days)

10,000

1000

100

10

1
0.1

1

10

100

1000

10,000100,000

Central value of aborted commands A.C.

Fig. 4. Correlation between aborted commands and the average lifetime.

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Average lifetime µ (in days)

10,000

1000

100

10

1
0.1

1

10

100

Central value of media failures M.E.

Fig. 5. Correlation between media failures and average lifetime.

Average lifetime µ (in days)

10,000

1000

100

10

0.1

1

1

10

100

Central value of connection failures L.F.

Fig. 6. Correlation between connection failures and average lifetime.

3.2.3 Validating the correlations
From Figs. 4, 5, and 6, it is not possible to determine whether the correlations are statisticallysignificant. Therefore, we examined the correlations
with a significance test using Pearson productmoment correlation coefficients. The test statistic t0
in the significance test is expressed by Equation 12.
t0 = R√n−2

√1−R2

(12)

Here, n is the sample size (number of data points) and
R is the sample correlation coefficient. Because t0 follows the t distribution with n – 2 degrees of freedom,the significance level P was found to be P=Pr{| t | >_t0}. The results are listed in Table 5. With the acceptable level of significance set to 5% (P <_ 0.05), thenumbers of aborted commands, media failures, andconnection failures were recognized as having statistically significant correlations with the average lifetime.
3.3 Method for estimating HDD lifetimes
Finally, we describe a method for estimating HDD
lifetimes. First, we ascertained the past failure statistics of the targeted cluster and examined the correlation
between HDD information and average lifetime.
When given a specific HDD, the method measures
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Table 5. Values of variables in significance test.
Sample size n

Correlation
coefficient R

Test statistic t0

Aborted commands

11−0.7555

3.460

0.03837

Media failures

4−0.9616

4.957

0.007162

Connection failures

7−0.7879

2.861

0.03538

Tested factor

Significance level P

Average lifetime µ (in days)

10,0001000

100

Trial period10
Remaining life
1
0.1

1

Actual result

10

100

Central value of media failures M.E.

Fig. 7. Estimation of remaining life of an HDD.

the HDD’s information, fits the information to known
correlations, and estimates the average lifetime. The
remaining life can be calculated by subtracting thetime in use to date. If an HDD has a short remaining
life, it is replaced with a new one.
For example, the lifetime of an HDD with four
media failures (measured value) is estimated using
Equation 10 to be 338 days, given the correlation
between media failures and average lifetime shown in
Fig. 7. If the HDD had been in use for 328 days at thetime of the measurement, it would be replaced with a
new HDD since its remaining life would be only 10
days.
The R&D cloud is managed and operated through asystem that displays resources such as power consumption. We plan to integrate this method with theresource visualization system, as illustrated in Fig. 8,
in the future to let cloud operators ascertain theremaining lives of HDDs at a glance.
4. Conclusion
In this article we described a method of handling
HDD failures that occur frequently in the operation ofVol. 10 No. 9 Sep. 2012

an R&D cloud. In Section 1, we indicated two problems associated with HDD failures: how to ascertain
the failure frequency of each HDD and how to ascertain the lifetime of a specific HDD.
In Section 2, we described an approach to the firstproblem. We began by examining the past HDD failure statistics of a high-failure-rate cluster (Cluster A).
We found the HDD lifetime distribution from thefailure statistics and fitted them to three lifetime distribution models. Using the Akaike information criterion, we demonstrated that the Weibull distribution(with shape parameter α of 1.508) was the best fit for
the HDD lifetime distribution. We then applied thisfinding in a method for estimating the necessary
number of replacement HDDs.
In Section 3, we described an approach to the second problem. We began by assuming that HDD failure physics follows the rate process model and examined the correlation between information about HDD
output by RAID controllers and the average lifetimein Cluster A, which was also used in Section 2. We
described how a statistically significant correlation
was recognized between specific HDD information(numbers of aborted commands, media failures, andRegular Articles

Rack #0

Rack #1

Rack #2

Rack #3

Rack #4

Rack #5

Rack #6

Rack #7

Rack #8# # # #
0 1 2 3

# # # #
0 1 2 3

# # # #
0 1 2 3

# # # #
0 1 2 3

# # # #
0 1 2 3

# # # #
0 1 2 3

# # # #
0 1 2 3

# # # #
0 1 2 3

# # # #
0 1 2 3

Positions#0−#9

Positions#10−#19

Positions#20−#29

Positions#30−#39

Fig. 8. Conceptual image of integration with visualization system.

connection failures) and the average lifetime using asignificance test with Pearson product-moment correlation coefficients. We then applied this finding in amethod for estimating the remaining life of HDDs sothat HDDs with short remaining lives can be replacedon a preventative basis during regular maintenance.
We also described the concept of integrating thiswork with resource visualization systems.
The results of this research will enable us to reduce
the operations for preparing replacement HDDs andthose for replacing HDDs when HDD failures occur.
Consequently, the operation of large-scale cloudcomputing environments, such as an R&D cloud, will
become even more efficient.



