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 non-perturbative 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. However, the vacuum field is able to
induce the symmetry breaking of a dispersively coupled probe qubit. Here we show, by
exploiting an input/output theory well designed for a coherent probe source, the
spectroscopical evidences of such a symmetry breaking of the selection rules. Furthermore,
for a very weak coherent probe, we also show that the method herein exploited can bring
semi-analytical formulas, very useful for a clear understanding of the physics behind the
observed effects.

Organizzatore: prof. Sandro Wimberger