Operation of an HTS SFQ logic circuit

Abstract

A Josephson junction biased below its critical current can switch to the voltage state by a thermal fluctuation. Thermal noise, rather than the critical temperature of high Tc materials, may limit the maximum operating temperature of superconductive digital circuits. Due to the technical difficulty of making the Tc digital circuits, the maximum operating temperature of these devices could only be estimated. Measurement of the effect of noise on switching a Josephson junction to the voltage state was made on a simple SFQ device constructed with resistively shunted Nb/AIOX/Nb. Good agreement was obtained between the measured data and the calculations based on thermal activation theory. Thermal activation theory could also explain the data obtained on a simple HTS SFQ device fabricated with YBCO grain boundary junctions formed on a STO bicrystal substrate. Large margins in SFQ circuits indicate the possible operation of high Tc SFQ circuits at high temperature.