R&D Spirits
Opening the Door to Quantum Computers
Dr. Hideaki Takayanagi
Director,
NTT Basic Research Laboratories
NTT R&D Fellow

The key to revolutionizing information processing can be found in the quantum computer. In
Japan, quantum computers are being heavily researched, and NTT Basic Research Laboratories
are becoming a core site for this research. To learn about the current state of progress and future
outlook of quantum computer research, we sat down with Dr. Hideaki Takayanagi, the head ofquantum computer research at NTT Basic Research Laboratories and an NTT R&D Fellow.

Performing astronomically large calculations
instantly using the basic principle ofquantum mechanical uncertainty
Editor: Dr. Takayanagi, what exactly is a quantum
computer, the subject of your research?
Takayanagi: The conventional computer operates on
the basis of Newtonian mechanics, which means thatit conforms to classical mechanics. We call it a “classical computer” for this reason. In contrast, a quantum computer operates on the basis of uncertainty, a
key feature of quantum mechanics, where “the movement of an object can be determined only as a probability.” In other words, while a bit can be clearly
expressed as either a 0 or 1 in a classical computer, aquantum computer is concerned with the probability
that a 0 or 1 state exists. Consequently, a single data
element might simultaneously be 0 and 1, that is, it
may take two different states at the same time, as longas it is not observed. This is called superposition.
Since n elements result in 2n states, we can see that
100 elements in a quantum computer could represent
2100 states simultaneously. This ability to represent ahuge number of states should make it possible to perform massively parallel computing in which ultrahigh speeds are achieved on the whole as opposed toeach individual element running at high speed. This is
the general principle behind the quantum computer.
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E: Why are quantum computers receiving so muchattention at this time?
T: In 1994, it was shown that unique factorization
could be solved in an extremely efficient manner byan algorithm discovered by Peter Shor of AT&T. This
discovery had a major impact on the information
sharing society. To put this in perspective, existing
encryption technology is based on the fact that it is
impossible for all practical purposes to uniquely factor a 100-digit or larger integer. Just a 20-digit integer, for example, would take about 10,000 years touniquely factor using today’s fastest computers. A
quantum computer, however, could solve the sameproblem in 1/1,000,000 of a second, which would
clearly shake the foundation of encryption technology. This, in turn, would have grave effects on Internet
business and national defense, which explains thesudden interest in quantum computers. I would like topoint out here that quantum computers are notexpected to replace classical computers—they will
supplement them.
E: What could be done with quantum computers?
T: First of all, it would be natural to consider applications in information communications since this is
the field in which research on quantum computers hasflourished. One example is quantum encryption. If
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Deep spacecommunications

Quantum computer

Signature/authentication security management

Instantaneous information retrieval

Bioinformatics
Quantum repeater

Drug-discovery/living-organism simulations

Full weather forecasts

Distribution/traffic simulations
Optical-quantum
info-communications path

High-speed, large-capacity
quantum file system

Ultra high-speed,
first-principles new materials simulations

Fig. 1. Future quantum computer society.

current encryption schemes can be solved by quantum computers, new encryption schemes must be
developed based on quantum mechanics. Research
has already begun on applying to encryption the phenomenon of superposition in which one element cantake on two different states simultaneously. On a
broader scope, the extent to which such a property
might be used in the world comes down to an algorithmic problem, so no one really knows. If researchin this area progresses, we might see a variety of
applications. For example, quantum computers might
be used in simulations concerning distribution, drug
discovery, and living organisms, in bioinformatics,
and in the preparation of full weather forecasts. If
such applications come true, we might see a futuresociety that is very dependent on quantum computers.
In any case, it will take several decades of R&D
before such applications can be realized (Fig. 1).

that they will be integrated in the future, we are limiting our studies to solid-state quantum bits to which
LSI technology can be applied. To be honest, we have
not yet reached the stage of a real quantum computer,but I can say that we have been successful in constructing a single quantum bit using a superconducting loop. We have shown that producing current in a
1-µm × 0.5-µm aluminum superconducting loop
under certain conditions can result in clockwise andcounterclockwise currents flowing simultaneously(Fig. 2). A superconducting loop, while small by
everyday standards, is large compared to atoms andmolecules. Nevertheless, constructing a quantum bitat this scale is highly significant. The ability to readthe state of a quantum bit by a “one-shot” measurement is also an outstanding achievement that I don’t
think has been reproduced yet anywhere else in theworld.

