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Quantum Computer

About Dr. Tsai & Mr. Nakamura

Dr.Tsai & Mr.Nakamura

We talked with research fellow Jaw-Shen Tsai and research fellow Yasunobu Nakamura about their past and present research activities and their everyday life.

What kind of interests did you have as a child and as a student?

[Tsai] Well, I think I have liked science fiction ever since I was small. I have a vague recollection that I liked the sound of the word "physicist" as an elementary school student and that I wanted to become a scientist at that time. But on entering junior high school, I developed an interest in architecture and philosophy, and during my first two years in university, I actually studied architecture and philosophy too for about a half a year. For some reason, however, I felt that these fields did not exactly suit me, and in my third year when I had to decide on my major, I decided to talk this over with a senior student. He said that "it is better for a person to do something that he enjoys," and on hearing that, I developed confidence and chose physics as my major. I must admit that closing the door on architecture was not an easy decision, but in the end, I felt that that the world of art as a subject of study has its limits since its criteria for evaluation is subjective in nature. The clear-cut evaluation criteria of science, on the other hand, appealed to me, and this was a major reason why I chose science.

[Nakamura] I also liked natural science as a child. There were mountains and fields near my house and I always enjoyed going outside to play. I also enjoyed reading books of various types including science-fiction stories and mystery novels. In elementary school, I wanted to become an archaeologist or astronomer, but in junior high school, I developed an interest in physics and chemistry that could teach me about the origin and behavior of things. In university, I majored in applied physics.

What made you decide to enter NEC?

Dr.Tsai's photo

[Tsai] First of all, I performed my undergraduate and graduate studies in the United States, and in graduate school, I researched nothing but the Josephson junction. This field was very popular at the time, and IBM had a large-scale research project going with a staff of about 200 people. By the time I completed my doctoral course, however, this boom was dying out and job offerings in the field had declined significantly. At that time, it was common for a graduate that had studied in the United States to seek employment in an American university or company.

But also at that time, I was going out with a Japanese girl (who is now my good wife and companion Yoshiko), and her parents became quite concerned about my situation. They said "Why don't you come to Japan?" As it turned out, I wrote a letter to NEC where by chance an acquaintance of my father's was employed. I wrote: "I have been researching the Josephson junction. Could I continue that research at NEC?" That acquaintance replied saying "We certainly are researching that area at NEC. Please come." With that my coming to work at NEC was decided. My job hunting turned out to be quite simple in the end. This was in October of 1983.

Strangely enough, on October 18, the day I reported to the company, I was contacted by a friend of mine at the National Institute of Standards and Technology (NIST) in the United States saying that "there's talk about IBM terminating their large Josephson-junction project!". Although I really didn't know much about NEC then (other than recalling that my family's first TV when I was living in Japan during my first or second year of elementary school was manufactured by NEC), I realized on hearing that news that my selection was quite fortunate.

On the other hand, there were no foreigners as regular "permanent" employees at NEC at that time and I became their first one. Hiring people midway through the year was also rare at that time, and I was treated as short-term staff for the first two years of my employment. To be honest, I cannot say that I had no complaints at all about my treatment in the beginning, but I also feel that treatment of foreign employees has become much better nowadays.

[Nakamura] When I started to look for work, I visited several companies. During my studies for a master's degree, I majored in high-temperature superconductivity, but in a company, I thought that I would like do research utilizing micro-fabrication technology or what is commonly called nanotechnology nowadays. I therefore chose NEC, a company that I thought was at the forefront of that field. Later, I really came to enjoy working in Tsukuba with its abundance of greenery.

[Tsai] The period that Mr. Nakamura entered NEC (April 1992) was still a seller's market as no one was sure whether the bubble had burst. Since many graduating students were switching to other companies even after informal promises of employment, I decided to visit his university research laboratory in the autumn to try to keep him from jumping ship. I remember well how that laboratory had a strong smell of chemicals! Of course, my objective in visiting him was not just to check on his commitment to NEC but also to see with my own eyes what kind of research he was working on.

[Nakamura] When Dr. Tsai came to visit my research laboratory, I was, to be honest, quite surprised. "Could it be true," I thought, "that a man from a company is actually coming all this way to see me in this kind of place?"

Please tell us more about those times when you entered the company.

[Tsai] The name of the area that I joined on entering the company was called the High-speed Device Research Laboratory of the Microelectronics Research Laboratories. At that time, in 1983, NEC was participating in the "Josephson Computer Research" project funded by the Ministry of International Trade and Industry (MITI), and I became involved in that work. Two years later, the name of this laboratory was changed to the Advanced Devices Research Division although the subject matter of our research remained unchanged. Since then, my research efforts have dealt with no other field but this one.

[Nakamura] On being assigned to Dr. Tsai's team in 1992, I found myself in the Advanced Devices Research Division of the Fundamental Research Laboratories. At that time, research on mesoscopic physics, where "mesoscopic" means the area between "macroscopic" and "microscopic," was quite active there.

