|Abstract:||Semiconductor nanowires are promising materials for studying novel quantum devices. Following proposals to use 1D nanostructure to realize Majorana zero modes, quasiparticles that are relevant for topological quantum computing, many groups have attempted to engineer these modes in hybrid semiconductor-superconductor devices. While evidence for Majorana bound states in nanowires has been shown, many of these devices show behavior far from suitable for quantum control of Majorana modes. Two major experimental obstables must be overcome to realize robust Majorana nanowire devices. First, interfacial inhomogeneity between a superconductor and nanowire leads to a “soft superconducting gap” that prevents the topological nature of Majorana states to be tested. Therefore, pristine semiconductor-superconductor interfaces must be developed. Second, a lack of ballistic 1D behavior can induce quantum-localized bound states that mimic Majorana signatures. Therefore, both the proximitized nanowire and neighboring readout nanostructures should support ballistic transport. This thesis focuses on the resolution of these problems by realizing ballistic superconducting nanowire devices.
The first set of experiment we perform focus on realizing nanowire quantum point contacts in InSb nanowires. In previous experiments on proximitized nanowires, a disordered, quantum localized region of a nanowire was used to probe conductance signatures of Majorana modes. However, the disordered behavior in the probe region complicates the interpretation of local conductance spectroscopy. Alternatively, a quantum point contact, which is a ballistic 1D constriction, provides non-ambiguous signatures in the form of Majorana conductance quantization. To realize quantum point contacts in InSb, we identify mechanisms that lead to localization in InSb nanowire constrictions and develop robust methods for the routine observation of conductance quantization consistent with a 1D constriction having radial symmetry. Additionally, conductance quantization is achieved while using superconducting leads, and proximity effect is identified through conductance signatures of Andreev reflection.
The second set of experiments, we focus on developing superconducting quantum wires through the development of selective-area epitaxy of Al to InSb nanowires. Epitaxial InSb–Al devices generically possess a BCS-like superconducting gap in tunneling measurements and demonstrate ballistic 1D superconductivity and near-perfect transmission of supercurrents in the single mode regime, requisites for engineering and controlling 1D topological superconductivity. Additionally, we demonstrate that epitaxial InSb–Al superconducting island devices, the building blocks for Majorana-based quantum computing applications, prepared using selective-area epitaxy can achieve micron-scale ballistic 1D transport. Our results pave the way for the development of networks of ballistic superconducting electronics for quantum device applications.
The last set of experiments focuses on the development of local tunnel probes for novel spectroscopy InSb nanowire. We demonstrate how to realize tunnel junctions on InSb nanowires, where superconducting tunnel spectroscopy confirms the uniformity of the tunnel barrier. Additionally, fabrication of the tunnel barrier does not disrupt ballistic transport, which is relevant for Andreev bound state or momentum-resolved tunneling spectroscopy. Lastly, we demonstrate the utility of local tunnel juntions to detect signatures of electron-electron interactions that conventional, local transport measurement cannot probe.