Successful reading of quantum bits—amilestone in achieving real quantum computers

E: What are some technical issues in this regard?

E: Please give us an overview of this research and its
current state of progress.
T: At NTT Basic Research Laboratories, we are currently focusing our research efforts on quantum-bit
molecules, optical-quantum bits, and superconducting loops. Quantum bits, by the way, come in various
types such as solid, gas, and ion, but as we anticipate
Vol. 1 No. 3 June 2003

T: I would say progress in nanotechnology and ultraprecise measurement technology. Although the quantum computer has been around for some time in termsof basic principles, a successful one has yet to be constructed despite the best efforts of many researchers.
This is not only because it has been impossible to fabricate on a very small level but also because it has
been impossible to exclude noise during measurements. It is said that the quantum mechanical super5

R&D Spirits

Superconducting ring

Quantum-bit element fabricatedwith a superconductor (inner ring)

Switching current (nA)

Clockwise
Clockwise-and counterclockwise-flowing
Counterclockwise
superconducting current
superconducting currents
superconducting current
Clockwise

4
2
0
-2
-4

Counterclockwise

0.490

0.495

0.500
Φqubit / Φ0

0.505

0.510

Simultaneous clockwise/counterclockwise
currents confirmed in the vicinity of 0.55µm

Fig. 2. Quantum-bit elements.

position state that I mentioned earlier can be maintained indefinitely as long as no observation is made,and conversely, that this state will be destroyed if an
observation is performed. Noise cannot help but bemeasured, however, and the simple addition of noise
can destroy a quantum bit. Reducing noise in measurements is therefore a major issue. Some progresshas been made here on entering the 21st century, andresearch on quantum computers has likewiseadvanced to some degree.
E: What kind of form do you think quantum computers will take three or five years down the road?
T: In about five years, we may reach the stage of practical quantum computers for quantum encryption. For
example, we may see small, several-bit quantum
computers for quantum encryption/decryption in networks. On the other hand, 100-bit and 1000-bit quantum computers are still a long way off. We, of course,
are involved in basic research, and cannot easily venture into such applications. For this reason, we feel a
bit awkward when asked what kind of results we
expect in 3- or 5-years time (smiles). The president of
NTT understands our predicament well, but in general, research is now expected to produce results quickly. If we push out results too fast, though, we may
experience the same fate as Bell Laboratories. Basic
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research, in particular, needs to take a long view ofthings.
E: What are some international and domestic trends
in quantum computer research?
T: The country that is expending the most effort on
quantum computers is the United States, and there isalso considerable research in Europe and Australia.
In Asia, there is noticeable activity in the IT nationsof Singapore and Japan. Quantum computer researchin Japan is being led by the private sector, however,and several projects are proceeding in a nearly independent manner. This is why I have promoted theestablishment of a government-level project for sometime, and just this year a move in that direction
appeared centered on the Japan Science and Technology Corporation. Perhaps I will be asked to cooperate.

