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

Wang, Shuai-Peng; Ridolfo, Alessandro; Li, Tiefu; Savasta, Salvatore; Nori, Franco; Nakamura, Y; You, J Q · Nat Commun · 2023

basic_science · Level V

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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.