A Proximitized Quantum Dot in Germanium

Hole spin qubits in strained planar germanium quantum wells (Ge/SiGe) [1] have emerged as a promising qubit candidate for quantum information processing and simulation evidenced by the demonstration of single qubit fidelities above 99.99% [2], tune-up of a 4x4 quantum dot array [3] and the execution of multi-qubit quantum logic [4]. Recently, hard-gapped superconductivity has been engineered for the first time in Ge/SiGe, in a wide range of mesoscopic devices [5]. The demonstration of an isotopically purifiable semiconductor platform that can host clean superconductivity places Ge/SiGe in a uniquely advantageous position, facilitating many opportunities for hybrid superconducting-semiconducting quantum dots, including high fidelity two-qubit gates [6], extended range qubit-qubit coupling without the need for resonators [7], and integration with circuit QED [8]. In this work we make initial steps toward these goals, by presenting a proximiti zed germanium quantum dot (QD) coupled to a superconducting lead (S). We show the emergence of a superconducting gap within the charging energy of a quantum dot via bias spectroscopy, with tunable S-QD coupling. We use this to demonstrate switching between singlet and doublet ground states. We further characterize critical magnetic field strength, finding remarkably robust critical field strength of 0.911 T along the out-of-plane axis. Finally, we study spin splitting of sub-gab states, finding an out-of-plane g-factor of 6.11, and a large anisotropy of the g-factor for in- and out-of-plane field orientations. These results constitute a promising first step towards hybrid superconducting-semiconducting quantum information processing with hole spin qubit in Ge/SiGe.