Thermal noise in digital Josephson devices

A method of accurately estimating the I/sub 0/ (critical current) of a Josephson junction (JJ) with thermal noise was developed by measuring the effective thermal noise temperature of a JJ. The effective thermal noise temperature of various JJ devices was measured and calculated. The JJ devices evaluated included inductively and resistively coupled logic devices and JJ devices in the presence of various noise sources. The noise sources included room-temperature resistors, switched JJ devices, and JJ devices in the linear I-V region beyond the gap. The R/sub NN/ compensator is shown to result in a noise temperature of about 6 K at an interferometer. The addition of a shunt junction lessens the noise penalty introduced by the compensator. A switched isolation interferometer in a two-input AND gate contributes negligible noise at the injection device. It is also shown that bandwidth connections to room-temperature equipment result in very large excess noise and require special input and output circuits on the chip.<>