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Applied Physics Letters

Thermal noise induced switching of Josephson logic devices

M. Klein, A. Mukherjee

Published Apr 15, 1982• 37 citations
Publisher

Abstract

Thermal noise induced switching of Josephson logic devices is studied, both theoretically and experimentally. The results are critical for the design of the operating bias points for these devices. An analysis of the relevant energy contours shows that a three-junction superconducting quantum interference device has the same noise switching rate as a point junction with the same maximum critical current. Experimental results, for the dependence of the thermal noise induced switching rate on the gate current bias ratio, are in very good agreement with the theory.

DOI
10.1063/1.93213

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Thermal noise induced switching of Josephson logic devices is studied, both theoretically and experimentally. The results are critical for the design of the operating bias points for these devices. An analysis of the relevant energy contours shows that a three-junction superconducting quantum interference device has the same noise switching rate as a point junction with the same maximum critical current. Experimental results, for the dependence of the thermal noise induced switching rate on the gate current bias ratio, are in very good agreement with the theory.
Superconducting quantum circuits (SQCs) are being explored as model systems for scalable quantum computing architectures. Josephson junctions are extensively used in superconducting quantum interference devices (SQUIDs) and in persistent-current qubit systems. Noise excitations, however, have a critical influence on their dynamics. Thus, the primary focus of this research was to investigate the effects of thermal ac-tivation on the superconducting properties of Josephson junctions. Specifically, ther-mal noise tends to result in a range of switching currents, values less than the critical current at which a junction switches from the superconducting to the normal state. First, a general review of superconductivity concepts is given, including a treatment of the Josephson phenomena. Next, I describe some of my work on characterizing the current-voltage traces of Josephson junctions tested at 4 K with a Multi-Chip Probe (MCP). Then, I describe thermal activation theory and examine the equations useful for modeling switching current distributions. The Josephson junctions of a SQUID with a ramped bias current were tested for numerous temperatures T < 4.5 K (and
To measure the effect of thermal noise on switching a Josephson junction to the voltage state a simple SFQ circuit constructed with resistively shunted Nb/AlO/sub x//Nb junctions was used. Based on a good agreement between the measured data and the thermal activation theory, the effect of thermal noise on the bit error rate of SFQ devices was studied, The effective thermal noise temperature of 10 K used to fit the data was higher than the bath temperature of 4.2 K. Careful circuit design to get large critical margins is necessary to achieve high operating temperature of SFQ logic devices.
Thermally induced vortex-to-vortex transitions between the zero-quantum vortex state and one- flux-quantum vortex state of a symmetric two-junction dc superconducting quantum interference device (SQUID) have been observed. The characterization of transitions of this type is an essential part of the modeling of Josephson junction logic and memory devices. The transition rates, measured as a function of bias current, applied flux, and temperature, are in excellent agreement with rates predicted by a thermal activation model for two-junction SQUID’s.
Noise which can produce erroneous transitions in Josephson logic devices can be treated in terms of the familiar model for thermal activation of a particle over a potential barrier. Noise curves are calculated which represent the effective threshold curves for a given transition rate; this approach can be applied to the mode-to-mode (superconducting) transitions as well as to the mode-to-voltage state switching. The analysis will be illustrated with the example of a three-junction current injection device. The practical consequences for design of Josephson logic circuits will be indicated.
An experimental investigation of the critical current noise in underdamped niobium based Josephson devices [junctions and Superconducting Quantum Interference Device (SQUID)] by a technique based on the switching current measurements is reported. By sweeping the junction with a current ramp we measure the critical current switching using the standard time of flight technique and analyze the data to extract the current noise. Measurements on Josephson junctions having an area ranging from (4 \ifmmode\times\else\texttimes\fi{} 4) to (40 \ifmmode\times\else\texttimes\fi{} 40) \ensuremath{\mu}m2{}^{2}2 in the temperature range from 4.2 to 1.2 K are reported. The experimental results show a linear behavior of the current white noise from both the junction area and the temperature. These measurements provide very useful information about the intrinsic noise of Josephson devices involving SQUIDs and qubits.
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.>