Bridge over water. Scientists have entangled photons across the Danube River, breaking the distance record for open-air entanglement.

Entangling Across the Danube

One befuddling rule of quantum mechanics is that particles can become "entangled," each dependent on the behavior of the other, even over long distances. Now scientists have set a new record distance for monitoring such links out in the open. Spanning the Danube River in Austria, researchers have detected entanglement far from the controlled confines of the lab.

Scientists hope to apply entanglement to quantum cryptography, beaming information securely from one place to another. But this is a tough task to accomplish. Researchers generate entangled pairs by firing photons--discrete packets of light--into crystals. Once inside, a photon spontaneously splits, leaving entangled pairs of photons that then leave the crystal. But these packets can easily become absorbed by the atmosphere, breaking the entanglement between the pairs. Until now, scientists have only achieved entanglement in the open air over a few meters. (Other studies have achieved 10-kilometer entanglement within optical fibers.)

Now quantum physicist Anton Zeilinger of the University of Vienna and his colleagues are the first group to test and demonstrate entanglement over 600 meters, out in the open. From the rooftops of two sewage system buildings on opposite banks of the Danube, Zeilinger's team shot waves from a gasoline-powered laser and split them into two infrared beams. One beam was sent to a telescope across the river, the second headed in a different direction to another telescope. The researchers then analyzed the data collected by the telescopes. They found that the electric fields of the individual photons that struck the two detectors were linked. If one was oriented vertically, then the other was oriented horizontally--just as entangled photons should be. This satisfied the definition of entanglement, they report online 19 June in Science.

"I think it's a significant step in an interesting direction," says physicist Nicolas Gisin of the University of Geneva in Switzerland. Although atmosphere, weather conditions, and Earth's curvature limit entanglement distances on the ground, if open-air entanglement can be extended sufficiently, photons could be stretched to and from space, says Zeilinger.

Related sites
Quantum physics at the University of Vienna
More about quantum entanglement