Supercurrent noise in quantum point contacts

TL;DR

This paper calculates the spectral density of current fluctuations in a superconducting quantum point contact, revealing unexpectedly large supercurrent noise caused by coherence, with a unique voltage dependence opposite to the current.

Contribution

It introduces a calculation of supercurrent noise in quantum point contacts, showing large fluctuations caused by coherence and a novel voltage dependence.

Findings

Large supercurrent noise caused by coherence

Noise decreases as bias voltage increases

Noise can be interpreted as shot noise of large charge quanta

Abstract

Spectral density of current fluctuations in a short ballistic superconducting quantum point contact is calculated for arbitrary bias voltages $V$. Contrary to a common opinion that the supercurrent flow in Josephson junctions is coherent process with no fluctuations, we find extremely large current noise that is {\em caused} by the supercurrent coherence. An unusual feature of the noise, besides its magnitude, is its voltage dependence: the noise decreases with increasing $V$, despite the fact that the dc current grows steadily with $V$. At finite voltages the noise can be qualitatively understood as the shot noise of the large charge quanta of magnitude $2\Delta /V$ equal to the charge transferred during one period of Josephson oscillations.