PT-symmetric circuit QED

A parity-time (PT)-symmetric system emerging from a quantum dynamics is highly desirable in order to understand the possible implications of PT symmetry in the next generation of quantum technologies. In this work, we address this need by proposing and studying a circuit-QED architecture that consists of two coupled resonators and two qubits (each coupled to one resonator). By means of external driving fields on the qubits, we are able to tune gains and losses in the resonators. Starting with the quantum dynamics of this system, we show the emergence of the PT symmetry via the selection of both driving amplitudes and frequencies. We engineer the system such that a non-number-conserving dipole-dipole interaction emerges, introducing an instability at large coupling strengths. The PT symmetry and its breaking, as well as the predicted instability in this circuit-QED system, can be observed in a transmission experiment.