Effect of Dipole-Dipole Interaction on Entanglement

The influence of the dipole-dipole interaction on the entanglement and spin squeezing in the two-level atom Dicke model driven by a coherent field and damped by a broad band squeezed vacuum is discussed. We use the negativity criterion to measure the degree of entanglement. It is shown that in the absence of a squeezed vacuum, weak dipole-dipole interaction enhances entanglement. However, in the presence of a squeezed vacuum and for intense dipole-dipole interaction, the Dicke model evolves to a two-atom squeezed state which is maximally entangled for N >> 1. We also compare the criterion of negativity with spin squeezing.