High-speed operation of SQUID–array-type interface circuits using a cryocooler

Abstract

We have evaluated the operation of high-temperature superconducting SQUID–array- type interface circuits with normal-metal control lines. Transimpedance amplification with the operating speed up to 1 Gbps was demonstrated by using a cryocooler. The influence of the number of SQUIDs connected in series and parallel on the output level from the SQUID–array interface circuits was examined by Josephson circuit simulation and Monte-Carlo simulation. It was found that the configuration of two parallel SQUID–array with 64 SQUIDs gives the highest output signals when the junction characteristics in the arrays have a certain spread. We have fabricated the interface circuits by the conventional interface-engineered junction process. The process reproducibility was 100 μA ± 25% for the junction I c, 3.02 pH ± 5% and 2.57 pH ± 17% for the sheet inductance values of the counter- and base-electrodes, respectively. The transimpedance at low frequencies reached 20 and 4 V/A for the 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 the 1 Gbps 2 15−1 pseudo random binary signal input at 40 K.