Expanding into international activities as arecognized authority on quantum computers
E: What kind of research have you personally been
involved in over the years?
T: In graduate school, I was researching SQUIDs(Superconducting Quantum Interference Devices),
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which means that I was already studying quantum
bits. This research, however, did not actually concern
the quantum computer but was rather a pursuit ofhigh-sensitivity detectors. After entering NTT, myresearch in this area expanded into superconducting
Josephson devices, and as part of this research, I
demonstrated, for the first time in the world, that theJosephson effect could be achieved by combining asemiconductor and superconductor to form a superconducting transistor. After that, superconductivity
research at NTT was unfortunately scaled down, butthree researchers including myself remained in thisarea, and I went on to do a lot more basic research insuperconductivity.
E: Why did you begin researching quantum computers?
T: As I mentioned, the backbone of my current
research goes back to my university days, and myinterest in the quantum computer began during mytime as a guest researcher at the University of California, Berkeley in 1988 and 1989. Quantum
mechanics for someone studying physics is the mostcorrect interpretation of the physical world at thistime, and as such, is the ultimate area of study. One of
my desires was to find out how far and in what way
quantum mechanics could be used. That was my firstencounter with computational principles and otherphenomena in relation to quantum mechanics. Later,a more specific encounter with the quantum computer concept occurred in 1996 when I was invited to aninternational NATO conference in Curacao. Although
I was actually giving a presentation on another subject, this conference marked the first time that I began
discussing quantum computer theory with otherresearchers. I found it utterly fascinating.
E: Are you involved in any international activities
related to your research at this time?
T: Actually, I am involved in many international
activities and have been making about 5 to 10 overseas trips a year. I could never say that I don’t enjoy
these trips, as any place that brings together manyleading researchers from around the world is quite
stimulating. I have been a member of the organizing
committees for various conferences including the10th International Conference of Narrowgap Semiconductors, 23rd International Conference of Low
Temperature Physics, and 15th International Conference on Electronic Properties of Two-Dimensional
Vol. 1 No. 3 June 2003

Systems. I have also been involved with various joint
research programs with overseas researchers, including two major programs sponsored by the New Energy and Industrial Technology Development Organization (NEDO). I have been a research partner first in
Sweden and France and later in Germany and theUnited States. I have since entered into a jointresearch contract with Delft University of Technology in the Netherlands and am also involved in programs in Switzerland, Italy, the United Kingdom, andDenmark. From this spring, moreover, I will be aguest professor at Nanjing University in China, and I
think I may be able to do some joint research there as
well. As you can see, I have been involved in all sortsof activities up to now, which can certainly be stressful, and recently, I think I have become a bit disoriented as a result (smiles). I am therefore trying to
make one long overseas trip a year so that I candevote more time to research without having to thinkof other duties. Everyone asks me if that is possible,
but if I declare that I am going somewhere, it usually
happens.
E: It has been said that you are a candidate for aNobel prize. What is your personal reaction to this?
T: Well, I have attended Nobel award ceremonies
twice and have been an invited speaker at Nobel symposiums three times. Looking back at those times, I
remember that many symposium speakers wereNobel-prize recipients and that the staff of the Nobel
Foundation were saying that much attention was
being given to Japan. Perhaps that is why I have been
considered a candidate. At the same time, I know thatobtaining a Nobel prize is not that easy. It is nevertheless a great honor even to be mentioned as a candidate. If I were to receive a Nobel prize, however, it
would be for past accomplishments and not for mycurrent research on quantum computers. As my past
research has been successfully completed, my mainconcern now is my present research.

Expanding beyond its corporate framework,
NTT is becoming a core research site forquantum information processing in Japan
E: What direction would you like your research totake in the years to come?
T: First of all, I would like to be involved in research
for the rest of my life. This is also the wish of NTT.
In fact, receiving the title of NTT R&D Fellow means
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exactly that, and the reality is that you gradually
move from pure research to management of research.
As a result, I have been able to develop an eye for the“big picture,” and with this in mind, I would like tomake a sincere suggestion as to how I might promote
basic research in Japan and quantum computer
research in particular. At present, Japan is in the earlystages of quantum computer research and even thegovernment is starting to become involved. Since I
am also involved in national research in various
forms, I feel I can add to this momentum and spur on
this research. At the least, I would like to be one of thepeople providing a driving force behind this research.
E: In the light of many and varied international activities, how do you think NTT is viewed overseas?
T: NTT Laboratories on the whole and Basic
Research Laboratories in particular have received
high praise. While it would be presumptuous to saythat we might take the place of Bell Laboratories,
whose fortunes are declining, it is widely acknowledged that NTT deserves to be noticed as a major private research institution. The number of papers published by NTT in such periodicals as Science andNature is impressive for Japan and proposals for joint
research are being received constantly.
E: As a manager of one of the NTT Laboratories,
what are your hopes and challenges for the future?
T: The scale of quantum computer research at NTT is8

