Entanglement bridges the Danube

04 Jul 2003

Scientists demonstrate that two photons can travel a total of 600 meters through free space and still remain entangled.

Physicists have broken another distance record for demonstrating the quantum properties of light. Markus Aspelmeyer and colleagues at the University of Vienna in Austria have shown that it is possible for two photons to travel a total of 600 meters through free space and still remain "entangled" (Science to be published). The previous record for entanglement in free space was a few meters.

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 meters 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 nm into pairs of photons with wavelengths of 810 nm. These photons then passed through optical fibers to "telescopes" that focussed them onto a second pair of telescopes. One of the receiving telescopes was 500 meters away on the opposite side of the Danube, while the other was about 150 meters 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 kilometers with optical fiber, but the losses incurred in such fibers mean that the maximum distance possible will be around 100 kilometers. 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 kilometers in free space and 100 kilometers along optical fiber. 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.