References
[1]

[2]

[3]

B. Schroeder and G. A. Gibson, “Disk Failures in the Real World:
What Does an MTTF of 1,000,000 Hours Mean to You?” Proc. of the
5th USENIX Conference on File and Storage Technologies(FAST’07), pp. 1–16, San Jose, CA, USA, 2007.
E. Pinheiro, W. –D. Weber and L. A. Barroso, “Failure Trends in a
Large Disk Drive Population,” Proc. of the 5th USENIX Conference
on File and Storage Technologies (FAST’07), pp. 17–28, San Jose,
CA, USA, 2007.
J. Hiwatashi, S. Iwamura, and K. Arai, “Proposal of an HDD Failure
Rate Model for Cloud Computing Environments,” Journal of Digital
Practices, the Information Processing Society of Japan, Vol. 2, No. 3,
pp. 227–234, 2011 (in Japanese).

NTT Technical Review

Regular Articles

Jin Hiwatashi

Senior Research Engineer, Platform Technology SE Project, NTT Software Innovation Center.
He received the B.S. and M.S. degrees in electrical engineering from Waseda University,
Tokyo, in 1993 and 1995, respectively. He joined
NTT Network Information Systems Laboratories
in Yokosuka in 1995. As a result of organizational changes in April 2012, he is now in NTT
Software Innovation Center. He is a member of
the Information Processing Society of Japan(IPSJ).

Vol. 10 No. 9 Sep. 2012

Sotetsu Iwamura

Senior Research Engineer, Supervisor, Platform Technology SE Project, NTT Software
Innovation Center.
He received the B.S. and M.S. degrees alongwith the Ph.D. degree in electronic engineeringfrom the University of Tokyo in 1989, 1991, and1994, respectively. He joined NTT Telecommunication Networks Laboratories in Musashino in
1994. As a result of organizational changes in
April 2012, he is now in NTT Software Innovation Center. He is a member of IPSJ.

10Global Standardization Activities

Establishment of New ITU-T Focus
Group Related to Disaster Relief
Hideo Imanaka and Noriyuki Araki
Abstract

At the last meeting of the Telecommunication Standardization Advisory Group (TSAG) of ITU-T(International Telecommunication Union, Telecommunication Standardization Sector) in January 2012,
it was agreed that three focus groups studying new standardization areas for a limited period would be
newly established in January 2012. This article provides a little background information about theirestablishment and touches on their study directions, especially for the focus group dealing with disaster
relief. In addition, this article also introduces the main results of the TSAG meeting.

1. Overview of ITU-T TSAG meeting
ITU-T TSAG (International Telecommunication
Union, Telecommunication Standardization Sector,
Telecommunication Standardization Advisory
Group) discusses strategic plans and working methods related to ITU-T standardization, unlike a Study
Group (SG) whose role is to make Recommendations. Dr. Hideo Imanaka of NTT’s R&D Planning
Department, one of the authors of this article, regularly participates in TSAG meetings. TSAG discusses
the SG structure of ITU-T for the next period, working methods for subjects that overlap two or moreSGs, and the establishment of Focus Groups (FGs),
which undertake intensive study of new standards.
As shown in Table 1, the TSAG meeting in January
2012 established three new FGs and two Joint Coordination Activities (JCAs). The FGs are related todisaster relief, service aspects of machine-to-machine(M2M) communication, and ways to bridge the standards gap for developing countries; the JCAs will
work on a structure for making ITU-T Recommendations based on the deliverables of two FGs that concluded their work last December, namely FG Smarton the Smart Grid and FG Cloud on cloud computing.
2. Establishment of FG on disaster relief
2.1 Background
The Great East Japan Earthquake and the accompa

