A quantum dot in germanium proximitized by a superconductor

TL;DR

This paper demonstrates a tunable germanium quantum dot proximitized by a superconductor, enabling control over its quantum states and revealing rich sub-gap physics, advancing the development of hybrid quantum devices.

Contribution

It introduces a germanium quantum dot with controllable superconductor coupling, ground state parity, and sub-gap phenomena, a novel platform for quantum information processing.

Findings

Tunable QD-SC coupling and gate control of the induced gap.

Control of the system's ground state parity between even and odd.

Observation of rich sub-gap spin splitting spectra.

Abstract

Planar germanium quantum wells have recently been shown to host hard-gapped superconductivity. Additionally, quantum dot spin qubits in germanium are well-suited for quantum information processing, with isotopic purification to a nuclear spin-free material expected to yield long coherence times. Therefore, as one of the few group IV materials with the potential to host superconductor-semiconductor hybrid devices, proximitized quantum dots in germanium is a compelling platform to achieve and combine topological superconductivity with existing and novel qubit modalities. Here we demonstrate a quantum dot (QD) in a Ge/SiGe heterostructure proximitized by a platinum germanosilicide (PtGeSi) superconducting lead (SC), forming a SC-QD-SC junction. We show tunability of the QD-SC coupling strength, as well as gate control of the ratio of charging energy and the induced gap. We further exploit this tunability by exhibiting control of the ground state of the system between even and odd parity. Furthermore, we characterize the critical magnetic field strengths, finding a critical out-of-plane field of 0.90(4). Finally we explore sub-gap spin splitting in the device, observing rich physics in the resulting spectra, that we model using a zero-bandwidth model in the Yu-Shiba-Rusinov limit. The demonstration of controllable proximitization at the nanoscale of a germanium quantum dot opens up the physics of novel spin and superconducting qubits, and Josephson junction arrays in a group IV material.