[Tsai] That's right. In fact, because the word "mesoscopic" was not readily understood, we tried using the word "nanoelectronics" instead of "mesoscopic" in an in-house research proposal that we made in 1986. But everyone, from the general manager on down, criticized this choice of words saying "What in the world is 'nanoelectronics'?" Of course, "nanoelectronics" is now a very popular term, but at that time, it was not commonly used.

By the way, the reason why our research division got involved with mesoscopic research was our desire to prove the prediction of a single-electron tunnel effect in a Josephson junction, which I talked about earlier. To this end, electron-beam exposure equipment for fabricating fine devices was an absolute necessity, but in 1986, there was only one set of this equipment in the laboratories and that was in another research division. In the end, we did succeed in experimentally confirming the occurrence of this tunnel effect in 1986, but the difficulty in procuring needed equipment continued. Finally, in 1991, our research division got its own electron-beam exposure equipment, and our research work took a great leap forward.

[Nakamura] The research theme that I took on after entering the company was the preparation of a single-electron transistor using that electron-beam exposure equipment. After about one year, I succeeded in doing so, but that transistor could only operate at low temperatures. For my next research theme, I would investigate methods that allow a single-electron transistor to operate at high temperatures much like a silicon transistor.

But during this research, I came to develop an interest in something completely different from those research themes. The single-electron transistor that we fabricated made use of an aluminum electrode, and at low temperatures, the aluminum was found to enter a superconducting state. I began to investigate closely how a single-electron transistor might behave in a superconducting state. I soon found that a superconducting single-electron transistor exhibits discrete energy levels just like an atom even when realized by an electrical circuit containing many electrons, and that such a single-electron transistor includes a function for reading that state. From 1995 to 1996, I performed experiments based on these characteristics to spectroscopically study the energy difference between the two lowest energy levels of a single-electron transistor by irradiating it with microwaves in the same way that an atom can be irradiated with light to study its absorption spectrum.

In 1997, I presented the results of these experiments in Physical Review Letters (PRL: a journal of the American Physical Society) in a manuscript titled "Spectroscopy of Energy-Level Splitting between Two Macroscopic Charge States Coherently Superposed by Josephson Coupling." In that report, the word "qubit" was not mentioned even once! But around that time, the expression "quantum computer" finally began to surface here and there, and on hearing those words, I began to wonder whether that research was similar to what I was working on. At that time, however, I was still nowhere near the stage of being able to actively control quantum states, so I refrained from using the expression "quantum computer."

[Tsai] In 1996, a project named "Quantum Effect Phenomena" was inaugurated as part of Core Research for Evolutional Science and Technology (CREST) under the Japan Science and Technology Corporation (now the Japan Science and Technology Agency). We participated in that project under the research theme of "Physics of Metallic Fine-Tunnel-Junction Systems and their Application to Devices." On applying for participation in this CREST project, we had to prepare a proposal and describe how we thought we could make a contribution. At the very end of that proposal, I wrote that "if we could harness the quantum coherence existing in the superconducting single-electron transistor, it might be possible to construct qubits, the basic components of a quantum computer." I wrote this thinking that a little exaggeration might help us in the selection process, but of course, I also worried whether it was really OK to write that at that time!

[Nakamura] Dr. Tsai, I had no idea that you mentioned the possibility of a quantum computer in the proposal for that project.

[Tsai] At that time when the CREST project began, the single-electron transistor was already being researched here and there in various degrees, and in applying for participation in the project, it was necessary to add some value to the proposal. For that reason, I proposed research on a single-electron transistor with the Josephson effect. Nevertheless, when you said "I want to observe the temporal behavior of quantum states" after you submitted your manuscript to PRL, I honestly became worried and thought "Can quantum states really be observed in a macroscopic manner?" All researchers working on superconducting themes at that time had been targeting the macroscopic observation of change in superposition states over time, but not one had succeeded in doing so.

[Nakamura] There were actually three reasons why I developed this strong desire to observe the temporal change of quantum superposition states. First, the series of research studies that I had been involved in were all motivated by a desire to confirm quantum mechanical behavior in a macroscopic system like a Josephson-junction electrical circuit. In an experiment that I reported in 1997 on the measurement of energy gap in the superconducting single-electron transistor, I achieved that confirmation in a certain sense. But on discussing this with other researchers, they pointed out that observing the behavior of quantum states in the time domain would be an even more convincing demonstration. This stimulated my desire to achieve that observation. On attempting to do that, however, some technical problems arose, and I started to doubt the possibility of doing that.

The second reason for wanting to observe this temporal change was that I had started to hear talk about quantum computers and qubits, as I mentioned earlier.

The third reason is that I learned that high-speed voltage-pulse transmission technology was being developed for other research purposes in the same research division, and I though that that technology might be able to solve some problems in my experiments. Finally, in November 1998, my experiment succeeded. In December of that year, Dr. Tsai gave an invited talk titled "Single Electron Devices and Quantum Coherence" at the Aspen Winter Conference on Quantum Criticality (sponsored by the Aspen Center for Physics).

[Tsai] I thought that if we reported your success simply as "macroscopic confirmation of quantum state," it would not be easily understood, and for this reason, I added that "this observation may lead to quantum computers."

[Nakamura] But we were still not yet at that stage, and I said to you "Is it really OK to write that?" You did, however, go ahead and write it.