Supercurrent Noise in a Phase-Biased Superconductor-Normal Ring in Thermal Equilibrium

TL;DR

This study directly measures supercurrent noise in a phase-biased SNS ring, confirming theoretical predictions of thermal fluctuations and dissipation in superconductor-normal-superconductor junctions, with implications for topological states.

Contribution

First experimental verification of supercurrent noise and admittance in a phase-biased SNS ring, demonstrating the fluctuation-dissipation theorem and phase-dependent conductance behavior.

Findings

Supercurrent noise exhibits 1/T temperature dependence.

Dissipative conductance aligns with fluctuation-dissipation theorem predictions.

Enhanced current correlations are observed at phase π, even in gapless spectra.

Abstract

In superconductor-normal-superconductor (SNS) junctions, dissipationless supercurrent is mediated via Andreev bound states (ABSs) controlled by the phase difference between the two superconductors. Theory has long predicted significant fluctuations, and thus a noise, of such supercurrent in equilibrium, due to the thermal excitation between the ABSs. Via the fluctuation-dissipation theorem (FDT), this leads paradoxically to a dissipative conductance even in the zero frequency limit. In this article, we directly measure the supercurrent noise in a phase-biased SNS ring inductively coupled to a superconducting resonator. Using the same setup, we also measure its admittance and quantitatively demonstrate the FDT for any phase. The dissipative conductance shows an 1/T temperature dependence, in contrast to the Drude conductance of unproximitized metal. This is true even at phase $\pi$ where the Andreev spectrum is gapless. Supported by linear response theory, we attribute this to the enhanced current correlation between the ABSs symmetrical across the Fermi level. Our results provide insight into the origin of noise in hydrid superconducting devices which may be useful for probing topologically protected ABSs.