nying tsunami devastated a large part of the east coastof Japan on March 11 2011. At the subsequent annual
CTO (Chief Technical Officer) meeting, which provides an opportunity for discussion of future ITU-T
study subjects by CTO-level personnel from ITU-T
member companies throughout the world, Japanesecompanies including NTT addressed the importanceof standardization studies in relation to safety confirmation systems, emergency communication, andinformation and communications technology (ICT)systems to be utilized during a disaster [1].
The experience of Japanese telecommunications
companies is expected to be useful for telecommunications operators worldwide because they faced hugedamage as a result of the earthquake, tsunami, andnuclear power plant accident. In addition, Japanese
telecommunications operators have a lot of experience regarding network resiliency to both earthquakes
and typhoons, and this experience is going to be put
to use after the flooding in Thailand in November
2011. At TSAG, the Japanese government led thediscussion on establishing an FG on disaster relief asrequested by the ITU-T director, and it was agreed toestablish a new FG in TSAG.
The new FG covers (1) disaster relief systems,
which provide relief by using ICT in the event of adisaster, (2) network resiliency, which provides
sustainable communication even during a disaster,and (3) network recovery, which restores network
facilities. Therefore, the new FG was named “Focus
Group on Disaster Relief systems, Network Resilience
NTT Technical Review

Global Standardization Activities

Table 1. Major results of TSAG meeting in January 2012.
StatusFormer FG

New organization

Chair

Description

New

FG DR&NRR

NTT (Japan)

Disaster relief

New

FG M2M

CATR (China)

e-Health (initial target)New

FG Innovation

NSN (Germany)

Gap analysis

Complete FG Smart

JCA-SG&HN

Lantiq (Germany)

Expansion of JCA-HN

Complete FG Cloud

JCA-Cloud

CISCO (USA)New WP in SG13
WP: Working Party

Disaster relief
systems

Disasterdetection

Healthcare
for victims

Emergency
alerts
Evacuation
assistance

Safetyconfirmation

Emergency telecommunications
Telecommunications in
disaster areas

Network
resiliency andrecovery

Restoration of damaged
base stations
Highly reliable telecommunication
network

Electric power
supplyTemporary telephone services

Highly reliable power supply
Emergency generatorsand batteries
Before disaster

Assured electric power supply
including refueling method
Disasterhits

AftermathAfterdisasterFig. 1. Expected study scope of FG-DR&NRR.

and Recovery (FG DR&NRR)” [2].
2.2 Objectives of FG DR&NRR
The expected study area of this FG is very wide.
Some examples of study items are shown in Fig. 1.
First, we must find a way of guaranteeing a highly
reliable electric power supply, for example, by using
multiple electrical distribution routes and redundant
electrical generation systems. We must also find away to ensure that electric power, including a methodof refueling generators, is available in the event of adisaster. Next, we must consider the subjects of network resiliency and recovery by both developing ahighly reliable network design such as multiple network routes and ensuring that emergency telecommunications is available by establishing temporary
telephone services when a disaster occurs and restorVol. 10 No. 9 Sep. 2012

ing damaged mobile base stations as soon as possible
afterward. As regards disaster relief systems, we muststudy emergency warning systems, systems to assist
evacuation by using digital signage, and safety confirmation systems that use the World Wide Web andvoice messages. In addition, we need to consider ahealthcare system for disaster victims who are livingin shelters as a disaster relief system that can help
ease mental stress. The FG may not cover all of thesesubjects, but its study area must nevertheless be very
wide ranging.
In this FG, the goal is to specify disaster categories
and use cases for disaster relief, describe the requirements for tackling disasters, and provide an analysisof the gap between standardization activities and subjects requiring standardization.

