R&D Spirits
Focusing on “Single Electrons” toward Quantum
Computing
Dr. Toshimasa Fujisawa
Distinguished Technical Member
Quantum Solid State Physics Research Group
Physical Science Laboratory
NTT Basic Research Laboratories

Research on quantum computing has been progressing throughout the world with the aim ofachieving ultrahigh-speed processing through parallel computation. As part of this world-wide
research effort, the Physical Science Laboratory at NTT Basic Research Laboratories is taking
the approach of using “single electrons.” With the research period expected to span several
decades, what stage is this research currently at and what is its final objective? We put thesequestions to Distinguished Technical Member Toshimasa Fujisawa, a key figure in this researchand the winner of the 2003 Sir Martin Wood Prize.

Taking on the nano-level world with a view
several decades into the future—Dr. Fujisawa, could you give us a brief outline ofyour current research?
Well, my research is part of a field that is generally
called “nanotechnology” and I work on determining
how accurately a single electron can be controlled
and measured on a short timescale of a certain duration [1]. Electrons have been used in electronicequipment and other devices as carriers of energy,charge, or information, and they have usually been
controlled in the form of “voltage” or “current.” But
if electrons can be confined to an extremely small
space, it should be possible to control them individually. Then, a single electron will start behaving as awave as opposed to a particle. The objective of ourresearch is to investigate the behavior of single electrons in semiconductors on the nanometer scale andto connect our findings to applications that can makeuse of the properties that we discover.
A variety of application themes can be considered
here, but at present, I am targeting two specific applications. The first is a relatively near-future application: an ultrahigh-precision ammeter that can count
6individual electrons. Since an ammeter is the mostbasic of measuring instruments, the ability to accurately measure an extremely small current, say on theorder of only 1000 electrons per second, would
undoubtedly be useful in many fields. The second
application is quantum computing that will treat asingle electron as one bit of information and exploit
its superposition state (Fig. 1). But computation cannot be achieved with only one electron—we need toincrease the number of electrons while maintaining
control of each and every one. As we have justreached the stage where a single quantum bit, orqubit, can be achieved, our research has just gotten
under way.
—In what way do you think this research can contribute to society?
Quantum computing should enable ultrahigh-speed
information processing through parallel computation(Fig. 2). At present, it is expected to be used mainlyin scientific fields such as for specialized databasesearches and quantum physics simulations. But thisresearch field has just begun, so achieving our overall
objective of a 1000-qubit-class quantum computer
could take several decades. While setting our final
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Classical picture

Quantum mechanical picture

101

e–
e–

—What technology is associatedwith controlling single electrons?

e–

or

0e–

Superposition state(candidate for quantum
computing devices)Fig. 1. Classical bit (left panel) and quantum bit (right panel) in a double quantum
dot. A quantum mechanical state can take any superposition of the twoclassical states. This superposition is a key to quantum computation.
Ordinary digital computer
Sequential data processing

0110...
AND/NOT...
transistors

calculating...
...100 years ?

Quantum digital computer
Parallel data procesing|01>+|10>
CNOT/ROT...
unitary

calculated!

Fig. 2. Comparison of ordinary digital (classical) computer (upper panel)and quantum computer (lower panel). In the quantum computing
scheme, data prepared in a superposition is processed in parallel.

Vol. 2 No. 10 Oct. 2004

target as quantum computing, ourplan is to develop applications likethe ammeter that can contribute tosociety in the mean time.

There are two main technicalaspects to our research. The first
concerns semiconductor nanofabrication technology, in which we tryto determine the extent to whichsmall structures can be fabricated atwill. The second concerns measurement technology, in which we try to
determine the maximum level atwhich precision measurements canbe made. Nanofabrication technology has been an NTT specialty for along time and our work is worldclass, but our measurement technology lags somewhat behind. This isbecause measurements of quantummechanical effects are made in an
extremely low-temperature environment to reduce noise as much as
possible, something with which
NTT did not have much experience.
Therefore, we set out to improve
our technical abilities through joint
research with European research
institutes having a history in thisfield. Through this effort, weachieved world-class measurement
capabilities in single-electron control.
—What are some current problems?
A major problem is the noise that
occurs during measurements. We
must connect the low-temperatureenvironment where the measurements are made and the laboratory
where the measurement equipment
is located. However, thermal fluctuations in the measurement equipment operating at room temperature
can be transmitted as noise over thewiring. Removing that noise is our7

