Entangled photons produced by a three-level atom in free-space

A driven three-level atom system in free-space is investigated. The quantum entropy between the three-level atom and its spontaneous field is calculated. The entanglement between them and the influence of the classical field Ω on the entanglement are studied. The result indicates that there is a steady entanglement between the three-level atom and its spontaneous field, and they cannot be disentangled even the classical field is very large. In addition, the entanglement of the k photons and q photons is studied, it shows the two field is entangled for short time evolution.