Current Fluctuations in Rough Superconducting Tunnel Junctions

TL;DR

This paper studies how tiny, highly transparent channels called pinholes affect current fluctuations and noise in superconducting tunnel junctions, revealing their significant role in shot noise and potential impact on qubit resonators.

Contribution

It demonstrates that pinholes, despite minimal conductance contribution, dominate low-voltage current fluctuations and contribute to shot noise through Andreev processes, offering new insights into junction noise mechanisms.

Findings

Pinholes significantly influence low-voltage shot noise.

Fully transparent channels contribute to noise via Andreev cycles.

Pinholes may act as microscopic two-level fluctuators.

Abstract

Intrinsic noise is known to be ubiquitous in Josephson junctions. We investigate a voltage biased superconducting tunnel junction including a very small number of pinholes - transport channels possessing a transmission coefficient close to unity. Although few of these pinholes contribute very little to the conductance, they can dominate current fluctuations in the low-voltage regime. We show that even fully transparent transport channels between superconductors contribute to shot noise due to the uncertainty in the number of Andreev cycles. We discuss shot noise enhancement by Multiple Andreev Reflection in such a junction and investigate whether pinholes might contribute as a microscopic mechanism of two-level current fluctuators. We discuss the connection of these results to the junction resonators observed in Josephson phase qubits.