Single-flux-quantum based coincidence circuits using Nb/AlO_x/Nb Josephson junctions with critical current density of 10 kA/cm²

Abstract

We demonstrated a coincidence circuit based on single flux quantum (SFQ) circuits with high-time accuracy for a two-photon interferometer using superconducting nanostrip photon detectors (SNSPDs). The coincidence circuit was designed to have 3 different time windows of 30 ps, 100 ps, and 200 ps, which can be selectable on demand. Furthermore, each time window can be fine-tuned by changing the applied bias currents. The circuit was fabricated by the AIST high-speed standard process based on Nb/AlO_x/Nb junctions with critical current density of 10 kA/cm². We experimentally evaluated the time window of the coincidence circuit at temperatures below 2.4 K in a 0.1-W Gifford-McMahon (GM) cryocooler.