Researchers from the University of Vienna beamed 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, with a twist
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
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. In terms of individual photons of light, it means that instead of spinning like the Earth around its own axis, their energy traces out a spiral. It is the same sort of momentum that sees the Earth orbit the sun, but the photons are also moving forward at the speed of light. That corkscrew-like motion is useful because instead of just having two possible directions like polarisation (clockwise or anticlockwise), it can turn in either direction with a potentially infinite number of twists — much like a screw with multiple threads. This is why physicists have been investigating whether twisted light could help transmit information very quickly: each twist configuration could be its own channel, just like different colours of light inside an optical fiber
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”
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