Evolution of a twin beam in active optical media

We study the evolution of a twin beam propagating inside active media, which may be used to establish a continuous-variable entangled channel between two distant users. In particular, we analyse how entanglement is degraded during propagation, and determine a threshold value for the interaction time, above which the state becomes separable. We explicitly calculate the fidelity for coherent state teleportation and show that it is larger than one half for the whole range of parameters preserving entanglement.