A Fast and Ultrasensitive Electrometer Operating at the Single-Photon Level

We demonstrate fast and ultrasensitive charge detection with a cavity-embedded Cooper pair transistor via dispersive readout of its Josephson inductance. We report a minimum charge sensitivity of $14$ $\mu e/\sqrt{\mathrm{Hz}}$ with a detection bandwidth on the order of $1$ MHz using $16$ attowatts of power, corresponding to the single-photon level of the cavity. This is the first ultrasensitive electrometer reported to operate at the single-photon level and its sensitivity is comparable to rf-SETs, which typically require picowatts of power. Our results support the feasibility of using this device to mediate an optomechanical interaction that reaches the single-photon strong coupling regime.