Exchange fields and relaxation times in quantum dot

本计画的目标是从理论上描述多量子点系统中的时变衰变，以及量子点自旋阀中的自旋电子交换效应，这两个问题都是表征奈米电子系统对外部控制与驱动的响应的重要问题。在子计画（A）中，我们将计算复合量子点系统在驱动下脱离平衡态的衰变模式，并确定如何使用通过一个组成量子点的电流来测量其他量子点的衰变模式。这种对时间依赖性响应的理解对于外部驱动量子点器件的物理和技术应用至关重要，例如，分别用于灵敏的电子计数和量子位读出。我们将建立测量装置和协议，以解释量子涨落的影响。特别地，我们将研究如何测量最近预测的量子点的费米子宇称衰变模式，由于量子力学的基本超选择规则，量子点对库仑相互作用和其他局域效应非常不敏感。在子项目（B）中，我们将计算通过耦合到偏置非共线铁磁体的量子点的输运，考虑到自旋相关限制引起的交换场与隧穿和库仑相互作用引起的交换场的竞争。这些机制是对自旋电子学和量子信息纳米结构中单自旋的时间依赖控制的基础。我们将对稳态和随时间变化的测量进行预测，旨在检测和解开这两种效应。特别地，我们将预测量子点和非共线铁磁体之间的隧穿打开后交换场如何随时间发展。对于这两个问题，我们将应用因果超费米子公式的实时图解技术。这种方法揭示了非平衡过程的新的基本方面，并允许我们以更有效的方式获得量子点的约化密度算符的时间演化，补充物理上更透明的量子动力学方程的耦合观测。一个功能的核心，这两个拟议的子项目是，我们可以系统地处理-在一个单一的方法-更高，序扰动的互补限制的任意相互作用和弱隧穿，以及，另一方面，任意隧穿和弱相互作用。一般来说，该方法简化了许多所需的计算。它已经导致了强相互作用量子点的新的精确结果，例如，建立了费米子宇称衰变模型，并显示出成功实现所提出的研究计划的巨大潜力。每个拟议的子项目都涉及一名外部研究人员，他们与领先的实验工作密切合作，带来了建模设备的专业知识。

The goal of the proposed project is to give a theoretical description of (A) time-dependent decay in multiple quantum-dot setups, and (B) spintronic exchange effects in quantum-dot spin valves, both of which are important problems characterizing the response of nanoelectronic systems to external control and driving.In subproject (A), we will calculate the decay modes of composite quantum-dot systems driven out of equilibrium, and determine how the current through one constituent quantum dot can be used to measure the decay modes of the others. This understanding of the time-dependent response is of key importance to physical and technological applications of externally driven quantum-dot devices, e.g., in sensitive electron-counting and qubit readout, respectively. We will establish measurement setups and protocols accounting for the effects of quantum-fluctuations. In particular, we will study how one can measure the recently predicted fermion-parity decay mode of a quantum dot, which is strikingly insensitive to Coulomb interaction and other local effects due to a fundamental superselection rule of quantum mechanics.In subproject (B), we will calculate the transport through a quantum dot coupled to biased noncollinear ferromagnets, accounting for the competition of the exchange field induced by spin-dependent confinement and the exchange field induced by tunneling and Coulomb interaction. These mechanisms are fundamental to the quest for time-dependent control over single spins in nanostructures for spintronics and quantum information. We will make predictions for stationary as well as time-dependent measurements that aim to detect and disentangle these two effects. In particular, we will predict how the exchange fields develop in time after switching on the tunneling between the quantum dot and the noncollinear ferromagnets.For both problems we will apply a causal superfermion formulation of the real-time diagrammatic technique. This approach reveals novel fundamental aspects of nonequilibrium processes and allows us to obtain the time-evolution of the reduced density operator of the quantum dot in a more efficient way, complementary to the physically more transparent quantum-kinetic equations for coupled observables.A feature central to both proposed subprojects is that we can systematically treat -- within a single approach - higher-order perturbations to the complementary limits of arbitrary interaction and weak tunneling, and, on the other hand, arbitrary tunneling and weak interaction. In general, the approach simplifies many of the required calculations. It has already led to new exact results for strongly interacting quantum dots, e.g., establishing the fermion-parity decay mode, and it shows great potential for successfully realizing the proposed research program. Each proposed subproject involves an external researcher who brings in expertise on modeling devices in close collaboration with leading experimental efforts.