Pairwise atomic states: Two-atom system in a three-dimensional squeezed vacuum field

We analyze the pairwise atomic states that are produced when a system of two atoms is irradiated by a squeezed vacuum field. Included in the analysis is the three-dimensional squeezed vacuum field propagated over a solid angle Ω, the interatomic separation ${\mathit{r}}_{12}$, the microscopic Fabry-Pérot cavity, and the role played by an imperfect coupling between the cavity modes and squeezed input modes. It is shown that the steady state of the system in free space is the pairwise atomic state when the atoms interact with a perfect squeezed dipole wave and the interatomic separations are much smaller than the resonant wavelength (Dicke model). With the interatomic separations comparable to the resonant wavelength or with an imperfect squeezed dipole wave, the system decays to a state that is not the pairwise atomic state. The effect of squeezed vacuum field on the system is now manifested by the selective population of the collective atomic states. For atoms inside a microscopic Fabry-Pérot cavity the pairwise atomic states are produced with an imperfect squeezed dipole wave. This can occur even for small solid angles Ω. However, this effect is sensitive to mode matching between internal and external fields. It is shown that with a Gaussian profile for the input squeezed modes the steady state is not the pairwise atomic states, but a significant reduction of the population in the symmetric state still can be observed.