Readout Circuit Based on Single-Flux-Quantum Logic Circuit for Photon-Number-Resolving SNSPD Array

A read-out circuit for photon-number-resolving superconducting nanowire single-photon detector (SNSPD) based on the single-flux-quantum (SFQ) circuit was designed and the basic performance of the fabricated circuit was confirmed. The current pulse output from an SNSPD array was converted to a SFQ pulse. The SFQ pulse triggered two stacks of double-flux-quantum (DFQ) amplifiers. A single block of a 40-series DFQ amplifier (DFQA) generated 2.3-mV voltage with a rising time of 400 ps for an operation frequency of 1.25 GHz, and the two stacks of DFQA generated 4.6-mV voltage when the SFQ pulse trains supplied both the DFQAs. The total voltage generated by twelve-series of 40-series DFQAs was expected to be 27.6 mV, with the same rising time of 400 ps. Using the fabricated circuit, the 40-series DFQA and the two stacks of the 40-series DFQA generated 2.3- and 4.6-mV voltage, respectively. The output voltage of the SFQ readout circuit was compatible to a room-temperature photon counter with a wide-band low-noise amplifier operating at low temperature.