The Josephson Microwave Photomultiplier

The current lack of single microwave photon counters -as opposed to microwave amplifiers- has become a problematic omission in the toolkit of available circuit QED devices. Hence, we propose a microwave photo-multiplier based on a modified phase qubit. Such a system, trapped in the metastable state, can be activated over its potential energy barrier by an incoming photon, creating an avalanche effect analogous to current photo-diodes. Linear coupling of the junction flux with the radiation field, in the weak damping regime, permits photodetection from an arbitrary quantum source in the GHz range. We model this device theoretically and investigate its sensitivity, bandwidth, efficiency, and dark-count rate using the Langevine stochastic differential equations and a path integral approach.