Refractive index of driven dense atomic gases

The optical properties of an atomic gas, including the dramatic reduction of the group velocity of light associated with electromagnetically induced transparency (EIT), usually grow with the density of atoms in the medium. However, in high density atomic gases, the resonant dipole-dipole interaction (DDI) generates atomic correlations. These correlations could significantly modify both linear and nonlinear optical properties of the medium [1, 2, 3], which may be beneficial for quantum information processing. We develop a theory that describes the influence of DDI on the optical properties of a dense gas driven by classical control fields. Our method combines dressed-states of quantum optics with nonequilibrium many-body theory based on Feynman diagrams and includes the effect of the atomic centerof- mass-motion, which is relevant for the study of different temperature regimes. This approach has the advantage that the control fields are treated nonperturbatively.