Circuit QED with a Nonlinear Resonator : ac-Stark Shift and Dephasing

TL;DR

This paper investigates how a nonlinear resonator affects a superconducting qubit's frequency and dephasing, revealing detailed insights into the nonlinear dynamics and quantum limits of the system.

Contribution

It provides a detailed spectroscopic analysis of a superconducting qubit coupled to a nonlinear resonator, highlighting the nonlinear effects on qubit properties and the quantum-limited dephasing behavior.

Findings

Qubit frequency shift accurately probes intracavity field.

Resonator nonlinearity characterized precisely.

Dephasing rate approaches quantum limit at low gain.

Abstract

We have performed spectroscopic measurements of a superconducting qubit dispersively coupled to a nonlinear resonator driven by a pump microwave field. Measurements of the qubit frequency shift provide a sensitive probe of the intracavity field, yielding a precise characterization of the resonator nonlinearity. The qubit linewidth has a complex dependence on the pump frequency and amplitude, which is correlated with the gain of the nonlinear resonator operated as a small-signal amplifier. The corresponding dephasing rate is found to be close to the quantum limit in the low-gain limit of the amplifier.