GAIN WITHOUT POPULATION-INVERSION IN A 2-LEVEL ATOM DAMPED BY A BROAD-BAND SQUEEZED VACUUM

We study the absorption spectrum of a two-level atom driven by a reson ant laser field and damped by a broadband squeezed vacuum. We show tha t for some values of the Rabi frequency of the driving field the centr al component of the absorption spectrum can switch from absorption to amplification. We find that the presence of the squeezed vacuum field is essential for the gain at the central component. Moreover, the gain is not attributed to any population inversion in both the atomic bare states and in the dressed atomic states. We show that the gain origin ates from the coherent population oscillations, which are significantl y enhanced when the atom is damped by a broadband squeezed vacuum.