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Younger Scientists Yasunobuyasunobu Nakamuranakamura the JSAPI Would Like to Introduce to You Mr Younger Scientists YasunobuYasunobu NakamuraNakamura The JSAPI would like to introduce to you Mr. Naturally, since the Al electrodes (which extrinsic broadening due to experimental fluc- Yasunobu Nakamura, who won the Young In- we also use in larger devices) become super- tuations. Hence, I felt that a time-domain ex- vestigator Award at the 1999 Japan Society of conducting below about 1 K, the temperature periment to observe the coherent oscillations Applied Physics Fall Meeting for the outstand- range in which we usually have to make mea- was necessary. ing quality of his research. Mr. Nakamura is an surements on these devices, we were also in- The problem was that the time-scale of assistant manager in the NEC Fundamental Re- terested in researching the characteristics of a the oscillation in our device was typically only search Laboratories working on the quantum- superconducting SET (S-SET). In superconduct- 100 ps. This appeared to be too fast for good state control of nanoscale superconducting ing electrodes, electrons form Cooper pairs control by an electric voltage pulse transmit- devices. which tunnel through junctions in a coherent ted from a room-temperature instrument to a manner by means of the Josephson effect. The device embedded in the cryostat. I happened There come exciting moments in every coherent superposition of two charge states, to hear, though, that one of my colleagues in researcher’s life when he or she wants to cry differing by just one excess Cooper pair in the the same group had succeeded in delivering out, “I’ve found it!” In my case, the most re- island (or ‘box’), thus becomes possible in an 10 GHz digital signals to a circuit in his cry- cent and biggest experience of this kind was S-SET. ostat, so I adopted his technique and carefully when I saw a particular oscillating signal (see My first naïve question was how we could designed an experimental set-up. At the very figure) on the monitor of our PC in the lab. obtain directly observed evidence of the super- first trial, that resulted in more beautifully de- The oscillation corresponded to the coherent position. My interest stemmed from the dual- fined observations of oscillating signals from time evolution between two quantum states ity of the Cooper-pair box and the SQUID ring, the quantum-state evolution than I had dared in a mesoscopic superconducting device known in which macroscopic quantum coherence be- expect. as a Cooper-pair box.1) This result demon- tween two flux states has long been sought. I In the last five years or so, quantum infor- had already learned that in quantum mechan- mation technology has become a very popular ∆ t(ps) ics a coherent two-level system would lead to field of study and the Cooper-pair box is now 400 the formation of two eigenstates with symmet- considered a candidate for use as a quantum ric and antisymmetric wave functions, and co- bit. Although I am still not sure to what extent 300 herent oscillations in the time-domain between we can push this and other technologies to- the two localized states. But how could we wards quantum computing, directly controlling make those eigenstates or oscillations visible? and measuring the quantum state of a physi- 200 In earlier experiments on the I-V charac- cal system is a new and challenging concept, ∆l(pA) 6 teristics of S-SETs, I had noticed that an S-SET particularly for solid-state devices. I hope to con- 3 100 0 can be regarded as a combined Cooper-pair tinue spending part of my time trying to unveil 0.4 0.6 0.8 1.0 box and voltage-biased probe junction in cer- and visualize some of the important and inter- Q0/e tain conditions. Although it is difficult to ob- esting phenomena that lie hidden in this field strated, for the first time, the coherent control serve clear evidence of coherent superposition of nature, while keeping in mind that the Little of quantum states of an artificial two-level sys- through dc I-V measurements, the concept of Prince said, “What is the most important thing tem in a solid-state device. I was able to reach detecting the charge states from the tunneling is invisible...” this goal, which is a preliminary step towards through the probe junction turned out to be quantum computing, because of a number of very useful in later experiments. References lucky encounters in the course of my career. The first good idea came from what 1) Y. Nakamura, Yu. A. Pashkin and J. S. Tsai, Nature 398, When I first joined the NEC Fundamental seemed initially to be an unrelated paper about 786 (1999). Research Laboratories eight years ago, I started semiconductor coupled quantum dots by Stoof 2) Yu. A. Pashkin, Y. Nakamura and J. S. Tsai, to study single-electron tunneling devices with and Nazarov of Delft University of Technology. Appl. Phys. Lett. 76, 2256 (2000). Dr. Jaw-Shen Tsai, who has been my boss and The authors had proposed using photon-as- 3) Y. Nakamura, C. D. Chen and J. S. Tsai, Phys. Rev. Lett. co-worker ever since. That was five years after sisted tunneling as a spectroscopic tool to study 79, 2328 (1997). Fulton and Dolan of Bell Laboratories had first the energy-level splitting between two coher- Nakamura received his demonstrated the feasibility of a single-elec- ently coupled states. On reading the paper, I BS degree from the tron transistor (SET). immediately drew an analogy with the S-SET Department of Ap- The operation of SETs at that time was still problem and started an experiment in which I plied Physics in the limited to temperatures below 1 K because the irradiated samples with microwaves. The ex- University of Tokyo, single-electron charging energy of the island periment successfully showed clear level- and received his electrode coupled to the source and the drain anticrossing due to the superposition of two Master’s degree, ma- via small tunnel junctions was so small. We charge states.3) joring in high-Tc super- therefore tried to make the island smaller. By Having obtained evidence of coherence, conductivity research, using electron-beam lithography and an anod- the next question was how long the coherence from the same university. He joined the NEC ization technique to fabricate an island of 20 could be maintained. In principle, a line-width Fundamental Research Laboratories in 1992 nm in size, we succeeded in raising the operat- analysis of the photon-assisted peak could pro- and was awarded the Nishina Memorial prize ing temperature to about 100 K. (We have re- vide some information about decoherence. and the Sir Martin Wood prize in 1999. cently achieved operation at room-tempera- However, it was hard to measure in practice ture.2)) the expected very sharp peak width without JSAP International No.2 (July 2000) 25.
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