High-speed operation of HTS SQUID-array interface circuits with a cryocooler

The authors evaluated the operation of high-temperature superconducting quantum interference device (SQUID)-array interface circuits (IFCs) with normal-metal control lines. Transimpedance amplification was obtained at an operating speed of 1 Gb/s using a cryocooler. The effect of the number of SQUIDs connected in series and the number of arrays connected in parallel on the level of output from the SQUID-array IFCs was examined by Josephson circuit simulation, and then the effect of statistical spreads of junction characteristics was evaluated by Monte Carlo simulation. It was found that the configuration of two parallel SQUID arrays with 64 SQUIDs gives the highest output when the junction characteristics in the arrays have a certain spread. The authors fabricated the IFCs by using the conventional interface-engineered junction process. The process reproducibility was 100 μA ±25% for junction I_c, and 3.02 pH ±5% and 2.57 pH ±17% for the sheet inductance of the upper and lower electrodes, respectively. The transimpedance at low frequencies reached 20 and 4 V/A for input levels of 20 and 100 μA, respectively. Output voltages as high as 4.4 mV at 4.2 K and 2.3 mV at 40 K were obtained. Furthermore, an output voltage of 600 μV was obtained for a 1-Gb/s 2¹⁵-1 pseudo-random binary signal input at 40 K.