Time-Resolved Measurement of a Charge Qubit

TL;DR

This paper presents a method for real-time monitoring of a charge qubit's coherent dynamics using high-frequency ac driving and phase measurement of the outgoing signal, enabling low backaction observation.

Contribution

It introduces an analytical and numerical approach to measure quantum coherence in charge qubits via phase response, enhancing time-resolution and reducing measurement disturbance.

Findings

Phase of outgoing signal correlates with system observable

Analytical solution matches numerical simulations

Method effectively monitors coherent oscillations

Abstract

We propose a scheme for monitoring coherent quantum dynamics with good time-resolution and low backaction, which relies on the response of the considered quantum system to high-frequency ac driving. An approximate analytical solution of the corresponding quantum master equation reveals that the phase of an outgoing signal, which can directly be measured in an experiment with lock-in technique, is proportional to the expectation value of a particular system observable. This result is corroborated by the numerical solution of the master equation for a charge qubit realized with a Cooper-pair box, where we focus on monitoring coherent oscillations.