Hard superconducting gap in germanium

TL;DR

This paper demonstrates a method to create a high-quality, hard superconducting gap in germanium-based devices, enabling advanced quantum circuits with improved coherence and scalability.

Contribution

It introduces a low-disorder, oxide-free interface between germanium and a superconductor, achieving a hard superconducting gap and phase control in germanium-based quantum devices.

Findings

Near-unity transparency in Josephson junctions

Hard induced superconducting gap in quantum point contacts

Phase control of a Josephson junction

Abstract

The co-integration of spin, superconducting, and topological systems is emerging as an exciting pathway for scalable and high-fidelity quantum information technology. High-mobility planar germanium is a front-runner semiconductor for building quantum processors with spin-qubits, but progress with hybrid superconductor-semiconductor devices is hindered because obtaining a superconducting gap free of subgap states (hard gap) has proven difficult. Here we solve this challenge by developing a low-disorder, oxide-free interface between high-mobility planar germanium and a germanosilicide parent superconductor. This superconducting contact is formed by the thermally-activated solid phase reaction between a metal (Pt) and the semiconductor heterostructure (Ge/SiGe). Electrical characterization reveals near-unity transparency in Josephson junctions and, importantly, a hard induced superconducting gap in quantum point contacts. Furthermore, we demonstrate phase control of a Josephson junction and study transport in a gated two-dimensional superconductor-semiconductor array towards scalable architectures. These results expand the quantum technology toolbox in germanium and provide new avenues for exploring monolithic superconductor-semiconductor quantum circuits towards scalable quantum information processing.