  Planar germanium quantum wells have recently been shown to host a hard-gapped
superconductor-semiconductor interface. 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 are a crucial ingredient towards topological superconductivity 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
robust critical out-of-plane field of 0.91(5) T. 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.
