Fig.1) Coherent oscillations observed in the first Cooper pair box experiment.

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.

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.