  A system of two two-level atoms interacting with a squeezed vacuum field can
exhibit stationary entanglement associated with nonclassical two-photon
correlations characteristic of the squeezed vacuum field. The amount of
entanglement present in the system is quantified by the well known measure of
entanglement called concurrence. We find analytical formulas describing the
concurrence for two identical and nonidentical atoms and show that it is
possible to obtain a large degree of steady-state entanglement in the system.
Necessary conditions for the entanglement are nonclassical two-photon
correlations and nonzero collective decay. It is shown that nonidentical atoms
are a better source of stationary entanglement than identical atoms. We discuss
the optimal physical conditions for creating entanglement in the system, in
particular, it is shown that there is an optimal and rather small value of the
mean photon number required for creating entanglement.
