Gate-Tunable Superconductivity

The work presented in this thesis was performed in the alternative qubits group at IBM Research Europe - Zurich. The results of three projects are presented in the following.
In chapter 2 we review important concepts which are fundamental for understanding the subsequent results. Our discussion ranges from BCS theory and quasiparticle disequilibrium in superconductors to hybrid superconductor-semiconductor interfaces, templated semiconductor epitaxy and the Aharonov-Bohm effect. Chapter 3 introduces the various fabrication methods used in this work, with special focus on patterning of superconducting nanowires, indium arsenide (InAs) epitaxy in hybrid templates, and device fabrication on selective-area-grown lead telluride (PbTe) structures. In chapter 4 we investigate gated metallic superconducting nanowire switches. We explain the mechanism for triggering the transition from the superconducting to the normal state, an effect that was controversially discussed in literature and could find use in superconducting signal routers and multiplexers in the near future. Chapter 5 presents a novel method for semiconductor-superconductor hybrid device fabrication which relies on InAs epitaxy inside superconducting, CMOS compatible templates. After further optimization, this method might enable the monolithic integration of classical cryogenic CMOS control electronics into scalable quantum processors. In chapter 6 we present the first transport experiments on selective-area-grown lead telluride (PbTe) structures on InP. High electron mobility and phase coherence length, together with extreme material properties, make PbTe a promising building block for topological qubits. In chapter 7 we summarize our findings and propose future directions for all three projects.