Experimental Demonstration and Numerical Analysis of Microampere Gray Zone Width with Enhanced Operating Margin in Shunted Quasi-One Junction Superconducting Quantum Interference Device Comparators

Abstract

We evaluated the relationship between the gray zone width and the operating margin for comparators composed of quasi-one-junction superconducting quantum interference devices (QOSs) with shunt resistors, which are often used as high-speed readout circuits in multiple superconductor detector systems. The gray zone width is a good measure of current sensitivity of a single-bit comparator. We numerically analyzed the gray zone width of a QOS comparator and determined the circuit parameters. The gray zone width obtained from the experiments concurred with the results of the numerical analysis and was 2-3 mu A at 4.2 K in a QOS comparator composed of three Nb/AlOx/Nb junctions with critical currents of less than 90 mu A. The experimentally obtained operating margin for the bias current provided to the comparator was +/- 15% at the bias current of around 140 mu A. These results show that QOS comparators are promising for readout circuits operating up to tens of GHz and imply that gray zone width is the thermal noise in the resistors at 4.2 K. (c) 2013 The Japan Society of Applied Physics