Single flux quantum (SFQ) circuits are superconducting digital circuits operating at cryogenic temperatures and have features of high-speed operation, low power consumption, and low timing jitter. We are developing cryogenic signal processors based on SFQ circuits using Nb/AlOx/Nb Josephson junctions (JJs) for superconducting strip photon detectors (SSPDs) made of NbTiN. The SSPDs have high sensitivity and detection efficiency at various wavelengths ranging from visible to mid-infrared light, high-speed response, low dark count rate, and low timing jitter, which is widely applied in quantum information and communications technology [1].
In our system, the SSPDs and SFQ circuits are installed in the 0.1-W Gifford-McMahon (GM) cryocooler and cooled to around 2.4 K. We designed some SFQ signal processors with lowered critical currents of JJs and bias voltages for SFQ circuits to reduce bias currents and power consumption, leading successful operation of SFQ circuits with larger number than 2000 JJs in 0.1-W GM cryocooler [3], [4]. However, larger or complicated SFQ circuits will be required for more advanced SSPD systems, and Joule heat by DC bias currents for SFQ circuits must be reduced in order to suppress the increase in temperatures of 0.1-W GM cryocooler. We developed a new package to supply large DC currents with small Joule heat, and the temperatures of cryocooler were found to be kept below 2.4 K with supplying DC currents of 400 mA. We plan to investigate the correlation between cooling power of cryocooler and Joule heat, and develop the large scale SSPD systems.