Guido Burkard, University of Konstanz
Both semiconductors and carbon each have their own advantages and problems as host materials for spin qubits, and both are currently being studied experimentally and theoretically. In the first part of my talk, I will concentrate on spin qubits in III-V semiconductors, where the hyperfine coupling to a large ensemble of nuclear spins brings up new and interesting questions. Among them is the possibility to use pulsed-gate techniques in a two-electron double quantum dot system to prepare a large number of nuclear spins, as well as to coherently manipulate a singlet-triplet T+ qubit. Both processes can be described theoretically based on Landau-Zener-Stückelberg transitions. The second part of the talk will be on spin qubits in carbon-based quantum dots. Carbon has recently emerged as an interesting alternative material for spin qubits, due to the low concentration of nuclear spins and relatively weak spin-orbit coupling. I will discuss the formation of quantum dots in extended graphene, being a non-trivial issue due to the absence of a band gap and the effect of Klein tunneling. Further topics include the role of the valley degeneracy for spin qubits, hyperfine interactions with 13C nuclear spins and their manifestation the spin-valley blockade, as well as spin-orbit induced spin relaxation for spin qubits in graphene.