Strong spin-orbit interaction, helical hole states, and spin qubits in nanowires and quantum dots

Semiconducting nanowires (NWs) and quantum dots (QDs) are promising platforms for spintronics and quantum computation. Great experimental and theoretical efforts have been made to continuously improve their performances, which is evident from the large variety of setups, material combinations, and operation schemes under investigation. With the work summarized in this PhD thesis, we want to contribute to a better understanding of some of these systems.
The main result of our work is the discovery of a novel spin-orbit interaction (SOI) of Rashba type that arises for holes in NWs in the presence of an electric field. In contrast to conventional Rashba and Dresselhaus SOI, this mechanism is not suppressed by the fundamental band gap and therefore unusually strong. As a consequence, we find that Ge/Si core/shell NWs can host helical hole states with remarkably large spin-orbit energies on the order of millielectronvolts. Furthermore, we propose a setup for universal and electrically controlled quantum information processing with hole-spin qubits in Ge/Si NW QDs. Single-qubit gates can be performed on a subnanosecond timescale; two-qubit gates can be controlled independently and over long distances; idle qubits are well protected against electrical noise and lattice vibrations (phonons).
Another key result follows from our analysis of the phonon-mediated decay of singlet-triplet qubits in lateral GaAs double quantum dots (DQDs). We find that two-phonon processes lead to strong dephasing when the DQDs are biased, and the predicted temperature dependence provides a possible explanation for recent experimental data. When the DQDs are unbiased, the dephasing is highly suppressed and the decoherence times of the qubits are by orders of magnitude longer than those for biased DQDs.
In the last part of the thesis, we present a technique for manipulating the emission polarization and the nuclear spins of a single self-assembled QD. Our scheme exploits a natural cycle in which an electron spin is repeatedly created with resonant optical excitation when the QD is tunnel coupled to a Fermi sea. Among other things, we find that the nuclear spin polarization and the effective electron g factor can be changed continuously from negative to positive via the laser wavelength, with a region of bistability near a particular detuning. An analogous behavior is observed for the average polarization of the spontaneously emitted photons. Our experimental results, some of which are counterintuitive, are very well reproduced with a quantitative model.
The thesis is organized as follows. In Chapter 1, we review experimental and theoretical progress toward quantum computation with spins in QDs, with particular focus on NW QDs, lateral QDs, and self-assembled QDs. In Chapter 2, we study the low-energy hole states of Ge/Si NWs in the presence of electric and magnetic fields. We also consider the shell-induced strain, which strongly affects the NW and QD spectra. In Chapter 3, hole-spin qubits in Ge/Si NW QDs are investigated. We find a highly anisotropic and electrically tunable g factor and analyze the qubit lifetimes due to phonon-mediated decay. A setup for quantum information processing with these qubits is proposed in Chapter 4, where we also present surprisingly simple formulas for the effective Hamiltonian of the qubits. A detailed analysis of the static strain and the low-energy phonons in core/shell NWs is provided in Chapter 5, completing the part on NWs and NW QDs. In Chapter 6, we investigate the phonon-mediated decay of singlet-triplet qubits in lateral DQDs. The developed technique for controlling the emission polarization and the nuclear spins of optically active QDs is discussed in Chapter 7. Supplementary information to Chapters 2-7 is appended.