Measurement-Driven Quantum Dynamics and the Quantum-Classical Transition with Ultracold Atoms

该项目旨在更好地理解超冷原子运动的连续量子测量。这里的核心问题是测量和量子系统的自然动态演化之间的相互作用。这种有条件的量子演化代表了一种具有挑战性的量子动力系统新体制，这既是量子技术的根本利益，也是量子技术长期成功的重要因素。本研究通过原子与经典激光场相互作用的自由空间动力学实验，刻画了连续量子测量的特征。本研究方向从研究原子自由空间运动的量子芝诺效应的实验开始，由于位置测量本身在空间中是局域化的。这种效应表现为来自测量区域的相干反射。这个实验将证明由量子测量的随机反作用力引起的相干演化：即使在强耗散状态下也显示出相干行为的原子镜子。下一个里程碑是演示一个时间和空间分辨测量单个原子的成像光从探针激光散射，实现连续的“海森堡显微镜”。这将允许对连续测量过程进行彻底的表征，包括研究测量记录中量子相关性和经典轨迹行为的出现。这个研究方向的延伸检查了海森堡显微镜的一些微妙之处，特别是在这个系统通常不会产生标准位置测量的情况下。具体来说，实验研究了由于测量反作用导致的从正常扩散到异常扩散的转变。这个研究方向的主要长期目标是测试最近的理论，即连续测量过程如何导致从量子行为到经典行为的过渡。量子-经典边界仍然是量子力学中最具挑战性和最不为人所知的方面之一，特别是对于经典混沌系统。虽然这是一个明确的基本兴趣主题，但对量子-经典跃迁的深刻理解也将为保持量子相干性至关重要的情况和技术提供有价值的见解。这些实验涉及到应用海森堡显微镜来观察单个原子在非调和、时变势下的动力学，这些势通常会引起孤立量子系统中所特有的混沌。主要目标是观察一个受控的、测量诱导的明显的量子系统向经典的混沌轨迹行为的转变。当连续测量迫使原子保持在相空间的局域时，就会发生这种情况，然后它就会沿着一个接近经典的轨道运行。该研究方向还研究了在量子-经典行为之间的过渡区域中丰富而复杂的行为，例如预测会发生“非经典混沌”。本研究项目研究量子力学领域内测量的基本物理，并研究扰动（量子力学预测的测量的基本副产品）如何影响量子系统的运动。因此，这项研究对跨学科的知识做出了重大贡献，包括量子信息、测量和控制领域；原子物理学;以及凝聚态物理。所有这些都是高度活跃的科学领域，具有重要的技术意义和根本利益。推进这些领域的基础知识有助于未来的量子技术，如量子计算和信息处理、量子通信、量子模拟和量子计量。这在获取量子信息和避免退相干（通过与量子系统周围环境耦合而破坏量子力学效应）或潜在地将其充分利用的关键领域尤其如此。教育也是该项目的一个组成部分。这项研究为至少两名研究生提供了论文主题，也涉及到几个本科生在一个令人兴奋的跨学科研究领域。这项研究也直接影响了俄勒冈大学的课程。在各个层次的讲座课程中，量子测量和耗散以及相关的原子光学技术为课程主题提供了相关性，激发了学生的兴趣。俄勒冈大学光学教学实验室也从实验的“技术转移”中受益，因为该实验中使用的模块化，易于复制的设备由教学实验室的学生构建，以建立新的，先进的实验室模块，包括磁光阱和光子下转换实验。

This project aims to achieve a better understanding of continuous quantum measurements of the motion of ultracold atoms. The core issue here is the interplay between measurement and the natural dynamical evolution of a quantum system. Such conditioned quantum evolution represents a challenging new regime of quantum dynamical systems that is both of fundamental interest and important for the long-term success of quantum technologies.This research characterizes continuous quantum measurements through experiments on the free-space dynamics of atoms interacting with classical laser fields. This research direction begins with an experiment to study the quantum Zeno effect for the free-space motion of atoms, due to a position measurement that is itself localized in space. This effect is manifested as a coherent reflection from the measurement region. This experiment will demonstrate coherent evolution induced by the stochastic, back-action force of a quantum measurement: an atom mirror that displays coherent behavior even in a regime of strong dissipation. The next milestone is the demonstration of a time- and space-resolved measurement of a single atom by imaging light scattered from a probe laser, a realization of a continuous "Heisenberg microscope." This will allow a thorough characterization of a continuous measurement process, including studies of the emergence of quantum correlations and classical-like trajectory behavior in the measurement record. An extension of this research direction examines some subtleties in the Heisenberg microscope, particularly in how this system does not normally give rise to a standard position measurement. Specifically, the experiment studies the transition from normal to anomalous diffusion due to the measurement back-action.The main long-term goal of this research direction is to test recent theories of how continuous-measurement processes can cause a transition from quantum to classical behavior. The quantum-classical boundary remains one of the most challenging and least understood aspects of quantum mechanics, especially for classically chaotic systems. While this is a topic of clear fundamental interest, a deep understanding of the quantum--classical transition will also provide valuable insight in situations and technologies where the preservation of quantum coherence is paramount. These experiments involve the application of the Heisenberg microscope to observe the dynamics of a single atom in anharmonic, time-dependent potentials, potentials that classically give rise to the chaos that is characteristically absent from isolated quantum systems. The major goal is to observe a controlled, measurement-induced transition of a manifestly quantum system to classical, chaotic-trajectory behavior. This occurs when continuous measurement forces the atom to remain localized in phase space, where it then traces out a nearly classical orbit. This research direction also studies the rich and complex behavior in the transition region between quantum-classical behavior, where, for example, "nonclassical chaos" is predicted to occur.This research project studies the fundamental physics of measurements within the realm of quantum mechanics, and investigates how the disturbances (fundamental by-products of measurements, as predicted by quantum mechanics) influence the motion of quantum systems. This research therefore contributes significantly to knowledge across disciplines, including the areas of quantum information, measurement, and control; atomic physics; and condensed-matter physics. All of these are highly active scientific fields of technological importance and fundamental interest. Advancing fundamental knowledge in these areas facilitates future quantum technologies, such as quantum computation and information processing, quantum communication, quantum simulation, and quantum metrology. This is particularly the case in the critical areas of acquiring quantum information and avoiding decoherence (the destruction of quantum-mechanical effects by coupling to the surroundings of a quantum system) or potentially putting it to good use.Education is also an integral component of the project. This research provides dissertation topics for at least two graduate students, and also involves several undergraduates in an exciting and interdisciplinary research area. This research also directly impacts coursework at the University of Oregon. In lecture courses at all levels, quantum measurement and dissipation, as well as related atom-optical techniques, provide relevance for course subject matter and stimulate student interest. The University of Oregon optics teaching laboratory also benefits from "technology transfer" from the experiment, in that modular, easily reproduced equipment designed for use in the apparatus is constructed by teaching-laboratory students to build up new, advanced-laboratory modules, including a magneto-optic trap and a photon-down-conversion experiment.