High fidelity gates, dynamic nuclear polarization and spin-orbit interaction in GaAs two-electron spin qubits.

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GaAs based two-electron-spin qubits have demonstrated great promise for quantum computing. An important requirement for further progress is to realize high fidelity gate operations for manipulating individual and multiple qubits. The principles for these control gates are well established, and the demonstrated long coherence time of our qubit compared to the gate duration promises remarkably low error rates. Yet, the gate fidelity has not been optimized or characterized in detail so far. Systematic errors, which can arise, e.g., from poorly calibrated control pulses and coupling to the qubit, are large for current approaches. One goal of this project is to completely eliminate systematic errors from single qubit gates, and to verify the achievable performance. Thus, we are aiming to reach the fidelity limit set by decoherence. Because of the way the quibt is controlled, standard Rabi pulses are not applicable. We will thus first identify suitable control pulses via sufficiently realistic simulations, and then fine tune them on the experiment in such a way that the desired gates are obtained. Reaching this goal is crucial for progress towards scalable quantum information processing, and will also be very useful for conducting more accurate and detailed experiments to understand decoherence in GaAs qubits, for example via dynamical decoupling.For further improvements of qubit performance and based on fundamental interest, it is also important to better understand the device physics. A particular relevant aspect for GaAs (and many other) spin qubits is the hyperfine interaction between electrons and nuclear spins, which, if not treated properly, is the dominating source of dephasing. A second goal of this project is to further refine some of the methods to reduce decoherence due to nuclear spins, and to answer pressing open questions regarding their effectiveness and the underlying mechanisms. Our particular focus will be on so called narrowing procedures, which reduce fluctuations of the nuclear spins via dynamical nuclear polarization and feedback. While these have been quite successful, their effectiveness leaves room for major improvements. We thus plan to improve them towards their fundamental limits, and to explore the nature of the latter. An important topic in this context is the intricate interplay between hyperfine and spin-orbit interaction, which we propose to characterize with advanced and partially novel measurement techniques.While the two components of this project pursue different goals, they are related by significant synergies both in terms of scientific results and practical aspects. Both can be performed on a single sample in one experimental setup, thus maximizing the utilization of existing resources.