Long-distance quantum cryptography with entangled photons

Abstract

Entanglement-based quantum cryptography has the appealing advantageof intuitively more evident security. While originally, weak laserpulse schemes were implemented earlier as technologically simpler,it is now possible to build entanglement-based quantum key distributionsystems on a technically equally advanced level. The existing polarizationbasedsystems as developed in Vienna now cover distances of the order of100 km in fiber and of 144 km in free space. In a recent fiber experiment,an asymmetric source is used such that one photon at the 1.550 nmtelecom wavelength is transmitted to Bob, while the other photonat 810 nm is locally measured by Alice. It turns out that polarizationentanglement is rather robust, certainly over distances of 100 kmin fibers. In a recent long-distance free-space experiment, one photonwas sent over 144 km from the Canary Island of La Palma to the islandof Tenerife, while again the other photon was measured locally. Thereceiving station uses the OGS telescope operated by the EuropeanSpace Agency ESA. This experiment opens up the possibility for futurequantum key distribution using satellites.