Andreev bound states in semiconducting double nanowires

Parafermions, which can be intuitively seen as fractional Majoranas, offer the advantage of a wider universal set of operations for topological quantum computing when braided. We set out with the ambitious goal of realising a parafermion-hosting double nanowire device. Working towards this goal led us to develop fabrication techniques and yielded measurements that allowed the investigation of multiple phenomena of a double wire semiconductor-superconductor hybrids.
In this thesis, we thus laid the focus on improving the fabrication process in order to reliably individually contacted the wires, as well as understanding the individual phenomena involved in generating the conditions required for the observation of the Majorana modes. We first present a summary presentation of the key theoretical concepts used in this thesis, followed by a chapter where the experimental techniques required for the fabrication and measurement of our devices are introduced. Then we present the core of this work, consisting of the results of three series of experiments. In the first, we design a double wire N-S junction, enabling straightforward access to the properties of our device which allows us to demonstrate the excellent quality of the wire to superconductor interface. In the second experimental chapter, we focus on the Josephson effect, analysing a double nanowire Josephson junction and characterising its RNIC product. In the last chapter we design and report on a quasiparticle trap, which dramatically enhances the critical current of a Josephson junction. We tentatively attribute this effect to the superconducting shell of the wire transitioning to a mixed state, trapping quasiparticles in normal regions close to the Josephson junction.