Persistent Currents in Fermionic Quantum Gases

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Quantum phenomena are generally most evident at microscopic scales, but there are some special materials where quantum effects are important at large scales. These materials often have extraordinary properties such as superconductivity that make them important for diverse applications such as electrical power distribution, electromagnets, precision sensors, and quantum computing. The many-body quantum physics of these "super" materials can be stunningly complex, and predicting material properties or even the existence of quantum phases of matter has proven to be surprisingly difficult. This award supports laboratory quantum simulation experiments with ultra-cold atoms that will improve our understanding of the mechanisms leading to the formation of superfluid phases, and the way they break down when disturbed in different ways. These experiments will expand present capabilities to create, study, and understand exotic quantum phases of matter that are both of fundamental scientific interest and important to the advancement of technology. In this research program, ultra-cold fermionic atoms (Li-6) will be confined to ring-shaped optical traps, and optical techniques will be used to create persistent currents in the superfluids of fermionic pairs which form when the system temperature is sufficiently low. The initial objective is to create and study persistent currents in a ring-shaped molecular BEC. The researchers will create rotatable Josephson junctions to probe the system and analyze the properties of the system as a function of interaction strength, temperature, and effective dimensionality. These studies will then be extended to include persistent currents in the strongly-interacting unitary limit, and as far into the BCS limit as permitted by the temperatures achievable in the system. The results of these experiments will be analyzed to extract information about the superfluid critical velocity, the transition from phonon-mediated dissipation to pair-breaking excitations, superfluid/condensate fractions, and otherwise validate key theoretical predictions for this important superfluid reference system. Measurements conducted above the superfluid transition temperature will help resolve questions raised by recent experimental observations of persistent currents in mesoscopic normal-metal rings. This program will also establish an essential foundation for longer-term efforts to study the low energy limits of mass and spin transport in unconventional superfluid phases that are less well understood, and/or whose existence has yet to be confirmed.