Qubit State Measurement using the Josephson Photomultiplier*

We describe results of high-fidelity qubit state measurement based on microwave photon counting. The protocol involves mapping the qubit state onto high-contrast microwave cavity pointer states in a linear readout resonator followed by subsequent photodetection using the Josephson Photomultiplier (JPM). Our technique provides access to the classical binary outcome of qubit state measurement at the millikelvin stage of a dilution refrigerator and does not use any non-reciprocal components between the qubit and JPM. We exploit the intrinsic damping of the JPM to “clean up” following photodetection so that the measurement is highly QND. This approach is well-suited for integration with proximal cryogenic digital logic.