Superconducting quantum devices with germanium nanowires

This thesis investigates the potential of germanium (Ge)-based nanowires as a platform for quantum information processing, particularly focusing on superconductor-Ge hybrid devices. In the past five years, hole spin qubits in Ge quantum wells have seen rapid advancements. Long coherence times, rapid operations and high gate fidelity have been achieved. Yet, challenges remain in establishing long-distance interconnections for semiconductor spin qubits. In the meantime, superconducting qubits stand out as another leading quantum computing platform. Their larger physical size and macroscopic quantum states facilitate remote coupling. Can we combine the advantages of both platforms? This is the central question that this thesis seeks to address. Our goal is to define and coherently manipulate qubits in Ge/Si core/shell nanowires (NWs), leveraging Ge’s unique properties and integrating Ge into circuit-QED architectures.
On the path of achieving this goal, several crucial steps must be undertaken. First and foremost, we established reliable and reproducible recipes to making superconducting contact to Ge NWs. To gain a deeper insight of the Andreev bound states (ABSs) in Ge/Si nanowire Josephson junctions (NW JJs), we conducted DC measurements. Our study included tunnelling spectroscopy of ABSs, and the current-phase relation measurements of Ge/Si NW JJs using superconducting quantum interference device (SQUID).
To develop qubits in the Ge hybrids system, it is necessary to integrate Ge/Si NW JJs into microwave circuits. In this context, we explored two types of qubits: superconducting qubits, which utilize collective bosonic modes, and Andreev qubits, defined in fermionic quasiparticles of the ABSs spectrum. In Chapter 7, the NW JJ is shunted by a large capacitor, forming a gatemon qubit. In Chapter 8, we embedded the NW JJ in a RF SQUID loop, aiming for a Andeev qubit.