Quantum communication takes a new twist

Communications of the future may be securely encoded in light that is twisted like fusilli pasta.

A long-distance transmission of particles of light above the Vienna skyline demonstrates a new way of relaying information using the tricks of quantum physics. Austrian researchers exploited the twistiness of light to establish a delicate quantum connection called entanglement between pairs of photons. The entanglement remained intact even after one photon from each pair traversed three kilometers.

The successful optical link, described online July 23 at arXiv.org, suggests that twisted light may play a key role in implementing quantum communication systems, which require preserving entanglement over great distances. “It allows you to pack more information into your transmission,” says Roger Andrews, a physicist at the University of the West Indies in St. Augustine, Trinidad and Tobago.

Anyone with an Internet connection has received bits of data (individual 1s and 0s) that have been encoded in light. But scientists envision building communication networks that rely on encoding information in photons’ fragile quantum properties and then using entanglement to help disseminate that data to the intended recipient. A secure key established between parties would prevent eavesdroppers from stealing information.

In recent years, physicists have routinely entangled photons via a property called polarization, the orientation of light’s oscillating electric field. Armed with the measurement of an entangled photon’s polarization, one can predict what the other photon’s polarization will be once it is measured. The challenge is finding other properties of light that can also be robustly entangled, which would enable each photon to transmit more quantum information over large distances.

Scientists have proposed encoding additional data in photons’ orbital angular momentum, the twistiness of a light beam. Successive light waves do not always line up like ocean waves along a straight coastline; looking at a beam head-on, waves can arrive in a corkscrew-shaped pattern that resembles screw threads or curly pasta. But many physicists expected that the turbulence of open air would degrade a beam’s twistiness and destroy the delicate entanglement state, thus losing the data, says Martin Lavery, a physicist at the University of Glasgow in Scotland.

To test the robustness of corkscrew-based entanglement, a team led by University of Vienna quantum physicist Anton Zeilinger started with pairs of photons (A and B) and entangled them via their polarization. Then the researchers sent a bunch of photon B’s through an instrument that converted their polarization into orbital angular momentum; now in each pair, photon A’s polarization was linked to photon B’s degree of twistiness. The researchers then beamed the photon B’s from a weather radar tower in Vienna to a rooftop detector. Despite traveling about three kilometers, the photons retained entanglement — and thus a link for exchanging information — with their partners.

Lavery says the demonstration is an important step toward developing high-speed quantum communication networks. While preserving entanglement over longer distances may not be possible, he says a range of a few kilometers would be sufficient to create quantum networks in densely populated cities such as Vienna.