Before one can take advantage of the existing fiber optic networks for quantum communication over long distances there is one crucial problem that needs to be solved first. Although the loss in current optical fibers is remarkably low, it is finite, and scales exponentially with distance. For classical communication, this problem is solved by repeated amplification of the light pulses that carry the information. In the quantum, however, such direct amplification is in general forbidden by the laws of quantum physics. Nevertheless, the concept of a quantum repeater exists, and many researches world-wide are working on it.
A review article on the subject, written by current and former members of our group, has recently been published in Reviews of Modern Physics. The article describes recent theoretical and experimental progress and compares different schemes in terms of transmission rates and fault tolerance.