the largest in the nation, I believe. Nevertheless, wecannot do everything alone and joint research andinformation exchange are absolutely necessary. Even
though this is private-sector research, we cannot getaround the fact that this field is not very competitive
within Japan. Against this background, my aim is tomake Basic Research Laboratories a center of excellence for quantum information processing in Japan.
These laboratories have been very open from thestart, but I would like make it even easier for people
to come and go. My challenge as a manager is to create an environment conducive to such openness.
E: Dr. Takayanagi, what is it like to work at NTT
Laboratories?
T: Since I am not familiar with all aspects of NTT
Laboratories, I can’t say in general, but with regard to
Basic Research Laboratories, freedom in research isgood, the caliber of people is high, and funding is sufficient. I don’t think you could find a place as wellrounded as these laboratories elsewhere in the world.
There is also little interpersonal friction. Basic
research, moreover, is primarily an individual pursuit, and for independent-minded researchers, Basic
Research Laboratories is a place where they candemonstrate their abilities. At the same time, a wonderful environment might also be the cause of overly
nice behavior in young researchers, and I would
rather see more researchers who have the guts to giveme a good fight when needed. I actually like that typeof researcher very much.

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Interviewee profile■ Career highlights
1975 B.S. from the University of Tokyo, Dept. of
Basic Sciences
1977 M.S. from the University of Tokyo, Graduate
School of Arts and Sciences
1977 Joined Musashino Electrical Communication
Laboratory, NTT
1987 Ph. D. (science) from the University of Tokyo
1988 Guest Researcher in the department of physics
at UC Berkeley (one year)
1995-present Guest Professor at Tokyo University of
Science1996 Guest Professor at Delft University of Technology (Netherlands)
1997 Guest Professor at Chalmers University ofTechnology (Sweden)
1998 Guest Professor at Denmark University ofTechnology
2000 NTT R&D Fellow
2003 Concurrent Professor at Nanjing University(China)2003 Guest Professor at the University of Tokyo■ Major awards
1986
NTT R&D Headquarters Award
1994·1996 NTT Basic Research Laboratories Award
2000
Nissan Science Award
2003
Superconductor Science and Technology
Award■ Committee memberships and other posts
Research coordinator in NEDO Grants for International Joint Research (1997-1999)
Organizing committee member of the Satellite conference to LT22 (Göteborg, 1999)
Organizing committee member of the 10th International Conference of Narrowgap Semiconductors(Kanazawa, 2001)
Organizing committee member of the Int. SymposiumVol. 1 No. 3 June 2003

on Superconducting Device Physics (Tokyo, 2001)
Organizing committee member of the 23rd International Conference of Low Temperature Physics(2002).
Organizing committee member of the 15th International Conference on Electronic Properties of TwoDimensional Systems (Nara, 2003)
Chair of the Int. Symp. on Mesoscopic Superconductivity (Atsugi, 2000)
Chair of the Int. Symp. on Mesoscopic Superconductivity and Spintronics (Atsugi, 2002)
Research member of NEDO Grants for International
Joint Research (2000-2002)
Adjunct member in the Quantum Phase Electronics
Center, the University of Tokyo (2001-)
Committee member of the Atom, Molecule and Optical Science, Science Council of Japan (2001-2004)
Committee member of the Fundamental PhysicalStandards, Science Council of Japan (2002-2003)
Member of the University Evaluation Committee in
National Institution for Academic Degrees (2002)
Technological Committee member in NEDO (2002-)
Research Leader of the CREST, JST (2002-2007)
Guest Researcher of the Nanoscience Special Projectin Tsukuba University (2003-2005)■ Publications
B. J. van Wees and H. Takayanagi, “The superconducting proximity effect in semiconductor-superconductor systems,” in Mesoscopic Electron Transport,
ed. by L. L. Shon et al., (Kluwer, Dordrecht, 1997)
p. 469.
H. Takayanagi, “A mysterious boundary between asuperconductor and a normal conductor,” Kyoritu
Shuppan, Tokyo, 2000 (in Japanese).
“Towards the controllable quantum states,” ed. by H.
Takayanagi and J. Nitta, World Scientific, Singapore,2003.

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