Bottom-up grown single-crystalline semiconducting nanowires (SNWs) are very versatile systems, since quantum dots can readily be defined using external finger gate electrodes. Due to the large g-factor and large spin-orbit interaction, SNWs are a hot topic today, since one-dimensional helical states can be formed in them. Furthermore, adding a superconductor gaps the helical state and yields topological end modes, so called Majorana bound states. With a pair of SNWs one can even go a step further and generate more exotic particles, such as Parafermions. Majorana fermions and Parafermions are non-Abelian particles. They are studied since they have been proposed as “robust” qubits in topological quantum computing. In collaboration with the group of J. Nygard from the Niels-Bohr Institute at Copenhagen, who provides InAs SNWs to us, with the group of S. Csonka from the Budapest University of Technology and Seigo Tarucha from University of Tokyo and Riken, we investigate SNWs with superconducting contacts to explore the transition from a normal wire state to the topological phase where exotic zero-modes appear.

Two semiconducting InAs nanowires, NW1 and NW2, are connected to a common superconductor on the right, but individually attached to normal metal contacts on the left side. This device is a first test towards the realization of Parafermions caused by Cooper-pair splitting.

Current challenges are to correlate the transition to a topological phase with a helical gap and further to enhance Cooper-pair splitting to allow for the formation of Parafermions.