Quantum transport in nanowire networks

Quantum technology is a promising area of research, with the quantum computer as the prime example. Quantum computers can perform calculations thought to be impossible by conventional means. The fundamental building block of a quantum computer is a qubit, which is a quantumsystem which can be used to process and store quantuminformation. Most qubits can hold on to this quantuminformation for only a short period of time due to environmental noise. The resulting errors can be mitigated by storing the information in multiple qubits. An alternative approach uses qubits which are insensitive to noise. This can be achieved by using topological quantumstates.
An example of a topological quantum state is the Majorana zero mode. Majorana zero modes can be realized in a 1D system with strong spin-orbit coupling and superconductivity, in an external magnetic field. Such materials are not known in nature, but can be engineered by coupling a semiconductor nanowire to a superconducting material. To use these Majorana zero modes as qubits, multiple nanowires have to be connected to each other in a 2D network. The experiments described in this thesis aim to develop such networks based on InSb (indium antimonide) semiconductor nanowires.
A few necessary theoretical concepts are briefly introduced. Subsequently, the nanofabrication and electrical measurement techniques used to study the nanowires are described, with emphasis on the challenges related to working with hybrid semiconductorsuperconductor (InSb-Al)materials. Two methods are then presented to realize nanowire networks. Transport experiments on these networks show strong phase coherence and a hard superconducting gap, demonstrating the high quality of the material.
In addition to the intrinsic quality of the material, the electrostatic environment plays an important role for the functionality of hybrid materials. The coupling between the superconductor (Al) and the semiconductor (InSb) is studied by applying an external electric field. This electric field influences material properties such as the spin-orbit coupling and the Landé g -factor. An essential property of the Majorana zero modes is the fact that their state cannot be described locally. Exploratory experiments with the aim of demonstrating this non-locality are described, followed by theoretical simulations demonstrating the limitations of common experimental practice based on local measurements. Finally, several suggestions for future experiments are made, aimed at demonstrating and manipulatingMajorana zero modes.