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biggest challenge at present. While electrodes are
essential for controlling qubits and for counting, they
are also factors in the generation of noise. There isconsequently a tradeoff between reducing noise andmaintaining control. This is a problem that bothers
not just us but also other researchers around theworld.
—What direction is this research taking?
Most of our energy is concentrated on creating 2qubit states. If this can be done, we expect very interesting research to follow even in physical terms.
Above all it will let us prove whether the quantummechanical behavior of electrons described by theory
can actually be measured and controlled. In thisregard, we are particularly interested in quantum
entanglement experiments. Electron spin (rotation) is
expressed by the direction of rotation with clockwise
rotation indicated as “up” and counter-clockwise
rotation as “down.” Given two electrons, there are
four electron-spin combinations: up-up, up-down,down-up, and down-down. In “quantum entanglement,” however, it is possible to create a state withcorrelation in which an up spin in one electron forces
the other electron to have a down spin. While
research on this state is progressing through experiments using photons, its behavior for electrons insolid materials is still unclear. It appears that quantum
entanglement is becoming a very important research
theme.
As for applications, the establishment of basic computation techniques between one qubit and a pair ofqubits can be envisioned. If this can be achieved, I
believe the door to quantum computing will be wide
open.

Participating in joint research with overseas
institutions and linking up with industry,government, and academia—Dr. Fujisawa, what trends are taking place in quantum computing in Japan and overseas?
Research with quantum computing as a specific target is being performed here and there, and academic
societies and research conferences concerned withquantum computing are increasing. Hardly a week
goes by without a meeting on quantum computing
being held somewhere.
As for world trends, many research institutions areengaged in quantum-computing research based on
8superconductors initiated by Japanese electronics
manufacturers. These include the Saclay researchlaboratories in France and the National Institute ofStandards and Technology (NIST) in the UnitedStates. In contrast, semiconductor-based research,which is our type of research, is not as prevalent, but
it does cover a wide range of approaches from the useof optical techniques to electrical techniques, the latter being our approach. I would say that the research
being conducted at Delft University in The Netherlands and at Harvard University in the United Statesis the most similar to ours in terms of electrically controlling electrons in semiconductors.
—Are you engaged in joint research with overseas
research institutions?
In recent years, basic research has taken on aninternational style where advances are made throughcollaboration. A single team is limited in what it canaccomplish by itself. We have also been collaboratingwith a number of research institutions. For example,
a student from Denmark is currently working in ourgroup, and we have a delicate relationship with Delft
University in The Netherlands that includes both collaboration and competition. I myself was a visitingresearcher at Delft University for just under a year,
six years ago.
—Have you been involved in any other activities orscientific exchanges outside NTT Laboratories?
Well, as part of a project linking industry, government, and academia, I have been serving as a visiting
associate professor in the graduate school of Tokyo
Institute of Technology. As for scientific exchanges,
last year I received the Sir Martin Wood Prize, which
entitled me in June of this year to be an observer andmake research presentations at four British universities: Oxford, Cambridge, Nottingham, andSouthampton. This prize, which is named after thefounder of Oxford Instruments, a U.K. manufacturerof scientific instruments, is given annually to a
researcher under the age of 40 working in condensed
matter science at a Japanese research institution. A
lecture trip to the UK is given as a supplementary
prize to promote scientific exchange and research
activities between the UK and Japan. By the way,something that I was particularly impressed with on
this visit to the UK was the importance attached totradition. This was true even for research equipment.
In addition to new equipment, I also saw a number ofNTT Technical Review

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traditional but amazing devices that were used carefully with great respect. At Oxford University, forexample, there is a pulse magnet that produces a magnetic field stronger than even that of a superconducting magnet, and it is also treated with much importance. I admire this attitude immensely.

structure. At NTT Laboratories, this field of study ispromoted, and I was fortunate to be able to enter thecompany and pursue the research that I desired. I
have been involved with this research ever since.
—What kind of research in particular have you been
engaged in up to now?

Advancing research step by step while
incorporating new ideas

At first, I was involved with semiconductor fabrication technology, and I researched means of creating
nanostructures by focused ion beam implantation. I
then moved on to materials-properties research inrelation to ballistic conduction, in which the reduced
level of semiconductor impurities enables electrons
to propagate without being scattered. These studies
expanded into research of quantum dots that completely confine electrons (Fig. 3).
Then, as the next step in this research, I consideredobserving what happens when a quantum dot is irradiated with light. In general, there are two approaches to quantum-dot research: using electrical conduction and using light. Nevertheless, I thought that Imight observe something interesting if I tried combining these two approaches. However, electricalconduction and light have completely different energy scales, which means that they cannot be combineddirectly. I therefore decided to use microwaves
instead of light. Because the energy intervals of quantum dots are exactly equivalent to microwave energy,I thought that I might be able to excite electronsinside a quantum dot by irradiating it with—Could you tell us about your major field of study atuniversity? And what was your objective in entering
NTT Laboratories?
My research theme in school was impurity levels insemiconductors. A semiconductor functions because
intentionally doped impurities contribute conductive
electrons. Impurities, however, can also give rise to a
defect level in which conductive electrons becometrapped and no longer contribute to conduction. Thisis called the “impurity level.” I researched it for fiveto six years until receiving my doctorate. Thoughboth concern semiconductors, my past research wasbased on a materials-oriented approach, which differsfrom my current research. I enjoyed studying impurity levels by this approach, but I also developed aninterest in mesoscopic transport at the same time. Theword “mesoscopic” means a level between themacroscopic level of typical semiconductors and themicroscopic level of atoms. It’s a research area thatexamines how electrons behave within a very small

Quantum dot structure
300 nmLSRDGate electrodes

No. of electrons:
N < 30e ee e eeFig. 3. Double quantum dot structure fabricated in a semiconductor heterostructure.

