Effect Of Atomic Coherence On The Two-mode Entanglement In A Cascade Three-level Atomic System

In the past two decades, quantum information becomes a new science which is the cross of the information and quantum theory. Entanglement is one of the most characteristic properties of the quantum information theory and plays an essential role in quantum information processing such as quantum teleportation, quantum computation, and quantum cryptography. Atomic coherence, as a basic feature of quantum systems, has attracted much attention in the research of quantum optics in the past decades. It can lead to various effects, such as electromagnetically induced transparency (EIT), correlated spontaneous emission laser, population trapping, and a laser without inversion and so on.In this paper, we investigate the effect of atomic coherence on the two-mode entanglement of three-level atoms in cascade configuration according to the entanglement criterion proposed by Duan et al. We show that, in the model of initially preparing the atoms in a coherent superposition of the states, atom coherence induces the entanglement of the two modes cavity fields. The degree of entanglement has a close relation with the gain coefficients and the detuning. In the model of driving atom coherence, the two- mode entanglement has a close relation with the pump intensity of the driving field. No entangled states can be built when the pump intensity is very weak. With the increasing of the pump intensity, the degree of the entanglement increases initially, then decreases.