Timing synchronization with photon pairs for quantum communications

In quantum communication experiments using entangled photon pairs far-distant measurement results are correlated on the sub-nanosecond level. For best performances the timing of the coincidence window and its width are critical. In contrast to lab experiments, where accurate timing could be adjusted easily by e.g. matched cable length, in distributed experiments a common clock more accurate than the coincidence time is requested by means of time stable synchronization channels or atom clocks synchronized by GPS. Synchronization of distant clocks is far from trivial. Efficient synchronization mechanisms are required to enable novel architectures like switched star shaped QKD-networks and facilitate quantum communication of a high number of potential users.