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Applied Physics Letters

Three-Josephson-Junction Interferometer

Don L. Stuehm, Carl W. Wilmsen

Published Jun 1, 1972• 15 citations
Publisher

Abstract

The three-Josephson-junction asymmetric-feed quantum interferometer has been analyzed and shown to have increased magnetic field sensitivity and amplitude deviation over the two-junction interferometer. The critical current at zero applied field is shown to be less than the critical current at zero flux in the interference loop.
DOI
10.1063/1.1654016

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An interferometric device is proposed in order to analyze the quartet mode in biased three-terminal Josephson junctions (TTJs), and to provide experimental evidence for the emergence of a single stationary phase, the so-called quartet phase. In such a quartet superconducting quantum interference device (quartet SQUID), the flux sensitivity exhibits period hc/4ehc/4ehc/4e, which is the fingerprint of a transient intermediate state involving two entangled Cooper pairs. The quartet SQUID provides two pieces of information: an amplitude that measures a total ``quartet critical current,'' and a phase lapse coming from the superposition of the following two current components: the quartet supercurrent which is odd in the quartet phase, and the phase-sensitive multiple Andreev reflection (phase MAR) quasiparticle current, which is even in the quartet phase. This makes a TTJ a generically ``$\ensuremath{\theta}$ junction.'' Evidence for phase MARs plays against conservative scenarios involving synchronization of AC Josephson currents, based on ``adiabatic'' phase dynamics and resistively shunted junction--like models.
We have calculated the total critical current as a function of applied magnetic flux for a superconducting interferometer consisting of many Josephson junctions in parallel. An enhancement and narrowing of the periodic principal maxima in the critical current versus flux characteristic is predicted as the number of junctions in parallel increases, even when the effects of finite self- and mutual inductances and nonuniformity of the junction critical currents are included in the calculations. The possible application of the superconducting quantum interference grating, or SQUIG, for the detection of magnetic flux is discussed.
The parallelism between diffraction and interference in optics and quantum interference in Josephson junctions is discussed and studied in details. The interdisciplinary character of the present work is highlighted through specific examples. The Fraunhofer-like pattern of the maximum Josephson current in a single Josephson junction and the periodic field dependence of the critical current in two-junction and in multi-junction quantum interferometers is analyzed and discussed in comparison with the homologous classical optical phenomena.
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Thin-film Josephson-junction quantum-interference devices require only a small amount of magnetic flux to switch them between the voltage state and the superconducting state. They are sensitive to any spurious magnetic flux present at the device. The magnetic flux may be trapped in the superconducting ground plane, particularly in ground plane holes. This paper analyzes the effects of the trapped magnetic flux on the threshold curves of three-junction Josephson interferometers. The threshold curves of the interferometer are calculated automatically by using a circuit optimization program. The automatic procedure of calculating the threshold curves of an interferometer can be applied to any Josephson-junction interferometer device. The threshold curves of both symmetric and asymmetric interferometers are calculated. We find that the threshold curves of a symmetric three-junction interferometer changes periodically as a function of the effective coupled trapped magnetic flux with a period of Φ0. The threshold curves are shifted and severely distorted in shape by the presence of the coupled trapped magnetic flux. The degeneracy of the threshold curves of a symmetric interferometer makes the unique quantitative determination of the trapped magnetic flux difficult. However, the degeneracy is removed in an asymmetric interferometer. The presence of the unbalanced trapped magnetic flux greatly shifts the entire threshold curve of an asymmetric interferometer.