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

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 intraresonator 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 the squeezing factor r. 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.