Entanglement is a property of quantum theory that allows two particles to display much stronger correlations than are possible in classical physics. For instance, two photons can be entangled such that if one is vertically polarized, the other is always horizontally polarized. Since the polarization of an individual photon is not known until it is measured, entanglement means that a measurement on one photon will automatically determine the polarization of the other photon -- even if it is hundreds of metres away.

This apparent action-at-a-distance led Einstein and other physicists to doubt the validity of quantum theory. However, entanglement has been demonstrated in countless experiments and is now being exploited, along with many of the other counter-intuitive predictions of quantum theory, in the blossoming field of quantum information.

The Vienna team used a crystal with nonlinear optical properties to split photons with a wavelength of 405 nanometres into pairs of photons with wavelengths of 810 nanometres. These photons then passed through optical fibres to “telescopes” that focussed them onto a second pair of telescopes. One of the receiving telescopes was 500 metres away on the opposite side of the Danube, while the other was about 150 metres away. By comparing the photons detected by the two receiving telescopes, the Vienna team was able to confirm that the photons had remained entangled over a distance of 600 metres in free space. There was no direct line of sight between the receiving telescopes.

Entanglement has been demonstrated over distances of up to 10 kilometres with optical fibre, but the losses incurred in such fibres mean that the maximum distance possible will be around 100 kilometres. Free-space techniques offer the possibility of using satellites to extend entanglement to longer distances. However, at present most free-space experiments are carried out at night because the background counts from sunlight are too high.

In similar experiments physicists have been able distribute quantum “keys” for cryptography over distances of 23.4 kilometres in free space and 100 kilometres along optical fibre. These experiments are less difficult than the entanglement experiments in that they involve the transmission and detection of single photons, rather than pairs of photons.