AC-Stark Shift and Dephasing of a Superconducting Qubit Strongly Coupled to a Cavity Field

TL;DR

This study investigates the ac-Stark shift and dephasing effects on a superconducting qubit strongly coupled to a cavity, revealing how measurement influences qubit coherence and energy levels.

Contribution

It provides the first spectroscopic measurement of energy level shifts and dephasing caused by strong coupling and measurement back-action in a circuit QED system.

Findings

Dispersive shift used for non-destructive qubit state measurement

Observation of 0.6 MHz ac-Stark shift per photon

Intrinsic qubit dephasing time exceeds 200 ns

Abstract

We have spectroscopically measured the energy level separation of a superconducting charge qubit coupled non-resonantly to a single mode of the electromagnetic field of a superconducting on-chip resonator. The strong coupling leads to large shifts in the energy levels of both the qubit and the resonator in this circuit quantum electrodynamics system. The dispersive shift of the resonator frequency is used to non-destructively determine the qubit state and to map out the dependence of its energy levels on the bias parameters. The measurement induces an ac-Stark shift of 0.6 MHz per photon in the qubit level separation. Fluctuations in the photon number (shot noise) induce level fluctuations in the qubit leading to dephasing which is the characteristic back-action of the measurement. A cross-over from lorentzian to gaussian line shape with increasing measurement power is observed and theoretically explained. For weak measurement a long intrinsic dephasing time of T_2 > 200 ns of the qubit is found.