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.
@article{arxiv.2209.05747,
title = {Probing the symmetry breaking of a light--matter system by an ancillary qubit},
author = {Shuai-Peng Wang and Alessandro Ridolfo and Tiefu Li and Salvatore Savasta and Franco Nori and Y. Nakamura and J. Q. You},
journal= {arXiv preprint arXiv:2209.05747},
year = {2023}
}