  High-fidelity, efficient quantum nondemolition readout of quantum bits is
integral to the goal of quantum computation. As superconducting circuits
approach the requirements of scalable, universal fault tolerance, qubit readout
must also meet the demand of simplicity to scale with growing system size. Here
we propose a fast, high-fidelity, scalable measurement scheme based on the
state-selective ring-up of a cavity followed by photodetection with the
recently introduced Josephson photomultiplier (JPM), a current-biased Josephson
junction. This scheme maps qubit state information to the binary digital output
of the JPM, circumventing the need for room-temperature heterodyne detection
and offering the possibility of a cryogenic interface to superconducting
digital control circuitry. Numerics show that measurement contrast in excess of
95% is achievable in a measurement time of 140 ns. We discuss perspectives to
scale this scheme to enable readout of multiple qubit channels with a single
JPM.
