Lasing without inversion: Realities and prospects

We review amplification and lasing without inversion (AWI and LWI) in homogeneously broadened closed three-level systems in the framework of both density-matrix and quantum-jump formalisms. With the density-matrix formalism: i) we study the nature of quantum interference in these systems; ii) determine analytically the conditions for AWI and locate the position of maxima and minima in the probe absorption spectrum; and iii) study various regimes of lasing, namely, continuous wave, self-pulsing and giant pulse lasing. Within the quantum-jump formalism we calculate the relative probabilities of the various physical processes responsible for probe field amplification and absorption.