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arXiv:1006.3181 · Physical Review Letters

Theory of Microwave-Assisted Supercurrent in Quantum Point Contacts

F. S. Bergeret, Pauli Virtanen, Tero Tapio Heikkilä, Juan Carlos Cuevas

Published Sep 8, 2010• 42 citations
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Abstract

We present a microscopic theory of the effect of a microwave field on the supercurrent through a quantum point contact of arbitrary transmission. Our theory predicts that (i) for low temperatures and weak fields, the supercurrent is suppressed at certain values of the superconducting phase, (ii) at strong fields, the current-phase relation is strongly modified and the current can even reverse its sign, and (iii) at finite temperatures, the microwave field can enhance the critical current of the junction. Apart from their fundamental interest, our findings are also important for the description of experiments that aim at the manipulation of the quantum state of atomic point contacts.

DOI
10.1103/physrevlett.105.117001

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We present a microscopic theory of the effect of a microwave field on the supercurrent through a quantum point contact of arbitrary transmission. Our theory predicts that (i) for low temperatures and weak fields, the supercurrent is suppressed at certain values of the superconducting phase, (ii) at strong fields, the current-phase relation is strongly modified and the current can even reverse its sign, and (iii) at finite temperatures, the microwave field can enhance the critical current of the junction. Apart from their fundamental interest, our findings are also important for the description of experiments that aim at the manipulation of the quantum state of atomic point contacts.
We present an extensive theoretical analysis of the supercurrent of a superconducting point contact of arbitrary transparency in the presence of a microwave field. Our paper is mainly based on two different approaches: a two-level model that describes the dynamics of the Andreev bound states in these systems and a fully microscopic method based on the Keldysh--Green function technique. This combination provides both a deep insight into the physics of irradiated Josephson junctions and quantitative predictions for arbitrary range of parameters. The main predictions of our analysis are: (i) for weak fields and low temperatures, the microwaves can induce transitions between the Andreev states, leading to a large suppression of the supercurrent at certain values of the phase; (ii) at strong fields, the current-phase relation is strongly distorted and the corresponding critical current does not follow a simple Bessel-function-like behavior; and (iii) at finite temperatures, the microwave field can enhance the critical current by means of transitions connecting the continuum of states outside the gap region and the Andreev states inside the gap. Our paper is of relevance for a large variety of superconducting weak links as well as for the proposals of using the Andreev bound states of a point contact for quantum computing applications.
Calibrated measurements of both the real and imaginary parts of the microwave impedance of proximity coupled superconducting weak links have been made. The measurements were compared with analytic calculations based on several different models for the supercurrent Is and quasiparticle current. For dc bias currents I1 less than the critical current Ic, the experimental results are consistent with the Josephson relation Is=Ic sinφ, where φ is the quantum phase angle. However, for I1≳Ic, the Fourier component of Is at the Josephson frequency is found to be significantly less (∼0.65±0.1) than would be predicted from Is=Ic sinφ, and consistent with Tinkham’s analysis of a phase slip process. The experimental results indicate that the phase-dependent part of the quasiparticle current, if it exists at all, is no more than 10% of the total quasiparticle current. These results do not necessarily contradict previous measurements made on true tunnel junctions because of the fundamental differences between the two types of devices.
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Spectral density of current fluctuations in a short ballistic superconducting quantum point contact is calculated for arbitrary bias voltages VVV. Contrary to a commonly held view that the supercurrent flow in Josephson junctions is a coherent process without fluctuations, we find extremely large current noise that is caused by the supercurrent coherence. At finite voltages, this noise can be qualitatively understood as the shot noise of the large charge quanta of magnitude $2\ensuremath{\Delta}/V$ equal to the charge transferred through the contact during one period of the Josephson oscillations.
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