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