Problem/issue being addressed:
Each particle has its antiparticle, and upon bringing them in close vicinity, they annihilate (they disappear). An interesting question is therefore: what happens if a particle is its own antiparticle? Any such particle can only be created as a pair and whenever one disappears, the other must disappear as well. In this sense, a pair of such particles are very robust if they are brought far apart from each other: no local perturbation can destroy it since, due to its locality, it does not affect its pair. Although such particles have not been reported, certain excitations in semiconductors provide the same features: creating the excitation occurs through the same operator as annihilating the excitation. Therefore, these excitations can also only exist in pairs, and creating them spatially separated protects them from local perturbations. Such semiconductor excitations are called Majorana fermions.
Importance for society
Once created in low-dimensional semiconductors, moving these Majorana fermions with respect to each other follows different laws than the three-dimensional world suggests. For example moving one particle around the other, in a two-dimensional world, is related to a phase acquisition. While this feature is in itself intriguing, and even more, it is also useful: so-called braiding (i.e. moving around with respect to each other) of Majorana fermions can be used to perform simple quantum operations. Eventually, Majorana fermions may be important building blocks for quantum computers.
Overall objectives
Majorana fermions are predicted to emerge at the ends of a one-dimensional semiconductor proximitized to a superconductor if the semiconductor hosts a large spin-orbit field perpendicular to the wire and it is subject to a Zeeman magnetic field perpendicular to the spin-orbit field. While these conditions have been fulfilled and some evidence for Majorana bound states (MBS) have been provided, several open questions remain. Importantly, most studies have been performed in one kind of material system, namely InAs. However, holes in germanium are a promising alternative. The objectives of this project are demonstration of controlled confinement of hole states (which is used to build a one-dimensional wire), observation of large spin-orbit interaction, and development of high-transparency superconductor-germanium contacts.