Global Standardization Activities

2.3 Plans for FG-DR&NRR
The chairman of this FG is Noriyuki Araki, theother author of this article, because Japan was deeply
involved in the establishment of this FG at TSAG
meetings, and NTT has a sense of responsibility that
motivates it to contribute to society.
The first meeting of this FG was held in Geneva
during the last week of June 2012. Following the firstmeeting, it is planned to hold an FG meeting once
every few months. If at all possible, the meetingshould be held in countries that have experienced ahuge disaster in recent years, such as Indonesia, Thailand, Turkey, and Chile.
3. Overview of other FGs and JCAs
3.1 FG M2M
It was agreed to establish an FG on the service layerof machine-to-machine communication. This groupis called FG M2M, as shown in Table 1. At first, it
seemed that it might be difficult to obtain agreement
to establish this FG because the difference between
M2M and the “Internet of Things” (IoT), which isalready being studied in ITU-T’s IoT-GSI (GSI:
Global Standards Initiative), is unclear and because itis related to “oneM2M”, whose establishment isbeing planned by several regional standardization
bodies such as the European Telecommunication
Standards Institute (ETSI). Finally, after consideration of the close connection between ITU and theWorld Health Organization (WHO), the establishment of the FG was agreed and the first target wasdetermined to be e-Health as an M2M application.
The chairman of FG M2M comes from the China
Academy of Telecommunication Research (CATR).
The first meeting was held in Geneva in April 2012
and the second meeting was held in Beijing in June
2012. In the last week of April 2012, a joint WHOITU workshop on e-Health was convened, and it discussed the need for e-Health standardization.
3.2 FG Innovation
To undertake a gap analysis of telecommunication
standards by analyzing the best practices of ICT innovation, it was agreed to establish an FG on innovation, called FG Innovation, on the basis of a German
proposal. The chairman of this FG is from a German
company. This FG will discuss how to deploy standardized technologies in developing countries.

standardization of the Smart Grid from 2010, and its
work was completed in December 2011. In this
TSAG, it was agreed to establish JCA-Smart Gridand Home Network (JCA-SG&HN), which will
coordinate standardization activities within ITU andwith other standards development organizations(SDOs) to make ITU-T Recommendations based on
the FG deliverables. This JCA is the former JCA-HN,
which coordinated home network standardization
work, and the smart grid issue is included in it
because of the close relationship between these subjects.
3.4 JCA-Cloud
An FG on cloud computing, called FG Cloud, completed its work in December 2010. It was agreed toestablish JCA-Cloud, which will coordinate standardization activities within ITU and with otherSDOs to make ITU-T Recommendations based on
the FG deliverables. It was also agreed to establish aworking party, which is a study organization within
an SG that handles several Questions, in SG13: theStudy Group that covers future networks.
4. Future work plan
The newly established FG DR&NRR will not only
study the standardization necessary to achieve networks that are robust in the event of disasters, but also
play the role of distributing effective ways of proceeding during a disaster anywhere in the world
based on the experiences of Japanese companies,
including the NTT Group, with a view to contributing
to society. NTT has been affected by many disasters
such as the Great East Japan Earthquake, the Hanshin-Awaji Earthquake, many huge typhoons, and thevolcanic eruption of Mt. Unzen. We believe that passing on what we have learned from these experiences
to the world will repay, in some small way, the many
countries that provided great help and huge support
after our recent disaster.
References[1][2]

CTO Meeting Geneva Communiqué.
http://www.itu.int/en/ITU-T/tsbdir/cto/Documents/111025/
Communique-final.pdf
FG DR&NRR.
http://www.itu.int/en/ITU-T/focusgroups/drnrr/

3.3 JCA-SG&HN
An FG on smart grids, called FG Smart, studiedNTT Technical Review

Global Standardization Activities

Hideo Imanaka

Senior Manager, NTT R&D Planning Department.
He received the B.E., M.E., and Ph.D. degrees
in electrical engineering from Mie University in
1985, 1987, and 2001, respectively. After joining
NTT Telecommunication Network Laboratories
in 1987, he was engaged in research on a fiber
optic access network architecture and network
operation process reengineering methods. From1996 to 2003, he worked on enterprise resource
planning (ERP) systems integration as a consultant in the Solutions Business Division of NTT
Communications. Since 2004, he has been
involved in NGN standardization work at ITU-T.
He was the Rapporteur of Question 1 of SG13
from 2007 to 2010. He has also played an activerole in IPTV standardization work at ITU-T. He
is currently in charge of standardization strategy
in the NTT Group. He received the ITU-AJ
Award from the ITU Association of Japan in
2009. He is a member of the Institute of Electronics, Information and Communication Engineers(IEICE) and the Society of Instrument and Control Engineers.

Vol. 10 No. 9 Sep. 2012

Noriyuki Araki

Senior Research Engineer, Access Media Project, NTT Access Network Service Systems Laboratories.
He received the B.E. and M.E. degrees in electrical and electronic engineering from Sophia
University, Tokyo, in 1993 and 1995, respectively. He joined NTT Access Network Systems
Laboratories, Ibaraki, in 1995. He then workedon the R&D of operation and maintenance systems for optical fiber cable networks. Since
2006, he has been engaged in standardization
work for outside plant in ITU-T SG6. He has
been the Rapporteur of Question 17 of ITU-T
SG15 since 2008. He has also been contributing
to the activities of IEC TC86, Fibre Optics, since2007. He is currently serving as the chairman of
ITU-T FG-DR&NRR. He is a member of
IEICE.



