Robustness of polarization entanglement for long distance QKD

Quantum key distribution (QKD) is on the brink of being deployed on a large scale. For a successful launch it is important though, that QKD devices can be incorporated into or use existing telecom networks. Although not at telecom wavelengths, QKD based on polarization entanglement has already been shown over 1.4 km outside the laboratory. However, it was thought that dispersion effects, mainly polarization mode dispersion (PMD), in optical fibers will limit the use of polarization entanglement for longer distances. We report the distribution of quantum correlations over a record length of 100 km in telecom fibers with such a high quality that it would be possible to extract a secret key without background subtraction.