Younger Scientists
Yasunobu Nakamura
The JSAPI would like to introduce to you Mr.
Yasunobu Nakamura, who won the Young Investigator Award at the 1999 Japan Society ofApplied Physics Fall Meeting for the outstanding quality of his research. Mr. Nakamura is anassistant manager in the NEC Fundamental Research Laboratories working on the quantumstate control of nanoscale superconducting
devices.
There come exciting moments in every
researcher’s life when he or she wants to cry
out, “I’ve found it!” In my case, the most recent and biggest experience of this kind waswhen I saw a particular oscillating signal (seefigure) on the monitor of our PC in the lab.
The oscillation corresponded to the coherent
time evolution between two quantum statesin a mesoscopic superconducting device known
as a Cooper-pair box.1) This result demon∆t(ps)
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1.0Q0/estrated, for the first time, the coherent controlof quantum states of an artificial two-level system in a solid-state device. I was able to reachthis goal, which is a preliminary step towards
quantum computing, because of a number oflucky encounters in the course of my career.
When I first joined the NEC Fundamental
Research Laboratories eight years ago, I started
to study single-electron tunneling devices withDr. Jaw-Shen Tsai, who has been my boss andco-worker ever since. That was five years after
Fulton and Dolan of Bell Laboratories had first
demonstrated the feasibility of a single-electron transistor (SET).
The operation of SETs at that time was still
limited to temperatures below 1 K because thesingle-electron charging energy of the island
electrode coupled to the source and the drain
via small tunnel junctions was so small. We
therefore tried to make the island smaller. By
using electron-beam lithography and an anodization technique to fabricate an island of 20
nm in size, we succeeded in raising the operating temperature to about 100 K. (We have recently achieved operation at room-temperature.2))

Naturally, since the Al electrodes (which
we also use in larger devices) become superconducting below about 1 K, the temperature
range in which we usually have to make measurements on these devices, we were also interested in researching the characteristics of asuperconducting SET (S-SET). In superconducting electrodes, electrons form Cooper pairs
which tunnel through junctions in a coherent
manner by means of the Josephson effect. The
coherent superposition of two charge states,
differing by just one excess Cooper pair in theisland (or ‘box’), thus becomes possible in anS-SET.
My first naïve question was how we could
obtain directly observed evidence of the superposition. My interest stemmed from the duality of the Cooper-pair box and the SQUID ring,in which macroscopic quantum coherence between two flux states has long been sought. I
had already learned that in quantum mechanics a coherent two-level system would lead tothe formation of two eigenstates with symmetric and antisymmetric wave functions, and coherent oscillations in the time-domain between
the two localized states. But how could we
make those eigenstates or oscillations visible?
In earlier experiments on the I-V characteristics of S-SETs, I had noticed that an S-SET
can be regarded as a combined Cooper-pair
box and voltage-biased probe junction in certain conditions. Although it is difficult to observe clear evidence of coherent superposition
through dc I-V measurements, the concept ofdetecting the charge states from the tunnelingthrough the probe junction turned out to bevery useful in later experiments.
The first good idea came from what
seemed initially to be an unrelated paper about
semiconductor coupled quantum dots by Stoofand Nazarov of Delft University of Technology.
The authors had proposed using photon-assisted tunneling as a spectroscopic tool to study
the energy-level splitting between two coherently coupled states. On reading the paper, I
immediately drew an analogy with the S-SET
problem and started an experiment in which I
irradiated samples with microwaves. The experiment successfully showed clear levelanticrossing due to the superposition of two
charge states.3)
Having obtained evidence of coherence,
the next question was how long the coherence
could be maintained. In principle, a line-width
analysis of the photon-assisted peak could provide some information about decoherence.
However, it was hard to measure in practicethe expected very sharp peak width without

extrinsic broadening due to experimental fluctuations. Hence, I felt that a time-domain experiment to observe the coherent oscillations
was necessary.
The problem was that the time-scale ofthe oscillation in our device was typically only
100 ps. This appeared to be too fast for good
control by an electric voltage pulse transmitted from a room-temperature instrument to adevice embedded in the cryostat. I happened
to hear, though, that one of my colleagues inthe same group had succeeded in delivering
10 GHz digital signals to a circuit in his cryostat, so I adopted his technique and carefully
designed an experimental set-up. At the very
first trial, that resulted in more beautifully defined observations of oscillating signals fromthe quantum-state evolution than I had daredexpect.
In the last five years or so, quantum information technology has become a very popular
field of study and the Cooper-pair box is now
considered a candidate for use as a quantum
bit. Although I am still not sure to what extent
we can push this and other technologies towards quantum computing, directly controllingand measuring the quantum state of a physical system is a new and challenging concept,
particularly for solid-state devices. I hope to continue spending part of my time trying to unveil
and visualize some of the important and interesting phenomena that lie hidden in this fieldof nature, while keeping in mind that the Little
Prince said, “What is the most important thingis invisible...”References
1) Y. Nakamura, Yu. A. Pashkin and J. S. Tsai, Nature 398,
786 (1999).
2) Yu. A. Pashkin, Y. Nakamura and J. S. Tsai,Appl. Phys. Lett. 76, 2256 (2000).
3) Y. Nakamura, C. D. Chen and J. S. Tsai, Phys. Rev. Lett.
79, 2328 (1997).

Nakamura received hisBS degree from theDepartment of Applied Physics in theUniversity of Tokyo,
and received his
Master’s degree, majoring in high-Tc superconductivity research,from the same university. He joined the NEC
Fundamental Research Laboratories in 1992
and was awarded the Nishina Memorial prize
and the Sir Martin Wood prize in 1999.

JSAP International No.2 (July 2000)

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