Parity detection of multiple superconducting qubits

We present schemes for detecting two-qubit parity. One involves preparing two qubits coupled to a common cavity such that the $\chi$ shifts representing odd parities overlap. Driving the cavity at this odd frequency then generates either a high photon occupation (for odd parity) or an oscillating photon occupation (for even parity) in the cavity, which can be discriminated with phase-insensitive photon detection. A second readout scheme involves taking advantage of cavity nonlinearity at high drive powers. In this strongly driven dispersive regime, we can perform a similar mapping of qubit parity to photon occupation, using a frequency-offset cavity drive during readout. Experimental results to realize both readout protocols using a Josephson Photomultiplier are discussed.