Qubit readout with Josephson Photomultipliers

Continued progress in superconducting qubits will require the development of scalable quantum-limited measurement tools. We have recently introduced a scalable superconducting qubit measurement protocol involving the state-selective ringup of a readout cavity followed by photodetection with the Josephson photomultiplier (JPM), a current-biased Josephson junction. Here we describe the experimental realization of this protocol. We discuss JPM optimization for high quantum efficiency, and describe integration of the JPM with a transmon qubit for high-fidelity dispersive readout. In addition, we discuss prospects for JPM readout of larger multi-qubit circuits.