Resonantly enhanced refractive index without absorption via atomic coherence

An enhancement of the index of refraction accompanied by vanishing absorption is shown to be possible in an ensemble of phase-coherent atoms (‘‘phaseonium’’). A survey of various possible schemes in which coherence is established by certain coherent or incoherent methods is given, and the main results are compared and contrasted. In particular, the influence of processes such as Doppler broadening that degrade coherence is discussed.