4thversion_modThere is an intense effort in information technology to find solutions to the problems emerging from the miniaturization of conventional complementary metal oxide semiconductor devices. In microelectronics, researchers are trying to create ever smaller and faster transistors by decreasing their dimensions or by choosing materials with different more promising properties. Researchers in basic research on the other hand are investigating new concepts which would allow information processing to operate on completely different principles. In this line, Loss and DiVincenzo suggested the use of electron spins confined in lithographically defined quantum dots (QDs) as elementary quantum bits (qubits) to realize a quantum computer. In the past few years Si and Ge have emerged as very promising hosts for the realization of spin qubits since they can be isotopically purified leading thus to very long coherence times.  In parallel to the development of spin qubits, there has been recently a huge wave of excitement in the prospect of using topological qubits (TQs) for quantum computation. Such TQs are predicted to be robust versus decoherence. In the main focus of these proposals are the so-called Majorana fermions, introduced by Majorana more than 70 years ago. Various studies have suggested the use of topological insulators and semiconductor nanowires for the realization of Majorana fermions. Indeed in the past few years first experimental signatures of Majorana fermions have been reported.
An intriguing perspective for quantum computation would open if one would be able to coherently transfer quantum information between spin qubits and TQs so as to combine the advantages of the two systems (spin qubits: easier manipulation and read out – TQs: more robust). In view of such a long term dream, Ge-based nanostructures emerge as a very promising system, since they demonstrate an enormous potential both for spin qubits and Majorana bound states.
In our group we study spin qubits in Ge based systems, self-assembled QDs and lithographically defined QDs in two dimensional hole gases. In parallel we aim to understand whether Majorana fermions can be realized and detected in a hole-type system. Finally hybrid InAs nanowire devices are studied in order to prove the exotic properties of Majorana fermions.