Fast high-fidelity charge readout by operating a cavity-embedded Cooper-pair transistor in the Kerr bistable regime

Operating the cavity-embedded Cooper-pair transistor (CCPT) in the Kerr bistable regime, we demonstrate single-shot resolution between two charge states that are $0.09e$ apart. The measurement is performed with $94\%$ fidelity in a duration of around $3\text{μ}s$. This is done with a drive power that corresponds to only 20 intracavity photons on average in the high oscillation amplitude state of the CCPT, which is orders of magnitude smaller than that in rf single electron transistors. We find that the limiting factor for this mode of operation of the CCPT is the spontaneous fluctuation-induced switching between the two metastable oscillation amplitude states. We present empirical data on the variation of the switching dynamics with drive parameters and CCPT dc bias.