Free-space transmission of quantum code

Free-space transmission of quantum code over a distance of 144 kilometers (89 miles) between two of the Canary islands has been demonstrated by a team of researchers in Europe. At the APS March Meeting in Denver, Anton Zeilinger of the University of Vienna described how he and his colleagues transmitted single photons from an astronomical observatory on La Palma to another one on Tenerife. The transmitted photons’ entangled polarization states formed the basis of a “quantum key,” a stream of information that could be used to decipher a longer encrypted message. To allow detection of potential eavesdroppers, the researchers further entangled the outgoing particles of light with photons kept at the transmitting station. The data transmission rate was low, only 178 photons in 75 seconds, but the experimenters overcame the difficulties imposed by long-distance propagation through a turbulent atmosphere. In a proposed experiment to be coordinated by the European Space Agency, which operates the Tenerife telescope, astronauts aboard the International Space Station would transmit an entangled key to two earthbound stations separated by distances 10 or more times greater than the two islands. (For a preprint, see R. Ursin et al., http://arxiv.org/abs/quant-ph/0607182 .)