Quantum interference and quantum-Zeno effect in amplification without inversion

We analyze amplification without inversion (AWI) in different homogeneously broadened closed three-level systems in the framework of both density-matr ix and quantum-jump formalisms. With the density-matrix formalism we study the nature of quantum interference in these systems; determine analytically the condition for AWI and locate the position of maxima and minima in the probe absorption spectrum. Within the quantum-jump formalism we calculate t he relative probabilities of the various physical processes responsible for probe field amplification and absorption, and analyze the role of the quan tum-Zeno effect in AWI. The result is that AWI in the h and p schemes is du e to one-photon processes while for V and Lambda schemes are due to two-pho ton processes. We also show that AWI in V and h schemes always occurs in th e quantum-Zeno regime, while this regime is not necessary for AWI in the La mbda and p schemes.