  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.
