Preparation of steady-state entanglement via a laser-excited resonant interaction

In this paper we propose a scheme, in which two-mode entanglement in a steady state is produced by using two lasers to resonantly drive a single four-level atom embedded inside a two-mode optical cavity. In this scheme, atomic coherence induced by a classical laser plays an important role in the process of preparing the entangled state. With the coupling of a strong control field, direct two-photon transition is generated and the relatively weak pump field induces the parametric interaction between two photons, which makes them entangled with each other. By numerical calculation, we find that the degree of entanglement depends strongly on the Rabi frequencies of the classical laser fields and the cavity losses.