Superconducting single-Cooper-pair box as quantum bit

Abstract

We demonstrated the first electronic control of 1-qubit achieved in a solid-state device by using a submicron electron device called a single-Cooper-pair box. The number of electrons in the box (typically 10 8) is quantized and they form a single macroscopic quantum charge-number state, corresponding to the number of excess electrons in the box. By making all the electrodes superconducting, we can couple two neighboring charge-number states coherently. In this way one can create an artificial two-level system. We attached an additional tunneling probe to the box to monitor the probability of one of the two states involved in the coherence. We applied a sufficiently fast voltage pulse to the gate to create a degenerated charge-number state, so that to force the two states to undergo a quantum oscillation. As a result, we indeed observed the coherent oscillation. This was the first time that the quantum coherent oscillation was observed in a solid-state device whose quantum states involved a macroscopic number of quantum particles. Multiple-pulse experiments were also carried out.