Ballistic supercurrent discretization and micrometer-long Josephson coupling in germanium

TL;DR

This paper demonstrates ballistic supercurrent transport over micrometer distances in germanium quantum wells, with supercurrent discretization, Fabry-Perot resonances, and clear Josephson effects, advancing hybrid quantum device potential.

Contribution

It reports the first observation of ballistic supercurrent and supercurrent discretization in germanium quantum wells with superconducting contacts, highlighting their suitability for quantum technologies.

Findings

Supercurrents extend over several micrometers in germanium quantum wells.

Discretization of supercurrent observed in superconducting quantum point contacts.

Clear Fraunhofer patterns and Shapiro steps confirm Josephson behavior.

Abstract

We fabricate Josephson field-effect-transistors in germanium quantum wells contacted by superconducting aluminum and demonstrate supercurrents carried by holes that extend over junction lengths of several micrometers. In superconducting quantum point contacts we observe discretization of supercurrent, as well as Fabry-Perot resonances, demonstrating ballistic transport. The magnetic field dependence of the supercurrent follows a clear Fraunhofer-like pattern and Shapiro steps appear upon microwave irradiation. Multiple Andreev reflections give rise to conductance enhancement and evidence a transparent interface, confirmed by analyzing the excess current. These demonstrations of ballistic superconducting transport are promising for hybrid quantum technology in germanium.