Images beamed across Vienna using 'twisted light'

In a first, scientists have beamed images across Vienna using the "twist" of a visible light beam - a technique that could revolutionise the way we communicate data.
It is the first time that twisted light has been transmitted over a large distance outdoors, and could enable researchers to take advantage of the significant data-carrying capacity of light in both classical and quantum communications.
Instead of using one wavelength of light as one channel of communication, the light can be theoretically twisted with an infinite number of turns, with each configuration acting as a single communication channel.
This twisting characteristic, known as orbital angular momentum (OAM), has been exploited by researchers in the past, with some showing that it can be used to transmit 2.5 terabits of data per second - the carrying capacity of more than 66 DVDs - through an optical fibre.

The researchers from the University of Vienna and the Institute for Quantum Optics and Quantum Information used a green laser beam to send twisted light through a lens on top of a radar tower in Vienna.
The researchers sent 16 different twisted configurations of a specific wavelength of light to a receiver 3 km away.

A camera was used to capture the beams of light and an artificial neural network was deployed to reveal the pattern and remove any possible disturbances that may have been caused by air turbulence.
After distinguishing and characterising the 16 different patterns, the researchers then encoded the light with real information - grey-scale images of Wolfgang-Amadeus Mozart, Ludwig Boltzmann and Erwin Schrodinger.

"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," co-author of study Mario Krenn said.
"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," Krenn said.
Krenn and co-authors also believe that the OAM of light can be used in quantum communication experiments, whereby a secret key made from a string of polarised, or "spinning", photons - individual particles of light - is passed between two individuals to protect data they want to share with each other.

The research was published in Institute of Physics and German Physical Society's New Journal of Physics.