This NSF award will support the development of a new suite of tools to control and measure physical systems whose behavior is governed by quantum mechanics. The project will extend current methods for single atom control to a many-body system associated with a large collection of atoms. The fundamental goal is to use light as a quantum bus to connect distant atoms, and thus gain direct control over the quantum state of the combined many-body spin without the need for individual atoms to interact directly. This will be achieved through coherent optical feedback, wherein a light beam is passed through the atom cloud to pick up information about the spin, and then returned in a second pass to transform the spin quantum state accordingly. One important measure of success is the degree to which one can reduce (squeeze) the fundamental quantum uncertainty of the many-body spin. Spin squeezing has direct application in quantum sensing and metrology, and may be a valuable resource for quantum communication and computing. Coherent optical feedback has the potential to produce squeezing that improves exponentially with the product of time and optical density of the atom cloud, and could be a game changer relative to existing schemes that improve only linearly. Experiments will be performed to determine the possible improvement in a real-world situation that includes decoherence, optical losses and imperfections. Numerical simulation suggest that a factor of ten reduction in quantum uncertainty should be feasible in the planned first generation experiment.
The quest for control of complex quantum systems is a grand challenge currently pursued across a broad spectrum of physics disciplines. The effort is motivated by applications that range from the simulation of quantum many-body physics to quantum-limited metrology and quantum information processing, all of which share the goal of harnessing uniquely quantum properties to perform tasks that are not otherwise possible. This project will contribute to the knowledge base of quantum information science, and to the training of future scientists in this highly interdisciplinary field. Students will be involved in all aspects of the project, including education, research, and the dissemination of results. The project will become a cornerstone of the NSF supported Center for Quantum Information and Control, co-located at the University of Arizona College of Optical Science and the University of New Mexico Department of Physics and Astronomy. Weekly video conferencing, an annual research retreat, and joint participation in conferences will enrich the educational experience and strengthen the connections between junior and senior participants at both institutions.
This project contributed new ideas and capabilities to the quest for high-performance atom-light quantum interfaces. Given such an interface, one can use light as a quantum bus to connect distant members of a large atom ensemble of atoms, and thus gain direct control over the atomic quantum many body state. One important application of such control is to generate spin-squeezed states, with which one can reduce quantum uncertainty and improve the accuracy of, for example, magnetometers, inertial sensors, and atomic clocks. In the course of three years of experimental and theoretical research, the project achieved a number of significant milestones towards that goal. The first complete theoretical model of an atom-light interface was developed for realistic three-dimensional configurations, allowing for the first time a quantitative understanding of the collective spin-wave that is measured and squeezed by a light beam of given shape. New protocols were developed that use quantum control of the internal atomic quantum state to strengthen the atom-light interface and enhance spin squeezing. On the experimental side, the project supported a complete rebuild of a unique laser cooling and trapping apparatus designed for quantum control of individual and collective atomic spins. This resource was initially shared with an experiment that developed MRI-like techniques to perform quantum gates on individual atoms in an optical lattice. Following that, experiments focused on the quantum interface between a single laser beam and a cloud of up to a million ultracold atoms confined in an optical trap. This work led to successful detection and strong conditional squeezing of quantum fluctuations in the collective atomic spin. Parallel efforts developed a novel toolbox for high-accuracy quantum control of internal atomic quantum states, thus setting the stage for future implementation of the above-mentioned protocols for stronger atom-light interfaces and improved spin squeezing. Finally, the project contributed to the setup of a separate laboratory dedicated to a new type of atom-light quantum interface based on atoms trapped near the surface of an optical nanofiber. If successful, this approach may eventually lead to atom-light quantum interfaces that can be incorporated directly into optical fiber networks. Significant results from the project were disseminated through peer reviewed publications and presentations at scientific conferences and workshops, thus contributing to the knowledge base and technology toolbox available to the broader Quantum Information Science community. The project also served as a cornerstone for the NSF-funded Center for Quantum Information and Control (CQuIC), thereby contributing to the education and training of a large group of PhD students and postdocs at the University of Arizona and the University of New Mexico.
