Cryogenic readout of superconducting nanowire single-photon detectors using high-sensitivity adiabatic quantum-flux-parametron circuits

We have been developing readout interfaces for superconducting nanowire single-photon detectors (SSPDs) using adiabatic quantum-flux-parametron (AQFP) logic. AQFP circuits operate with low power consumption, low bias currents, and high sensitivity, and thus are suitable as readout interfaces for large SSPD arrays. In this study, we develop a high-sensitivity AQFP interface, consisting of a current transformer, comparator, and rising-edge detector. We systematically investigated the current sensitivity of the AQFP interface by operating an NbTiN SSPD with the interface in a 0.1 W Gifford-McMahon cryocooler. We compared the outputs from the AQFP interface with the direct outputs from the SSPD, thereby demonstrating a sensitivity of 3.5 mu A, which is much smaller than that of the single-flux-quantum interfaces that we developed before.