Progress in the materials science of hybrid nanowires for topological devices

Semiconductor nanowires are a backbone in proposals for topological quantum computing based on manipulation of Majorana quasiparticles. Experimentally, various techniques exist for synthesis of semiconductor nanowires for quantum transport. For most applications, the fabrication method is not important once the semiconductor growth conditions and quality have been optimised. There are also different routes for forming heterostructures, branched geometries and other advanced nanowire materials. However, we will here focus on a unique process that is particularly promising for topological devices: epitaxial metal/semiconductor heterostructures based on Molecular Beam Epitaxy. Under vacuum conditions, an aluminum shell is grown in-situ onto InAs nanowires, leading to an epitaxially matched interface between the semiconducting core and the metal coating that acts as a superconductor at low temperatures. The perfect superconductor-semiconductor interface results in proximity induced superconductivity with a hard gap. The technique is compatible with formation of branched nanostructures and opens up for new directions in nanowire based quantum devices, e.g. transmon qubits and topological systems