Single-Flux Quantum Readout Circuits for Photon-Number-Resolving Superconducting Nanowire Single-Photon Detectors

Superconducting nanowire single-photon detectors (SNSPDs) with series-parallel meander-type configurations are demonstrated to have photon-number-resolving capabilities and are expected to be used for an evaluation system of single-photon generators and/or an energy-resolved fast detector for a new-generation time-of-flight mass spectrometry. In this paper, we present the design, fabrication, and confirmation of correct operations of two kinds of single-flux-quantum (SFQ) digital readout circuits for an SNSPD with a series-parallel meander-type configuration. Assuming a six-parallel meander-type configuration, we designed: a six-level current discriminator using six different magnetically coupled DC/SFQ converter circuits; and a Δ-type A/D converter using a quasi-one-junction SQUID (QOS) comparator with an SFQ feedback loop. For the design of the magnetically coupled inputs, we used a 3-D inductance extraction program FastHenry and the designed circuits were fabricated using a HYPRES 4.5-kA/cm² Nb process and a National Institute of Advanced Industrial Science and Technology (AIST) standard 2.5-kA/cm² Nb process.