Population inversion and stationary lineshapes in a driven two-atom system in a squeezed vacuum

We study the effect of a broadband squeezed vacuum on the dynamics of a two-atom system driven by a near resonant laser field. We show that only in the absence of the driving field are the collective atomic levels selectively populated. Otherwise all collective atomic levels are populated independently of the initial conditions and time. Moreover, we show that the mode correlations, characteristics for the squeezed vacuum, induce a population inversion which, in contrast to the selective population of the collective levels, is not destroyed by the driving field. The stationary lineshapes for the two atomic transitions are also studied. We show that in the squeezed vacuum the population in the upper collective level can exhibit three distinct peaks centred at the frequencies omega and omega +or- Omega , where omega is the atomic transition frequency and Omega is the dipole-dipole interaction potential. The central peak is induced by the mode correlations, whereas the sidebands are induced by the driving field and show a strong dependence on the relative phase between the squeezed vacuum and the driving field. The population in the intermediate level shows a distinct peak at the frequency omega + Omega , and holes at omega and omega - Omega