Coherent control of macroscopic quantum states in a single-Cooper-pair box

TL;DR

This paper demonstrates the coherent control and measurement of a superconducting qubit based on a single-Cooper-pair box, showing its potential for solid-state quantum computing.

Contribution

It provides the first time-domain observation of coherent quantum-state evolution in a superconducting two-level system using voltage pulses.

Findings

Successful manipulation of quantum states with voltage pulses

Observation of coherent superpositions in a macroscopic system

Measurement of quantum state evolution via tunneling current

Abstract

A small superconducting electrode (a single-Cooper-pair box) connected to a reservoir via a Josephson junction constitutes an artificial two-level system, in which two charge states that differ by 2e are coupled by tunneling of Cooper pairs. Despite its macroscopic nature involving a large number of electrons, the two-level system shows coherent superposition of the two charge states, and has been suggested as a candidate for a qubit, i.e. a basic component of a quantum computer. Here we report on time-domain observation of the coherent quantum-state evolution in the two-level system by applying a short voltage pulse that modifies the energies of the two levels nonadiabatically to control the coherent evolution. The resulting state was probed by a tunneling current through an additional probe junction. Our results demonstrate coherent operation and measurement of a quantum state of a single two-level system, i.e. a qubit, in a solid-state electronic device.