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

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.

14 pages, 9 figures

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• Quantum Phase Transitions in Optomechanical Systems
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• Critical quantum metrology in a stabilized two-photon Rabi model
• Linear Ultrastrong Optomechanical Interaction
• Strong coupling between a single-photon and a two-photon Fock state
• One-step implementation of nonadiabatic geometric fSim gate in superconducting circuits
• Upgrading Quantum Metrology by Combined Sensitivity Resources in Mixed Linear-Nonlinear Light-Matter Interactions with Bias Field
• Globalized Nonlinear Critical Quantum Metrology by Two-photon Rabi-Stark model
• Negative Wigner function by decaying interaction from equilibrium
• Circuit QED Spectra in the Ultrastrong Coupling Regime: How They Differ from Cavity QED
• From Few to Many Emitters Cavity QED: Energy Levels and Emission Spectra From Weak to Deep-Strong Coupling