The superconducting single-flux-quantum (SFQ) circuit is the ultimate energy-efficient digital circuit technology. The SFQ circuits are very promising for applications not only to huge systems such as supercomputers and/or high-end routers but also to the signal processing circuit operating with superconducting detectors. We introduce the recent progress in the SFQ circuit technology focusing on the application combined with superconducting single-photon detectors (SSPDs). The four-pixel SSPD array was successfully operated with the SFQ signal processing circuit in the 0.1 W Gifford McMahon refrigerator.
