Collaborative Research: Optically Created Metastable Mesoscopic Nuclear Spin States: Glassy Transitions and Properties Beyond Electron Decoherence in Quantum Dots

非技术摘要量子点由砷化铟等半导体制成，用作人造原子，已在许多当前的光电子器件中得到广泛应用。然而，由于这些点中电子的激发态寿命相对较短，它们在量子计算、通信和传感方面的应用受到了限制。最近，我们的团队发现，极化这些点中的核自旋导致寿命惊人地戏剧性地增加了许多数量级，为在下一代量子电子学中使用量子点打开了可能性。这个项目将从实验和理论上探索寿命急剧增加背后的基础物理。这项研究将支持在包括博士后、研究生和本科生在内的各级高等教育中培养对纳米和量子技术基础设施至关重要的训练有素的人才(包括NSF-Imes-Moore Bridge项目的学生)。这一研究结果不仅将在量子电子学中开辟潜在的应用，还将为磁共振成像和从摩尔定律过渡到后CMOS时代的新范式的发展提供新的途径。技术摘要在以前NSF支持的工作中，我们在单个和耦合的InGaAs量子点(QD)中发现了动态核自旋静止(DNSQ)，这是由光学耦合到e-h自旋而产生的，并伴随着明显的动态核自旋极化(DNSP)。结果表明，DNSQ表征的介观亚稳核自旋组态既有局域产生，又有非局域产生。这种效应持续时间很长(1秒)，反映了包含10,000个原子核的介观亚稳核态的产生和锁定。潜在的物理机制尚不清楚，但显然是由光学驱动的e-h自旋和量子点的原子核之间通过超精细耦合实现的非线性耦合所致。与只有一个原子核的简单原子不同，大的激子玻尔半径导致电子-空穴自旋与几乎所有原子核的相互作用。这一结果对于量子点或其他结构在基于自旋的量子电光器件中的应用具有非常重要的意义，例如量子中继器(e-自旋用作存储器，自发发射的光子用作飞行量子比特)，以及核系综态在量子计量、经典存储甚至改进磁共振成像中的潜在用途。这项研究的重要性在于，在没有动态干预的情况下，锁定的DNSQ将电子自旋相干时间增加了三个数量级以上。到目前为止，理论研究还没有提供与所有数据一致的统一图景，静止的核自旋系综态的性质仍然是一个谜。该理论主要研究了e自旋光学控制下的静态核态相对于热核态的性质。自旋玻璃的概念和方法将适应介观性质和系综的自旋动力学，为全面理解物理学制定详细的理论。这一结果可能会在长时间尺度上推动电子-光子信息处理的进步，例如量子中继器或测量过程。原子核的静止状态，类似于自旋玻璃，也可能为经典计算机科学和更远的领域提供实验室。该提议有两个科学目标：1.测量并从理论上理解导致各种介观亚稳态DNSQ态的两个不同量子系统(核系综自旋和e-自旋)之间相互作用的动力学。这包括确定这些态中核自旋的纵向和横向弛豫速率，包括确定在转换后核自旋态的量子相干能级；以及2.测量和理论预测作为各种介观核态的单个和耦合量子点中的e-自旋退相干。这项工作将提高对非平衡微观量子系统(几个e自旋)和介观量子系统(核自旋数目热力学极限)之间相互作用的理解。光学控制电子自旋态的机制的复杂性源于由此产生的修正介观核态对光学控制的电子自旋的反向作用。对于量子计量学来说，这是一种通过电子态来测量介观系统(核)的性质的范例系统，或者作为作为可控环境的核系综影响下的关联电子系统的量子测量。对于潜在的应用，量子物理的宽带(THz)实现了没有连接问题的高速光学控制，并在4-10K下运行。

Nontechnical AbstractQuantum dots, formed from semiconductors such as Indium Arsenide, act as artificial atoms and have seen widespread use in many current optoelectronic devices. However, their use for applications in quantum computing, communication and sensing has been limited by the relatively short lifetime of the excited state of the electrons in these dots. Recently, our team have discovered that polarizing the nuclear spins in these dots leads to a surprising and dramatic increase of the lifetime by many orders of magnitude opening the potential for using quantum dots in the next generation of quantum electronics. This project will explore the fundamental physics behind this dramatic increase in lifetime both experimentally and theoretically. The research will support the development of highly trained people, (including students from the NSF-Imes-Moore Bridge-program in Applied Physics) critical to the infrastructure of nano and quantum technology, at all levels of higher education including postdocs, graduate and undergraduate students. The results of this research will not only open up potential applications in quantum electronics but could provide advances in areas such as magnetic resonance imaging and the development of a path for transitioning from the Moore's law to the new paradigm of a post CMOS era.Technical AbstractIn previous NSF supported work, we discovered dynamic nuclear spin quieting (DNSQ) in single and coupled InGaAs quantum dots (QDs) produced by optical coupling to the e-h spin that is accompanied by clear dynamical nuclear spin polarization (DNSP). The results show both local and nonlocal creation of mesoscopic metastable nuclear spin configurations characterized by DNSQ. The effect is long lived (1sec) and reflects creation and locking of a metastable mesoscopic nuclear state involving 10,000 nuclei. The underlying physics is not clear but is obviously mediated by a nonlinear coupling between the optically driven e-h spins and the nuclei of the quantum dot through hyperfine coupling. The large exciton Bohr radius results in interaction of the electron-hole spins with nearly all the nuclei in the dot, unlike a simple atom with one nucleus. The results are highly significant for application of QDs or other structures to spin based quantum electro-photonic devices, such as quantum repeaters (e -spin serves as memory and the spontaneously emitted photons serves as the flying qubit) and for the potential use of the nuclear ensemble states for quantum metrology, classical memory and perhaps even improving MRIs. The importance of this research is rooted in that locked DNSQ increases e- -spin coherence time by more than three orders of magnitude , without dynamic intervention. The theoretical studies have, thus far, not provided a unified picture consistent with all the data, and the nature of the quiescent nuclear spin ensemble states remains a mystery. The proposed theory focuses on the properties of the quiescent nuclear states vis-a-vis the thermal nuclear states under optical control of the e-spin. Spin glass concepts and methodology will be adapted to the mesoscopic nature and the spin dynamics of the ensemble, to formulate a detailed theory for a comprehensive understanding of the physics. The results could lead to advances in electronic-photonic information processing on a long time scale, such as a quantum repeater or measurement processes. The nuclear quiescent state, analogous to spin glass, may also provide a laboratory for classical computer science and beyond. The proposal has two scientific objectives: 1. Measure and theoretically understand the dynamics of the interaction between the two distinct quantum systems (the nuclear ensemble spin and the e- -spin) leading the various mesoscopic metastable DNSQ states. This includes determining both longitudinal and transverse relaxation rates of the nuclear spin in these states including determining the level of quantum coherence in the nuclear spin states following switching; and 2. Measure and theoretically predict the e- -spin decoherence in single and coupled QDs as a function of the various mesoscopic nuclear states. This work will result in improving the understanding of the interaction between a non-equilibrium microscopic quantum system (a few e-- spins) and a mesoscopic quantum system (number of nuclear spins thermodynamic limit). The complexity of the mechanism of optical control of the e --spin states stems from the back action of the resulting modified mesoscopic nuclear state on the optically controlled e- -spins . For quantum metrology, this is a paradigm system for measuring the properties of the mesoscopic system (nuclear) via the electron states or as a quantum measurement of the correlated electron systems under the influence of the nuclear ensembles as controllable environment. For potential applications, the broad bandwidth (THz) of the quantum physics enables high-speed optical control without connectivity problems and operates at 4-10 K.