External Awards
COIN2012 IEEE ComSoc JC Young Engineer Award
Winner: Takafumi Tanaka, NTT Network Innovation Laboratories
Date: May 31, 2012
Organization: IEEE Communications Society Japan Chapter
For “Comparative Study of Optical Networks with Grid Flexibility
and Traffic Grooming”.
We comparatively investigated different optical network architectures characterized by grid flexibility and sub-wavelength traffic
grooming. Simulation shows that the elastic models are more effective than fixed-grid models in terms of spectrum efficiency, but thetrade-off between grooming gain and its additional cost should beconsidered, especially in elastic models.
Published as: T. Tanaka, A. Hirano, and M. Jinno, “Comparative
Study of Optical Networks with Grid Flexibility and Traffic Grooming,” Proc. of the 10th International Conference on Optical Internet(COIN2012), Yokohama, Japan, 2012.
ISMM 2012 Best Poster Award
Winners: Yuzuru Iwasaki, Tsutomu Horiuchi, Michiko Seyama,
Toru Miura, and Emi Tamechika, NTT Microsystem Integration
Laboratories
Date: June 12, 2012
Organization: 2012 The 4th International Symposium on Microchemistry and Microsystems (ISMM)
For “Quick and Simple Measurement for Antibody Array in Disposable Microfluidic Device Using Surface Plasmon Resonance”.
We have developed a disposable microfluidic device that is used inconjunction with a surface plasmon resonance (SPR) instrument toperform a quick, simple immunoassay. The key technologies we havedeveloped are a data processing method for finding the locations ofactive antibodies, in-situ measurement of the flow velocity, and adisposable microfluidic device with an integrated capillary pumping
system. They make it possible to detect antigens in milk every 10
minutes.
2012 TTC Distinguished Service Award
Winner: THironori Ohata, NTT Cyber Solutions Laboratories
Date: June 21, 2012
Organization: The Telecommunication Technology Committee(TTC)
For achievement of standardization activity concerning multimedia systems.
2012 TTC Distinguished Service Award
Winner: Yoshiyuki Mihara, NTT Cyber Solutions Laboratories
Date: June 21, 2012
Organization: The Telecommunication Technology Committee(TTC)

Second Place in the Hans Gros New Investigator Award
Poster Presentation Competition
Winners: Dan Mikami†1, Toshitaka Kimura †1, Kouji Kadota†2,
Makio Kashino†1, and Kunio Kashino†1†1	 NTT Communication Science Laboratories†2 Osaka University

Date: July 6, 2012
Organization: 30th International Conference on Biomechanics inSportsFor “INTER-TRIAL DIFFERENCE ANALYSIS THROUGHAPPEARANCE-BASED MOTION TRACKING”.
The purpose of this study is to develop a method for quantitativeevaluation and visualization of inter-trial differences in the motion of
athletes. Previous methods for kinematic analysis of human movement have required attaching specific equipment to a body segment
or can only be used in an environment designed for analyses. Therefore, they are difficult to use for observing motions in real games. To
enhance the applicability to real-game situations, we propose appearance-based motion tracking. Our method only requires an image
sequence from a camera. From the image sequence, automatic detection of trials and a difference analysis of them are conducted. We
applied our method to the analysis of pitching motions in actual baseball games. Though we have no quantitative evaluations yet, theexperimental results imply the efficacy of our method.
Published as: D. Mikami, T. Kimura, K. Kadota, M. Kashino, and K.
Kasino, “INTER-TRIAL DIFFERENCE ANALYSIS THROUGHAPPEARANCE-BASED MOTION TRACKING,” Proc. of the 30
International Conference on Biomechanics in Sports, Melbourne,
Australia, 2012.
Best Paper Award
Winners: Hiroyuki Shindo†1, Yusuke Miyao†2, Akinori Fujino†1, andMasaaki Nagata†1†1	 NTT Communication Science Laboratories†2 National Institute of Informatics

Date: July 10, 2012
Organization: The 50th Annual Meeting of the Association for
Computational Linguistics
For “Bayesian Symbol-Refined Tree Substitution Grammars for
Syntactic Parsing”.
Published as: H. Shindo, Y. Miyao, A. Fujino, and M. Nagata,“Bayesian Symbol-Refined Tree Substitution Grammars for Syntactic Parsing,” Proc. of the 50th Annual Meeting of the Association for
Computational Linguistics, pp. 440–448, Jeju, Korea, 2012.

