Hybrid single-electron turnstiles with thick superconducting electrodes for improved quasiparticle relaxation

TL;DR

This paper presents a fabrication method for high-quality hybrid single-electron turnstiles with thick superconducting electrodes, enhancing quasiparticle relaxation and paving the way for more accurate quantum current standards.

Contribution

It introduces a shadow evaporation technique for creating ultrasmall junctions with thick superconducting electrodes, improving quasiparticle thermalization in hybrid single-electron devices.

Findings

Successful fabrication of high-quality Al-based tunnel junctions

Demonstration of charge pumping in a hybrid single-electron transistor

Potential for improved metrological current quantization

Abstract

We demonstrate shadow evaporation-based fabrication of high-quality ultrasmall normal metal -- insulator -- superconductor tunnel junctions where the thickness of the superconducting electrode is not limited by the requirement of small junction size. The junctions are formed between a film of manganese-doped aluminium acting as the normal conducting electrode, covered by a thicker, superconducting layer of pure Al. We characterize the junctions by sub-gap current--voltage measurements and charge pumping measurements in a gate-driven hybrid single-electron transistor, operated as a turnstile for single electrons. The technique allows to advance towards turnstiles with close to ideally thermalized superconducting reservoirs, prerequisite for reaching metrological current quantization accuracy in a hybrid turnstile. We further present an alternative way to realize small junctions with thick Al leads based on multi-angle deposition. The work enables the future investigation of turnstiles based on superconductors other than Al, and benefits various other Al tunnel junction devices for which quasiparticle thermalization is essential.