Phase diffusion and suppression of the supercurrent by quantum-mechanical fluctuations of the Josephson plasma

TL;DR

This paper investigates how quantum fluctuations in Josephson junctions lead to phase diffusion and suppress supercurrent, especially in small-capacitance point contacts, with experimental validation on classical superconductors.

Contribution

It demonstrates the significant impact of quantum-mechanical zero-point fluctuations on supercurrent suppression in Josephson junctions with small capacitance, extending the RCSJ model to new regimes.

Findings

Quantum fluctuations cause supercurrent suppression in small-capacitance junctions.

Finite contact resistance arises due to quantum tunneling effects.

Experimental results on classical superconductors support the model predictions.

Abstract

The RCSJ model of resistively and capacitively shunted Josephson junctions is used to describe superconducting point contacts over a wide range of resistances up to the metallic -- tunneling transition. Their small dynamic capacitance of order $C = 0.1 $fF due to the point-contact geometry results in a huge plasma frequency. The critical current is then strongly suppressed and the contact resistance becomes finite because of quantum-mechanical zero-point fluctuations of the Josephson plasma and the rather large escape rate out of the zero-voltage state due to quantum tunneling. We test the predictions of the RCSJ model on the classical superconductors lead, indium, aluminum, and cadmium.