Qubit Readout with the Josephson Photomultiplier

The realization of a large-scale fault-tolerant quantum processor will require scalable high-fidelity readout of multiqubit parity operators. Here we describe development of a scalable qubit measurement approach based on microwave photon counting. The measurement protocol involves mapping the qubit state to photon occupation of bright and dark cavity pointer states, followed by photodetection using the Josephson photomultiplier (JPM). We discuss use of the qubit as a calibrated source of photons to measure JPM quantum efficiency, and we describe global optimization of the measurement protocol. Finally, we discuss prospects for interfacing the JPM output to single flux quantum circuits to allow low-latency classical postprocessing of the qubit measurement result.