Optical interpretation of linear-optics superradiance and subradiance

Super-and subradiance are usually described in the framework of Dicke collective states, which is an "atomic picture" in which the electromagnetic field only provides an effective interaction between the atoms. Here, we discuss a complementary picture, in which we describe the propagation and scattering of light in the atomic medium, which provides a complex susceptibility and scatterers. This "optical picture" is valid in the linear -optics regime for disordered samples and is mainly relevant at low density, when the susceptibility and scattering cross section can be computed from simple textbook formulas. In this picture, superradiance is a dispersion effect due to a single scattering event dressed by an effective refractive index, whereas subradiance is due to multiple scattering. We present numerical and experimental data supporting our interpretation.