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Twisted light carries data across Vienna

13 Nov 2014

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A team of researchers from Austria has twisted beams of light to send data across the rooftops of Vienna.

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.

The results of the experiment have been published in the Institute of Physics and German Physical Society's New Journal of Physics, and are accompanied by a video abstract, which can viewed here below.

Previous research has shown that if a beam of a certain colour, or wavelength, of light is twisted into a corkscrew shape, the number of channels that data can be transmitted through can be drastically increased. 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 2.5Tbits of data per second—the carrying capacity of more than 66 DVDs—through an optical fibre.

Yet optical fibres are not always suitable, or available, for certain types of communication where light is used—such as Earth to satellite communications—so researchers have been trying to send twisted light over free space whilst at the same time avoiding disturbances from air turbulence. So far, this has only been achieved over small distances in the lab.

In the current study, 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 at the Central Institute for Meteorology and Geodynamics in Vienna.

The researchers sent 16 different twisted configurations of a specific wavelength of light to a receiver 3km away at the University of Vienna. 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 Schrödinger.

Co-author of study Mario Krenn said: "We have shown for the first time that information can be encoded onto twisted light and sent through a 3km intra-city link with strong turbulences.

"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."

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