For achievement of standardization activity concerning home network topology identifying protocol.

NTT Technical Review

Papers Published in Technical Journals andConference Proceedings
Explicit beat structure modeling for non-negative matrix
factorization-based multipitch analysis
K. Ochiai, H. Kameoka, and S. Sagayama
Proc. of ICASSP 2012, Vol. 1, No. 1, pp. 133–136, Kyoto, Japan.
This paper proposes model-based non-negative matrix factorization (NMF) for estimating basis spectra and activations, detectingnote onsets and offsets, and determining beat locations, simultaneously. Multipitch analysis is a process of detecting the pitch and onsetof each note from a musical signal. Conventional NMF-based
approaches often lead to unsatisfactory results very possibly becauseof the lack of musically meaningful constraints. As music is highly
structured in terms of the temporal regularity underlying the onset
occurrences of notes, we use this rhythmic structure to constrain
NMF by parametrically modeling each note activation with a Gaussian mixture and derive an algorithm for iteratively updating model
parameters. It is experimentally shown that the proposed model outperforms the standard NMF algorithms as regards onset detection
rate.

Comparative evaluations of various harmonic/percussive
sound separation algorithms based on anisotropic continuity of spectrogram
H. Tachibana, H. Kameoka, N. Ono, and S. Sagayama
Proc. of ICASSP 2012, Vol. 1, No. 1, pp. 465–468, Kyoto, Japan.
In this paper, we explore several algorithms to find the best performing algorithm for harmonic and percussive sound separation(HPSS) based on anisotropic continuity of a spectrogram throughcomparative evaluation of their experimental performance. Separating harmonic and percussive sounds is useful as a preprocessor for
many music analysis purposes, including chord estimation andrhythm analysis, and other music information retrieval tasks. We haveintroduced a method called HPSS (Harmonic/Percussive Sound
Separation) that decomposes a music signal into two components byseparating the spectrogram into horizontally continuous and vertically continuous components, which roughly correspond to harmonicand percussive sounds, respectively. Though there are many possible
ways to achieve the HPSS algorithm on the basis of this concept, it
has not been known which algorithm performs best. This paper
describes five different HPSS algorithms and compares their performances for real music signals.

Constrained and regularized variants of non-negative
matrix factorization incorporating music-specific constraints
H. Kameoka, M. Nakano, K. Ochiai, Y. Imoto, K. Kashino, and S.
Sagayama
Proc. of ICASSP 2012, Vol. 1, No. 1, pp. 5365–5368, Kyoto,
Japan.
Music spectrograms typically have many structural regularities
that can be exploited to help solve the problem of decomposing agiven spectrogram into distinct musically meaningful components. In
this paper, we introduce new variants of the non-negative matrix
factorization concept that incorporate music-specific constraints.

Vol. 10 No. 9 Sep. 2012

RoF-DAS over WDM-PON using bandpass-sampling andoptical TDM techniques as universal network for broadband wireless access
K. Tsukamoto, T. Iwakuni, K. Miyamoto, T. Higashino, S. Komaki, T. Tashiro, Y. Fukada, J. Kani, N. Yoshimoto, and K. Iwatsuki
Proc. of the 31st Progress In Electromagnetics Research Symposium, Vol. 1, No. 1, pp. 491–495, KL, Malaysia, 2012.
With the explosive growth of mobile communication traffic, the airinterface of mobile services has rapidly improved, and new services
are being provided in new radio frequency bands. Moreover, to effectively use frequency or network resources and avoid their bottlenecks, the use of femto-cell or pico-cell architectures has been started. Backhaul networks accommodating a huge number of thesepico- and femto-cell stations have become a more important issue.
Broadband fixed access networks such as fiber-to-the-home (FTTH)
systems will play an important role as an entrance network for broadband wireless access. Radio-over-fiber (RoF) technologies can transparently transmit various types of radio services, and their use as
entrance networks achieves an effective universal platform for newly
arriving air interfaces. RoF-DAS (DAS: distributed antenna system)
over a wavelength-division-multiplexing passive optical network(WDM-PON) using bandpass-sampling and optical time-divisionmultiplexing (TDM) techniques has been proposed as a universal
entrance network for broadband wireless access. This paper introduces its proposed architecture and reports the latest results ofexperimental and theoretical investigation of the transmission performance.

