NEC Fundamental Research Laboratories
We demonstrated quantum coherence in a single Cooper pair box in 1999, which was the first realization of a superconducting qubit as well as a solid-state qubit.
We started our research on single electron transistors in 1992. As the devices were made of aluminium, they became superconductive at low temperatures below 1K. Naturally, we became interested in Cooper pair tunnelling in those devices and studied the parity effect, which discriminates the even-odd parity of the number of electrons in an electrode, as well as photon-assisted Cooper pair tunnelling under microwave irradiation around 1995-1996.
During that time, we learned the concept of macroscopic quantum coherence, coined by Tony Leggett in 1980, and the following experimental challenges using SQUID circuits. Macroscopic quantum tunnelling was demonstrated in the 1980s. However, coherence and superposition in macroscopic systems had been elusive.
This led us to an experiment of quantum superposition between two distinct charge number states in the Cooper pair box device. In 1997, we published a paper in Phys. Rev. Lett. on the observation of the energy-level anti-crossing as evidence of the superposition. The result was obtained in photon-assisted tunnelling spectroscopy.
Fig.1) Coherent oscillations observed in the first Cooper pair box experiment.
As a next step, we decided to do a time-domain experiment to demonstrate quantum state control of the charge-number superposition. That was also the time when ideas of quantum information processing and quantum computing started to spread around the community. We needed to develop a few new experimental techniques, such as the ultrafast gate-voltage pulses for the control and the time-ensemble current measurement for the readout, but finally succeeded to observe coherent oscillations (Figure 1) between charge number states, in autumn 1998. The paper was published in Nature, in April 1999. It was well accepted and triggered research on solid-state quantum computing.