The BCS–BEC Crossover and the Unitary Fermi Gas

Abstract

There has been great excitement about the recent experimental and theoretical progress in elucidating the Bardeen-Cooper-Schrieffer (BCS) to Bose Einstein condensation (BEC) crossover in ultracold Fermi gases. Prior to these cold atom experiments, all known, and reasonably well understood, superconductors and superfluids were firmly in one of the two limits. Either they were well described by the celebrated BCS theory of pairing in Fermi systems, or they could be understood in terms of the BEC of bosons, with repulsive interactions. For the first time, the ultracold Fermi gases exhibited behavior that, with the turn of a knob, could be made to span the entire range from BCS to BEC. While such a crossover had been theoretically predicted, its actual realization in the laboratory was a major advance [, ], and led to intense investigation of the properties of the very strongly interacting, unitary regime that lies right in the middle of the crossover. We now understand that the unitary Fermi gas has remarkable universal properties, arising from scale invariance, and has connections with fields as diverse as nuclear physics and string theory.