Multiphonon transitions in a quantum electromechanical system

TL;DR

This paper demonstrates the observation of multiphonon transitions in a superconducting qubit coupled to a quantum acoustic system, revealing nonclassical features and phonon quantization, with spectral shifts under high driving amplitudes.

Contribution

It provides the first experimental evidence of multiphonon transitions and phonon counting in a quantum acoustic system, highlighting nonclassical phonon states.

Findings

Observation of multiphonon transitions

Detection of nonclassical features in phonon spectrum

Spectral line shifts due to dressing effects

Abstract

We investigate a superconducting qubit coupled to a quantum acoustic system in a near resonant configuration. In our system we measure multiphonon transitions, whose spectrum reveals distinctly nonclassical features and thus provides direct evidence of quantization of GHz sound, enabling phonon counting. Additionally, at a high driving amplitude comparable to the qubit-oscillator coupling, we observe a shift of the multiphonon spectral lines owing to dressing of many multiphonon transitions.