Data transmitted across Vienna using twisted beams of light

The city of Vienna has hosted a laser light show with a twist with University of Vienna scientists having tested a new way of transmitting data over a light beam. The technique, which exploits classical and quantum mechanics, promises to provide the ability to send much more information through the air much more securely.

One of the perennial challenges of telecommunications has been how to cram more information into any given transmission line. No sooner was telegraphy invented than engineers started working on multiplexing to carry more than one message at a time. Over the years, messages ranging from the trivial to the tremendous have been crammed into coaxial cables, microwave conduits, fiber optics, and along laser beams.

The latest twist is based on the Orbital Angular Momentum of light or OAM, which allows a beam of a particular color – or wavelength – to be twisted into a corkscrew shape to increase the number of potential communication channels available. So rather than one wavelength of light serving as a single channel, each of the theoretically infinite number of turns acts as a separate communication channel.

Imagine a screw that, instead of having one thread, has many threads running parallel. Or, if you're a vinyl fan, think of one of those aggravating novelty records that played a completely different track every time the needle dropped. The only difference is that in a light beam the number of tracks are theoretically infinite.

The technology has been under development since OAM was first observed in the 1990s, and has already demonstrated the ability to carry up to 2.5 terabits per second, or the equivalent of 66 DVDs. However, up until now it's generally only been used to transmit data over fiber optic cables, with transmission over free space limited to small distances in a laboratory environment. According to the Vienna team, the Viennese demonstration is the first time that OAM has been used for a long distance open-air transmission.

For the demonstration, the University of Vienna and the Institute for Quantum Optics and Quantum Information used a green laser mounted on a radar tower at the Central Institute for Meteorology and Geodynamics, which was aimed at a receiver at the University of Vienna 3 km (1.8 mi) away.

The light beam was configured into 16 patterns corresponding to binary numbers. These were used to encode grey-scale images of Wolfgang-Amadeus Mozart, Ludwig Boltzmann, and Erwin Schrödinger, which were the subjects of the transmission. At the receiver, a camera picked up the beam, which was fed into an artificial neural network to filter out atmospheric interference.

The team sees a number of applications for the technology, including satellite and other open air channels. In addition, the quantum nature of the light twists would make eavesdropping very difficult. Encryption keys, for example, could be sent securely because trying to read the beam in flight would alter its quantum state and destroy the data.

"We have shown for the first time that information can be encoded onto twisted light and sent through a 3 km intra-city link with strong turbulences," says team member Mario Krenn. "The OAM of light is theoretically unbounded, meaning that one has, in theory, an unlimited amount of different distinguishable states in which light can be encoded. It is envisaged that this additional degree of freedom could significantly increase data-rates in classical communication.”

The team’s results were published in the New Journal of Physics.

Below is a video abstract of the team’s paper.

Source: Institute of Physics