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

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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.