  We propose a scalable readout interface for superconducting nanowire
single-photon detector (SSPD) arrays, which we call the AQFP/RSFQ interface.
This interface is composed of adiabatic quantum-flux-parametron (AQFP) and
rapid single-flux-quantum (RSFQ) logic families. The AQFP part reads out the
spatial information of an SSPD array via a single cable, and the RSFQ part
reads out the temporal information via a single cable. The hybrid interface has
high temporal resolution owing to low timing jitter in the operation of the
RSFQ part. In addition, the hybrid interface achieves high circuit scalability
because of low supply current in the operation of the AQFP part. Therefore, the
hybrid interface is suitable for handling many-pixel SSPD arrays. We
demonstrate a four-pixel SSPD array using the hybrid interface as proof of
concept. The measurement results show that the hybrid interface can read out
all of the pixels with a low error rate and low timing jitter.
