Parity Measurement in Ultrastrong Coupling Regime

The measurement of the parity of two qubits is a primitive of quantum computing that allows creating deterministic entanglement. In the field of circuit quantum electrodynamics, a scheme to achieve parity measurement of two superconducting qubits has been proposed and analyzed under the usual rotating-wave approximation (RWA). We show that the same scheme can be carried over beyond this approximation, to the regime of ultrastrong coupling, with an improvement in the fidelity.