Optical response of a superfluid state in dilute atomic Fermi-Dirac gases

We theoretically study the propagation of light in a Fermi-Dirac gas in the presence of a superfluid state. BCS pairing between atoms in different spin levels increases the optical linewidth and line shift of a quantum degenerate Fermi-Dirac gas already at very low densities and introduces a collisional local-field shift that may dramatically dominate the Lorentz-Lorenz shift. These optical properties could possibly signal the presence of the superfluid state and determine the value of the BCS order parameter.