Lasing without population inversion: towards X-ray continuous wave laser emission

This paper is devoted to lasing without population inversion in three and multi-level atomic configurations in interaction with a driving laser. First we discuss the interest and the current state-of-the-art of this phenomenon. Then, the basic physics of lasing without inversion is described in more detail and analyzed in terms of quantum interference between the dressed-states built up by the driving field. In addition, the quantum-jump formalism is used to introduce generalized Einstein B coefficients accounting for stimulated emission and absorption processes in these schemes. As a result, lasing without inversion is explained in terms of non-reciprocity between stimulated emission and absorption coefficients. Likewise, this formalism allows us to analyze lasing without inversion in terms of the quantum Zeno effect. Some particular problems of lasers without inversion are also investigated, such as, laser emission with a driving field of frequency smaller than the one we want to generate without population inversion; lasing without inversion in the absence of an external driving field; and electromagnetically induced transparency as a first step to amplification without inversion. Finally, some dynamical features of these lasers are explored such as the inversionless laser emission in a selfpulsing regime or the generation of giant pulses of laser light.