BCS-BEC Crossover and the Unitary Fermi Gas

TL;DR

This paper reviews the BCS-BEC crossover in Fermi gases, emphasizing recent experimental and theoretical advances in the strongly interacting unitary regime, and explores connections to other physics areas.

Contribution

It provides a comprehensive overview of recent progress in understanding the strongly interacting unitary Fermi gas, combining experimental insights with new theoretical developments.

Findings

Experimental realization of BCS-BEC crossover in ultracold gases

Insights into thermodynamics, transport, and spectroscopy of the unitary Fermi gas

Connections to quark-gluon plasmas, gauge-gravity duality, and high-temperature superconductivity

Abstract

The crossover from weak coupling Bardeen-Cooper-Schrieffer (BCS) pairing to a Bose-Einstein condensate (BEC) of tightly bound pairs, as a function of the attractive interaction in Fermi systems, has long been of interest to theoretical physicists. The past decade has seen a series of remarkable experimental developments in ultracold Fermi gases that has realized the BCS-BEC crossover in the laboratory, bringing with it fresh new insights into the very strongly interacting unitary regime in the middle of this crossover. In this review, we start with a pedagogical introduction to the crossover and then focus on recent progress in the strongly interacting regime. While our focus is on new theoretical developments, we also describe three key experiments that probe the thermodynamics, transport and spectroscopy of the unitary Fermi gas. We discuss connections between the unitary regime and other areas of physics -- quark-gluon plasmas, gauge-gravity duality and high temperature superconductivity -- and conclude with open questions about strongly interacting Fermi gases.