We develop a rigorous theoretical framework for interaction-induced phenomena in the waveguide quantum electrodynamics (QED) driven by mechanical oscillations of the qubits. Specifically, we predict that the simplest setup of two qubits, harmonically trapped over an optical waveguide, enables the ultrastrong coupling regime of the quantum optomechanical interaction. Moreover, the combination of the inherent open nature of the system and the strong optomechanical coupling leads to emerging parity-time ($\mathcal{P}\mathcal{T}$) symmetry, quite unexpected for a purely quantum system without artificially engineered gain and loss. The $\mathcal{P}\mathcal{T}$ phase transition drives long-living subradiant states, observable in the state-of-the-art waveguide QED setups.

• Received 4 July 2020
• Accepted 21 September 2020

DOI:https://doi.org/10.1103/PhysRevLett.125.183601