Cooper pair polaritons in cold fermionic atoms within a cavity

TL;DR

This paper develops a BCS theory for quasiparticles in a Fermi gas strongly coupled to cavity photons, revealing a novel BCS to BEC crossover involving atom-hole excitations as polaritons.

Contribution

It introduces a BCS framework for cavity polaritons in a Fermi gas, describing a new crossover scenario between BCS-like pairs and molecular states.

Findings

Identification of cavity polaritons as quasiparticles in a Fermi gas.

Description of a BCS to BEC crossover involving atom-hole excitations.

Proposal of an alternative crossover scenario in cavity QED systems.

Abstract

We formulate a Bardeen-Cooper-Schriffer (BCS) theory of quasiparticles in a degenerate Fermi gas strongly coupled to photons in a optical cavity. The elementary photonic excitations of the system are cavity polaritons, which consist of a cavity photon and an excitation of an atom within the Fermi sea. The excitation of the atom out of the Fermi sea leaves behind a hole, which together results in a loosely bound Cooper pair, allowing for the system to be written by a BCS wavefunction. As the density of the excitations is increased, the excited atom and hole become more strongly bound, crossing over into the molecular regime. This thus realizes an alternative BCS to BEC crossover scenario, where the participating species are quasiparticle excitations in a Fermi sea consisting of excited atoms and holes.