Effect of source-motion and self-motion on the resettingof auditory scene analysis
H. Kondo, D. Pressnitzer, I. Toshima, and M. Kashino
Acoustics 2012, Journal of the Acoustical Society of America, Vol.
131, No. 4, p. 3268, Hong Kong, China.
Auditory scene analysis needs to parse the incoming flow of acoustic information into perceptual streams, such as distinct musicalmelodies or sentences from a single talker. Previous studies havedemonstrated that the formation of auditory streams is not instantaneous: rather, streaming builds up over time and can be reset by sudden changes in the acoustics of the scene. The present study examined the effect of changes induced by voluntary head motion on
streaming. A telepresence robot in a virtual-reality setup was used todisentangle all potential consequences of head motion: changes inacoustic cues at the ears, changes in apparent sound location, andchanges in motor processes. The results showed that self-motion
induced resetting of auditory streaming. An additive model analysis
further revealed that resetting was largely influenced by acoustic cues
and apparent sound location rather than by non-auditory factors
related to head motion. Thus, low-level changes in sensory cues canaffect perceptual organization, even though those changes are fully
accounted for by self-motion of the listener. These results may reflect
a widely distributed neural architecture for the formation of auditory
streams.

One-by-one H2 bubble counting during water electrolysis
with a chemical electrometer
N. Clement, K. Nishiguchi, J. -F. Dufreche, D. Guerin, A. Fujiwara,Papers Published in Technical Journals and Conference Proceedings

and D. Vuillaume
MRS Meeting, Materials Research Society, San Francisco, CA,
USA, 2012.
We demonstrated the detection of hydrogen bubbles by using an
ion-sensitive field-effect transistor (FET) based on a silicon transistor. Hydrogen bubbles are generated during electrolysis of an electrolyte and change the Nernst potential. Since the FET has high charge
sensitivity, a small change in the Nernst potential caused by bubbles
with a diameter of a few micrometers can be detected by the FET.

Layered boron nitride as a release layer for mechanicaltransfer of GaN-based devices
Y. Kobayashi, K. Kumakura, T. Akasaka, and T. Makimoto
Nature, Vol. 484, pp. 223–227, 2012.
Nitride semiconductors are the materials of choice for a variety ofdevice applications, notably optoelectronics and high-frequency/
high-power electronics. One important practical goal is to realize
such devices on large, flexible, and affordable substrates, on which
direct growth of nitride semiconductors of sufficient quality is problematic. Several techniques—such as laser lift-off—have been investigated to enable the transfer of nitride devices from one substrate toanother, but existing methods still have some important disadvantages. Here we demonstrate that hexagonal boron nitride (h-BN) canform a release layer that enables the mechanical transfer of gallium
nitride (GaN)-based device structures onto foreign substrates. The
h-BN layer serves two purposes: it acts as a buffer layer for thegrowth of high-quality GaN-based semiconductors, and it provides ashear plane that makes it straightforward to release the resulting
devices. We illustrate the potential versatility of this approach byusing h-BN-buffered sapphire substrates to grow an AlGaN/GaN
heterostructure with electron mobility of 1100 cm2 V−1 s−1, an
InGaN/GaN multiple-quantum-well structure, and a multiple-quantum-well light-emitting diode. These device structures, ranging inarea from 5 mm × 5 mm to 2 cm × 2 cm, are then mechanicallyreleased from the sapphire substrates and successfully transferred
onto other substrates.

First-principles study of nonclassical effects in siliconbased nanocapacitors
H. Kageshima and A. Fujiwara
Phys. Rev. B, Vol. 85, No. 20, 205304, 2012.
Properties of silicon-based nanocapacitors are studied from firstprinciples. The nanocapacitor consists of electrodes of the siliconbased material planar polysilane. Nonclassical effects are analyzed
by changing both the electrode spacing and the applied bias simultaneously. Even when the electrode spacing is fixed, the effective
electrode spacing decreases with applied bias because of the quantum
capacitance effect. In addition, when the electrostatic capacitance isanalyzed in detail, it is also found that the effective electrode surfacechanges in a complicated manner with electrode spacing and appliedbias because of the dielectric polarization effect of the electrodematerial. The dielectric polarization effect is one order of the magnitude smaller than the quantum capacitance effect, which is due to thenature of the electrode material, planar polysilane. It is clarified thata nanocapacitor is governed by the detailed properties of the electronic states of the electrode materials as well as the geometry.

