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

基于砷化镓的双电子自旋量子比特在量子计算中显示出巨大的前景。进一步发展的一个重要要求是实现操作单个和多个量子比特的高保真门操作。这些控制门的原理已经确立，与门的持续时间相比，我们的量子比特的相干时间很长，这保证了非常低的错误率。然而，到目前为止，门的保真度还没有得到详细的优化或表征。对于当前的方法来说，可能产生的系统误差很大，例如，由校准不良的控制脉冲和耦合到量子比特引起。该项目的目标之一是完全消除单个量子比特门的系统误差，并验证其可实现的性能。因此，我们的目标是达到退相干设定的保真度极限。由于Qubt的控制方式，标准的Rabi脉冲不适用。因此，我们将首先通过足够逼真的模拟来确定合适的控制脉冲，然后在实验中对它们进行微调，以获得所需的门。实现这一目标对于量子信息处理的可伸缩性是至关重要的，也将非常有助于进行更精确和详细的实验来理解GaAs量子比特中的退相干，例如通过动态去耦合。为了进一步提高量子比特的性能并基于基本兴趣，更好地理解器件物理也是非常重要的。对于GaAs(和许多其他)自旋量子比特，一个特别相关的方面是电子和核自旋之间的超精细相互作用，如果处理不当，这是退相的主要来源。该项目的第二个目标是进一步改进一些减少核自旋引起的退相干的方法，并回答有关其有效性和潜在机制的紧迫公开问题。我们将特别关注所谓的窄化过程，它通过动态核极化和反馈来减少核自旋的波动。虽然这些措施相当成功，但成效仍有重大改善的余地。因此，我们计划改进它们，使之接近它们的基本极限，并探索后者的性质。在这一背景下，一个重要的主题是超精细和自旋-轨道相互作用之间的复杂相互作用，我们建议用先进的和部分新颖的测量技术来表征这一相互作用。虽然这个项目的两个组成部分追求不同的目标，但它们在科学结果和实践方面都有显著的协同效应。这两种方法都可以在一个实验装置中的单个样品上进行，从而最大限度地利用现有资源。

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.