Probing the symmetry breaking of a light--matter system by an ancillary qubit

TL;DR

This paper demonstrates the experimental observation of symmetry breaking in an ancillary qubit caused by a deep-strongly coupled resonator, revealing novel quantum-vacuum effects in hybrid quantum systems.

Contribution

It provides the first experimental evidence of parity symmetry breaking induced by a deep-strongly coupled resonator in a hybrid quantum system.

Findings

Observation of parity symmetry breaking in an ancillary qubit

Evidence of vacuum-induced symmetry breaking effects

Exploration of quantum-vacuum phenomena in deep-strong coupling regime

Abstract

Hybrid quantum systems in the ultrastrong, and even more in the deep-strong, coupling regimes can exhibit exotic physical phenomena and promise new applications in quantum technologies. In these nonperturbative regimes, a qubit--resonator system has an entangled quantum vacuum with a nonzero average photon number in the resonator, where the photons are virtual and cannot be directly detected. The vacuum field, however, is able to induce the symmetry breaking of a dispersively coupled probe qubit. We experimentally observe the parity symmetry breaking of an ancillary Xmon artificial atom induced by the field of a lumped-element superconducting resonator deep-strongly coupled with a flux qubit. This result opens a way to experimentally explore the novel quantum-vacuum effects emerging in the deep-strong coupling regime.