Hole spins in Ge/Si nanowires

Single hole spins confined in quantum dots (QDs) in Ge/Si core/shell nanowires (NWs) combine several properties which make them potentially very unique qubits. The natural abundance of non-zero nuclear spins in both Si and Ge is small and can be further reduced to a negligible amount by isotopic purification. Furthermore, hole spins have no contact hyperfine interaction due to their p-type wavefunction. These properties make hole spin qubits in Si and Ge resilient against dephasing via interaction with nuclear spins.

A particularly promising feature of hole spins in Ge/Si core/shell NWs is the nature of spin-orbit interaction (SOI) in this system. Confinement to one dimension gives rise to an SOI in the valence band, which is predicted to be both strong and electrically tunable. This promises electrical gating of the SOI, allowing to switch to a large SOI for high interaction strengths and fast quantum operations, or to turn off SOI for increased qubit coherence.

We demonstrate and characterize single, double and triple QD arrays in Ge/Si NWs, all with low hole occupation numbers. In the double QD configuration, we observe Pauli spin blockade and see indications of a sizeable SOI causing spin flips. Finally, we report progress towards electrically tuning the SOI.