Inversion-free amplification of a probe field in a medium consisting of two-level degenerate atoms in the presence of a magnetic field

The propagation of a probe field in a medium consisting of two-level atoms, degenerate in relation to the J_g = 1/2 → J_e = 1/2 transition, is examined. The atoms are pumped by a circularly polarised resonant field in the presence of a static magnetic field. It is shown that under these conditions one of the normal waves may be amplified in the absence of inversion in the medium. In strong optical and magnetic fields, inversion-free amplification is possible for a pump with a finite spectral width, which may be much greater than the natural width of the excited level.