Charge fluctuations in a quantum point contact attached to a superconducting lead

TL;DR

This paper develops a method to calculate charge noise spectra in mesoscopic conductors with superconducting leads and examines charge fluctuations in a quantum point contact, revealing doubled fluctuations and enhanced noise near conductance channels.

Contribution

It introduces a calculation framework for charge noise in hybrid normal-superconductor mesoscopic systems and analyzes charge fluctuations in a QPC with superconducting contacts.

Findings

Doubling of equilibrium charge fluctuations in the presence of superconducting leads.

Significant enhancement (>2) in current noise spectrum when a QPC channel opens.

Comparison showing differences between normal and superconducting lead configurations.

Abstract

We show how to calculate the charge noise spectrum in a normal mesoscopic conductor, which is capacitively coupled to a macroscopic gate, when this conductor is attached to L normal leads and M superconducting leads, the only restriction being that the superconducting leads must be at the same chemical potential. We then proceed to examine results for a quantum point contact (QPC) in a normal lead connecting to a superconductor. Of interest is the fluctuating current in a gate capacitively coupled to a QPC. The results are compared with the case when all leads are normal. We find a doubling of the equilibrium charge fluctuations and a large enhancement (>2) in the current noise spectrum to first order in |eV|, when a channel in the QPC is opening.