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

由砷化铟等半导体形成的量子点作为人造原子，已广泛应用于许多当前的光电器件中。然而，它们在量子计算、通信和传感方面的应用受到了这些点中电子激发态相对较短的寿命的限制。最近，我们的团队发现，在这些点中极化核自旋导致寿命惊人地增加了许多数量级，这为在下一代量子电子学中使用量子点打开了潜力。该项目将从实验和理论两方面探索寿命急剧增加背后的基础物理学。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（包括来自NSF-Imes-Moore bridge项目的学生）。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。在之前NSF支持的工作中，我们发现了通过光耦合与e-h自旋产生的单个和耦合InGaAs量子点（QDs）中的动态核自旋安静（DNSQ），并伴有明显的动态核自旋极化（DNSP）。结果显示了用DNSQ表征介观亚稳核自旋构型的局域和非局域产生。这种效应持续时间很长（1秒），反映了包含10,000个原子核的亚稳态介观核态的产生和锁定。潜在的物理机制尚不清楚，但显然是由光学驱动的e-h自旋与量子点原子核之间通过超精细耦合的非线性耦合介导的。大的激子玻尔半径导致了电子-空穴自旋与点中几乎所有原子核的相互作用，而不像一个只有一个原子核的简单原子。这些结果对于将量子点或其他结构应用于基于自旋的量子光电器件，如量子中继器（e -自旋作为存储器，自发发射的光子作为飞行的量子位），以及将核系综态用于量子计量学、经典存储器甚至改进核磁共振成像的潜在用途具有重要意义。本研究的重要性在于，锁定的DNSQ在没有动态干预的情况下，将电子自旋相干时间提高了三个数量级以上。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景，静止核自旋综态的性质仍然是一个谜。提出的理论侧重于在电子自旋的光学控制下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于系综的介观性质和自旋动力学，为全面理解物理学制定详细的理论。这一结果可能会导致电子-光子信息处理在长时间尺度上的进步，比如量子中继器或测量过程。类似于自旋玻璃的核静止状态，也可能为经典计算机科学及其他领域提供一个实验室。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。这包括确定这些状态下核自旋的纵向和横向弛豫率，包括确定核自旋状态下的量子相干性水平；和2。测量和理论上预测单量子点和耦合量子点中电子自旋退相干作为各种介观核态的函数。这项工作将有助于提高对非平衡微观量子系统（几个e-自旋）和介观量子系统（核自旋数量热力学极限）之间相互作用的理解。电子自旋态光学控制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，这是一个通过电子态测量介观系统（核）性质的范例系统，或者是在核系综作为可控环境的影响下，作为相关电子系统的量子测量。对于潜在的应用，量子物理的宽带（太赫兹）可以实现高速光控制，没有连接问题，工作在4-10 K。

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.