Experimental Demonstration of Spatial Correlation Reduc-tion Effect at MIMO Channel in RoF-DAS over WDM-PON
SystemT. Iwakuni, K. Miyamoto, T. Higashino, K. Tsukamoto, S. Komaki, T. Tashiro, Y. Fukada, J. Kani, N. Yoshimoto, and K. Iwatsuki
Proc. of APMP 2012, p. PA-14, Kyoto, Japan.
A radio-over-fiber distributed antenna system (RoF-DAS) over a
wavelength-division-multiplexing passive optical network (WDMPON) with multiple-input multiple-output (MIMO) has been proposed. The DAS has the effect of reducing the spatial correlation
between MIMO channels. This paper describes an experimental
demonstration of the spatial correlation reduction effect in an RoFDAS system compared with a conventional concentrated antenna
system.

A Proposal of Multi-Carrier Channel Control for Mobile
Satellite Communication Systems to Make Maximum Useboth of Transponder Bandwidth and Transmission PowerK. Nakahira, T. Sugiyama, H. Nishiyama, and N. KatoIEICE Trans. on Communications (Japanese Edition), Vol. J95-B,
No. 5, pp. 662–676, 2012.
This paper proposes a novel channel control technique for applying the propagation variation condition of mobile satellite earth stations. It is based on adaptive multi-carrier channel allocation(AMCA) that adjusts a modulation method carrier-by-carrier to flexibly utilize the satellite bandwidth and power simultaneously. Since
the proposed technique controls transmission power and re-allocates
AMCA channels in accordance with the earth station’s propagation
gain, it enables each of the channel resources needed for communication between stations to be maintained at a minimum level. Simulation evaluation shows that the proposed technique utilizes satellite
resources more efficiently than a conventional method that fixes thetransmission power for all earth stations. As a result, it enhances thesystem capacity by about 1.6 times.

Phase sensitive amplification with noise figure below the
3 dB quantum limit using CW pumped PPLN waveguide
M. Asobe, T. Umeki, and O. Tadanaga
Optics Express, Vol. 20, No. 12, pp. 13164–13172, 2012.
The noise figure (NF) of a phase sensitive amplifier (PSA) basedon a continuous-wave (CW) periodically poled LiNbO3 (PPLN)
waveguide was evaluated in the optical and electrical domains. Phasesensitive amplification was realized using degenerate parametricamplification in the PPLN waveguide, which was pumped by thesecond harmonic frequency of the signal. Second harmonic pumping
enables direct observation of the intrinsic amplified spontaneousemission (ASE), which determined the NF of the PSA. An NF below
the 3 dB quantum limit was obtained by observing the intrinsic ASE.
The low NF was also confirmed via the noise floor measurement of acascaded PSA and erbium doped fiber amplifier in the electrical
domain. The PSA was used as a preamplifier for detecting a 40 Gbit/s
phase shift keying signal. The low noise characteristics were confirmed by the improved sensitivity.

Extendable point-to-multi-point protocol processor for
10G-EPON MAC SoCs
N. Miura, A. Miyazaki, J. Kato, N. Tanaka, M. Urano, M. Nakanishi, and T. Shibata
Proc. of the 2012 IEEE International Symposium on Circuits and

NTT Technical Review

Papers Published in Technical Journals and Conference Proceedings

Systems (ISCAS), Vol. 2012, No. 1, pp. 1464–1467, Seoul, Korea.
This paper presents a software-hardware co-operative protocol
processor for media-access-control (MAC) system-on-a-chip (SoC)
devices in a 10 Gigabit Ethernet passive optical network (10GEPON), designed with a software-hardware division technique that
focuses on the throughput and timing-accuracy requirements. This
protocol processor consists of frame-processing hardware to meet thetiming requirements, an interface to absorb the speed difference

Vol. 10 No. 9 Sep. 2012

between software and hardware, and software to implement additional protocols. This protocol processor enables network operators
to install additional service protocols adaptively for their own services. The 10G-EPON system with this protocol processor worked
properly in the experiment.

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