Entanglement degradation in continuous-variable quantum teleportation

The influence of losses in the transmission of continuous-variable entangled light through linear devices such as optical fibers is studied, with special emphasis on Gaussian states. Upper bounds on entanglement and the distance to the set of separable Gaussian states are calculated. Compared with the distance measure, the bounds can substantially overestimate the entanglement and thus do not show the drastic decrease of entanglement with increasing mean photon number, as does the distance measure. In particular, it shows that losses give rise to entanglement saturation, which principally limits the amount of information that can be transferred quantum mechanically in continuous-variable teleportation. Even for an initially infinitely squeezed two-mode squeezed vacuum, high-fidelity teleportation is only possible over distances that are much smaller than the absorption lengths.