Quantum Computer

I'm Yuri Pashkin of Institute of Physical and Chemical Research.
After the successful demonstration of the first solid-state qubit, the next important step was coupling of qubits.
Without coupling, none of the quantum computing schemes or gates can be built. For our charge qubits, electrostatic, or capacitive, coupling was the easiest and the most natural way to go.
When we were starting this work everything seemed very easy and we ware very sure in our rapid success. To couple our qubits, we decided to use an extra island, prepared in a separate lithography step, that would overlap each Cooper pair box and provide desired coupling. Although our samples looked nice in the SEM we could not observe any traces of coupling in the measurements. We modified the sample layout many times and increased the overlap area, however, the result was the same. We kept on trying for about a year and our desperation was growing.
Finally, we decided to try a different approach. The coupling island was fabricated in the same lithography step as our qubits. Although there were tunnel junctions between the Cooper pair boxes and the coupling island, the inter-qubit coupling was purely electrostatic because all the tunneling processes between them were completely suppressed. The very first sample fabricated in this way was successful.
As for the future plans, it would be nice to demonstrate controllable coupling between our qubits. Although everything is clear from the theoretical point of view, it is not easy to do experimentally because there are some details that are not taken into account in the theoretical model.
Once controllable coupling is demonstrated, it will open a possibility to do quantum algorithms using a few coupled qubits. With the controllable coupling, it is much easier to do quantum algorithms(e.g., Deutsch-Jozsa or Grover algorithms) compared to the fixed coupling case.
Work at NEC is a real pleasure for me. I find the atmosphere in the lab very friendly. I enjoy discussions with my colleagues and can always rely on their help. The lab has state-of-the-art fabrication facilities and measurement instruments. All the machines are well maintained and in a perfect condition. I can hardly imagine better working conditions.

I am Tsuyoshi Yamamoto of NEC's Fundamental and Environmental Research Laboratories.
I entered NEC in April 2001 and have been engaged in the research of superconducting qubits since then.
Last year, we were able to announce research results on the coupling of two superconducting qubits. I have often been asked what difficulties did we come up against in that experiment, but to be honest, I don't have the impression that we encountered one difficulty after another before finally succeeding.
To be sure, we did have some problems in finding out how to couple those qubits as Dr.Tsai pointed out. Although it was clear that two qubits could be coupled by capacitance, we could not get that capacitance as large as we wanted. Yuri Pashkin and I studied this problem from various angles, and exactly at the time that we ware struggling with no solution in sight, Oleg Astafiev joined our group. Based on a proposal that he made with regard to our problem, we tried changing to a simple fabrication method.
As it turned out, his proposal was brilliant and solved ouur capacitance problem. After that, the experiment ran smoothly. For both the entanglement experiment and the 2-qubit-gate experiment, almost everything went as we had calculated and designed enabling me to feel the truth of quantum mechanics for myself. This was a very moving experience. I think I was very fortunate to be a part of these successful experiments.
Through this research, moreover, I came to ponder, study, and learn many things. For me, that newfound knowledge is precious above and beyond actual achievements, and I must of course put this knowledge to good use in the future.

I'm Oleg Astafiev of Institute of Physical and Chemical Research.
I have joined the group led by Dr.Tsai of the NEC Basic Research Laboratory on April 2002, when they were working on experiments with double qubits. This work is a continuation of experiments with single Josephson qubit done by Nakamura et al. and the first step towards creating integrated circuits of quantum computers. Taking part in these experiments, I was excited very much by this interesting work.
However, we were experiencing a plenty of difficulties as nobody has ever done such type of measurements in solid state devices - among them how to reliably control the system with many adjustment parameters, how to present the experimental data to convince scientists that we really manipulate with two-qubit state and some others. At that time, we had many informal discussions, which helped us to understand how and what we really need to do.
We found, as we think, convincing way for the data presentation, namely, we demonstrated frequency splitting in spectra of coherent quantum oscillations, which is a result of two qubit interaction. I am happy that we made a work that was highly estimated by scientific society and has been finally published in Nature.
I would like to point out that NEC Fundamental Research Laboratory is one of the best places in the world for doing research work. The work can be efficiently and successfully carried out because everything is very well organized here. Fabrication and experimental facilities are very good, we have almost everything we need, so that research results depend only on research ability of everyone. Important part of research work is scientific discussions, which extremely helpful for understanding physics and choosing proper research directions. High level of researchers in NEC FRL makes such discussions extremely useful and importan. In addition, I would like to mention that the laboratories are located in comfortable scientific town Tsukuba with clean environment, which is also quite important for successful work.
Now, I am planning to continue experiments on qubits. We still have a lot of interesting experiments in this direction, for instance, implementation of the single-shot readout for double qubits, which allows one to measure directly multi-qubit states, use of the single-shot readout for further study of the qubit decohernce, we are also thinking about some alternative qubit circuits, which may help to improve the qubit performance. The small problem that I see now in our research is a limited time. As each experiment requires long time needed for fabrication, measurements, data treatment and etc., we have to choose the most interesting experiments out of many exciting possibilities.

[Nakamura] I, as well, do not know for sure since my stay in The Netherlands was at a university and not at a company and since I have also worked only at NEC in Japan. At Delft University of Technology, however, I was very impressed not only with the number and high level of research members but also with the high respect given to the technical staff in charge of electric shop and machine shop.
NEC also has staff that work on these matters, however, while I hear that this is the case at other companies in Japan, I know there is also a trend toward outsourcing. I have some misgivings about that, since I think that research efficiency varies according to the distance between researchers and good support staff. I also had the impression that students at Delft University of Technology had a strong desire to learn and that they developed quickly.