  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.
