Beam splitter in a quantum entanglement experiment. Entanglement is a property of a pair of particles in which a change to one has an instantaneous effect on the other, no matter how far apart they are. Researchers at Vienna University, Austria, split an entangled photon pair from a laser source, and sent each photon to a separate receiver. There was no line of sight between the receivers, which were 600 metres apart, outside, on either side of the River Danube. Entanglement was confirmed by the detectors. It is hoped that this entanglement will find uses in cryptography, quantum computing and satellite communications. Photographed in 2003.