Collaborative Research: Optically created metastable mesoscopic nuclear spin states: Glassy transitions and properties beyond electron decoherence in quantum dots

量子点，由半导体如砷化铟形成，作为人工原子，在目前的许多光电器件中得到了广泛的应用。然而，它们在量子计算、通信和传感中的应用受到这些点中电子激发态寿命相对较短的限制。 最近，我们的团队发现，将这些量子点中的核自旋极化，会导致量子点的寿命惊人地增加许多数量级，这为在下一代量子电子学中使用量子点开辟了可能性。 该项目将探索实验和理论上这种寿命急剧增加背后的基本物理学。该研究将支持训练有素的人的发展，（包括应用物理学的NSF-Imes-摩尔桥计划的学生）对纳米和量子技术的基础设施至关重要，包括博士后，研究生和本科生在内的各级高等教育。 这项研究的结果不仅将开辟量子电子学的潜在应用，而且可以在磁共振成像等领域取得进展，并为从摩尔定律过渡到后CMOS时代的新范式开辟道路。技术摘要在NSF之前支持的工作中，我们发现，在单个和耦合的InGaAs量子点（QD）中，通过与e-h自旋的光耦合产生的动态核自旋平静（DNSQ）伴随着明显的动态核自旋极化（DNSP）。结果表明，本地和非本地创建介观亚稳核自旋配置DNSQ的特点。这种效应是长寿命的（1秒），反映了涉及10，000个原子核的亚稳介观核态的产生和锁定。其背后的物理机制尚不清楚，但显然是由光驱动的电子自旋与量子点原子核之间的超精细耦合引起的。大的激子玻尔半径导致电子-空穴自旋与点中几乎所有的原子核相互作用，不像一个简单的原子有一个原子核。这些结果对于QD或其他结构应用于基于自旋的量子电光器件（诸如量子中继器（e-自旋用作存储器，并且自发发射的光子用作飞行量子比特））以及对于核系综态用于量子计量、经典存储器以及甚至可能改进MRI的潜在用途是非常重要的。这项研究的重要性在于，锁定DNSQ将e-自旋相干时间增加了三个数量级以上，而无需动态干预。到目前为止，理论研究还没有提供一个与所有数据一致的统一图像，静态核自旋系综态的性质仍然是一个谜。所提出的理论着重于电子自旋的光学控制下的静态核态与热核态的性质。自旋玻璃的概念和方法将适用于介观性质和系综的自旋动力学，以制定一个详细的理论，全面理解物理学。这些结果可能会导致电子-光子信息处理在长时间尺度上的进步，例如量子中继器或测量过程。类似于自旋玻璃的核静态也可能为经典计算机科学及其他领域提供实验室。该提案有两个科学目标：1。测量并从理论上理解导致各种介观亚稳态DNSQ状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。这包括确定在这些状态下的核自旋的纵向和横向弛豫速率，包括确定在切换之后的核自旋状态中的量子相干性水平;以及2.测量并理论预测了单量子点和耦合量子点中电子自旋退相干随各种介观核态的变化。这项工作将导致提高非平衡微观量子系统（几个电子自旋）和介观量子系统（核自旋热力学极限数）之间的相互作用的理解。e-自旋态的光控机制的复杂性源于由此产生的修改的介观核态对光控e-自旋的反作用。对于量子计量学，这是一个范例系统，用于通过电子态测量介观系统（核）的性质，或者作为在作为可控环境的核系综影响下的相关电子系统的量子测量。对于潜在的应用，量子物理学的宽带宽（THz）能够实现高速光学控制而没有连接问题，并在4 - 10 K下工作。

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.