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Physical review. B, Solid state

Observability of Josephson pair-quasiparticle interference in measurements of flux entry into a superconducting interferometer

Robert I. Gayley, T. C. Wang

Published Oct 1, 1977• 1 citation
Publisher

Abstract

We show that the pair-quasiparticle interference current in Josephson junctions will probably have a large effect on the amount of flux that will break into a superconducting interferometer. This shows promise as a new more-accurate way to measure this interference current. For this and other reasons, we argue that the calculation of this flux entry should be done more accurately than heretofore.

DOI
10.1103/physrevb.16.3270

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We show that the pair-quasiparticle interference current in Josephson junctions will probably have a large effect on the amount of flux that will break into a superconducting interferometer. This shows promise as a new more-accurate way to measure this interference current. For this and other reasons, we argue that the calculation of this flux entry should be done more accurately than heretofore.
Pair-quasiparticle interference current has a measurable effect on the tuning characteristics of a superconducting interferometer operating in the nonhysteretic regime: $\ensuremath{\epsilon}\ensuremath{\equiv}(\frac{2\ensuremath{\pi}}{{\ensuremath{\Phi}}_{0}}){L}_{R}{I}_{c}<1$ where ${\ensuremath{\Phi}}_{0}=\frac{h}{2e}$, LR{L}_{R}LR​ is the inductance of the interferometer ring, and Ic{I}_{c}Ic​ is the critical current of the interferometer's weak link. In the limit $\ensuremath{\epsilon}\ensuremath{\ll}1$ the interferometer will have a zero crossing in its magnetic flux sensitivity versus frequency characteristic that is above, on, or below resonance depending whether the coefficient of the interference current term is positive, zero, or negative, respectively. For larger $\ensuremath{\epsilon}$, screening currents will also shift the zero crossing. Results for the magnitude of this additional shift are presented and discussed.
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.
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.
Quasiparticle tunneling across a Josephson junction sets a limit for the lifetime of a superconducting qubit state. We develop a general theory of the corresponding decay rate in a qubit controlled by a magnetic flux. The flux affects quasiparticles tunneling amplitudes, thus making the decay rate flux-dependent. The theory is applicable for an arbitrary quasiparticle distribution. It provides estimates for the rates in practically important quantum circuits and also offers a new way of measuring the phase-dependent admittance of a Josephson junction.
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.