An Ultrasensitive Electrometer Operating at the Single-Photon Level*

We operate the cavity-embedded Cooper pair transistor (cCPT), a nonlinear charge and flux tunable microwave cavity, as an electrometer. This mode of operation is dispersive in that the charge gating the CPT island is encoded in the resonant frequency of the cCPT, which is then determined by reflection measurements. We find minimum charge sensitivities on the order of 10 μe/√Hz using input powers corresponding to a single intracavity photon. This is comparable to the sensitivity of rf-SETs while using many orders of magnitude less power. We discuss the significance of these results, compare with theory, and discuss the factors limiting the charge sensitivity in practice.