Superconducting qubit as a probe of quantum fluctuations in a nonlinear resonator

TL;DR

This paper demonstrates how a superconducting qubit coupled to a nonlinear resonator can serve as a sensitive probe for quantum fluctuations, including squeezing and quantum heating, with theoretical models validated by experimental data.

Contribution

The paper develops an effective master equation incorporating resonator squeezing effects, enabling the use of qubits to detect quantum fluctuations in nonlinear resonators.

Findings

Qubit excitation spectrum reveals squeezing and quantum heating effects.

Theoretical predictions match experimental observations.

Effective master equation accounts for resonator squeezing.

Abstract

In addition to their central role in quantum information processing, qubits have proven to be useful tools in a range of other applications such as enhanced quantum sensing and as spectrometers of quantum noise. Here we show that a superconducting qubit strongly coupled to a nonlinear resonator can act as a probe of quantum fluctuations of the intra-resonator field. Building on previous work [M. Boissoneault et al. Phys. Rev. A 85, 022305 (2012)], we derive an effective master equation for the qubit which takes into account squeezing of the resonator field. We show how sidebands in the qubit excitation spectrum that are predicted by this model can reveal information about squeezing and quantum heating. The main results of this paper have already been successfully compared to experimental data [F. R. Ong et al. Phys. Rev. Lett. 110, 047001 (2013)] and we present here the details of the derivations.