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microwaves. This marked the beginning of myresearch on electron behavior when giving a quantum
dot a time-dependent potential by microwaves.
—What brought about your research on quantum
computing?
My research on quantum dots using microwaves
got under way in 1998, just around the time that quantum computing came to be discussed in the researchcommunity. As various quantum computing proposals were made and basic experiments performed, I
had a vague feeling that maybe my research could beconnected to quantum computing, and I began
research on achieving quantum bits.
—What have you been pursuing through yourresearch?
Well, to be honest, I can’t say that we began ourresearch with a clearly defined vision. Rather, in thearea that we were interested in, we developed ourtheme one step at a time while incorporating new
ideas and solving any problems that we encounteredalong the way. That is how we arrived at the present.
In our present research, we first devoted all our energies to achieving control of individual electrons. Thishas since become commonplace, although it is still
not perfect. As we work to improve the quality ofsuch control, perhaps we will make new discoveries,
even if only qualitative in nature.

Drawing the future by searching out
unforeseen fields—Dr. Fujisawa, how do you think your research willdevelop five and ten years down the road?
The kind of basic research that we deal withrequires a relatively long time to produce tangible
results, so I don’t think five or ten years is reallyenough time for such results to appear. On the otherhand, basic research gives researchers the possibilityof “drawing” a blueprint for the future, which can bequite exciting. But as the next step in our research, weare considering the application of scanning probe
microscope technology to control electrons both temporally and spatially. So far, we have been able tocontrol the number of electrons and control them
temporally, so our next task is to control them spatially. Our ultimate research goal is to establish various control technologies so that we can make elec10

trons do whatever we want.
—Is there anything of special concern for you as aresearcher?
Well, to begin with, I think researchers tend to confine themselves to narrow fields in order to establish
a specialty. Of course, it is important to decide on a
specialized field of study, but I don’t think yourresearch will have much impact if you do not keep aneye on the outside world. Even research results produced after much effort will not penetrate very far if
their value cannot be conveyed to society at large. For
this reason, I have recently been striving to express
specialized concepts in an easy-to-understand manner so that a greater number of people can comprehend our research activities.
—How has NTT Laboratories been advantageous foryou?
To be able to become involved in both basic
research and application-oriented research has been
truly wonderful for me. I think it would not be anexaggeration to say that NTT is currently the only private enterprise in the world pouring so much effort
into basic research. While amazing research can beperformed at universities and other research institutions, it is private enterprise that excels at returning
the fruits of research to society. We are always working to return something to society, even if that “something” is in the distant future. Another advantage of
NTT Laboratories is the freedom that is generally
given to researchers in their work. I feel very lucky tohave worked in such an environment.
—In closing, could you tell us something about yourfuture aspirations?
Yes, of course. As I touched upon earlier, researchsites are increasingly interacting with the industrial,
government, and academic sectors and with theircounterparts overseas. NTT Laboratories is likewise
promoting many collaborative projects with industry,government, and academia. My position as a guest
associate professor and lecturer at Tokyo Institute ofTechnology has enabled me to provide guidance tostudents. In addition to my desire to continue with myown research, I would love to see my research used as
educational material for students, the researchers oftomorrow. My hope is that activities such as thesewill help to expedite the spread of research results
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that are beneficial to society. That would be wonderful.
Reference[1]T. Fujisawa, “Quantum Information Technology based on Single
Electron Dynamics,” NTT Technical Review, Vol. 1, No. 3, pp. 4145, 2003.

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Interviewee profile■ Career highlights
Toshimasa Fujisawa received the M.S. and Ph.D.
degrees in engineering from Tokyo Institute of Technology in 1988 and 1991, respectively. His Ph.D. thesis was entitled “DX centers in III-V semiconductors”.
He joined NTT Basic Research Laboratories in 1991.
He became a Senior Research Scientist in 1995 and aDistinguished Technical Member in 2001. He was avisiting researcher at Delft University of Technologyfrom Dec. 1997 to Aug. 1998. He has been a Guest
Associate Professor at Tokyo Institute of Technology
since June 2003.
■ Major awards
Sir Martin Wood Prize, Millennium Science Forum,
Nov. 2003.

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