Hole spins in Ge-Si nanowires

In a universal quantum computer, coherent control over the state of a quantum mechanical two-level system is needed. This requires interactions of the quantum state with its environment. Inherently, such interactions also lead to decoherence and thus limit the performance of the quantum computer. A profound knowledge of the relevant interaction mechanisms is therefore key to the realization of a quantum computer. In this thesis we use Ge-Si core-shell nanowires to investigate holes confined to one dimension. Mixing of heavy and light hole states leads to a strong, anisotropic spin-orbit interaction in this system. We define highly stable quantum dots of different lengths in the nanowire and controllably split up longer quantum dots into double quantum dots. The effective g-factor in these one-dimensional hole quantum dots is found to be highly anisotropic with respect to the nanowire axis as well as the electric-field axis. In double quantum dots, we observe shell filling of new orbitals and Pauli spin blockade of the second hole entering the orbital. The leakage current in the spin-blocked state is highly anisotropic with spin-flip cotunnelling as the dominant leakage mechanism. At finite magnetic fields, we also find signatures of leakage current induced by spin-orbit coupling and anisotropic Coulomb effects.