For the first time ever, information has been transmitted outdoors using the "twist" of a visible light beam. It's a breakthrough that could allow for rapid and secure communication across vast distances.
For the experiment, researchers from the University of Vienna sent a beam of twisted light across a distance of 3 km above the city. The light, which is twisted like a wrung towel, allows for wicked-fast communication by encoding separate channels of information, which can be sent simultaneously.
The technique was tested by sending three greyscale images of famous Austrians — physicists Ludwig Boltzmann and Erwin Schroedinger, and composer Wolfgang Amadeus Mozart. The images were broken down into pixels and transmitted through the Vienna night inside a green laser been.
The BBC explains:
The twisting of light, technically described as its "orbital angular momentum" (OAM), was first demonstrated in the 1990s and so would probably have surprised the two famous physicists as well.
Rather than polarised light waves, which are restricted in the directions that they can "wiggle", light with this type of momentum twists through space like a corkscrew.
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 fibre.
But no cables were used for this study. Rather, a green laser was emitted by a spatial light modulator, which put two different twists on the light beamed across the city. It was detected by a specific structure 3 km away. The beam became a ring of dots, and the number of dots was determined by the degree to which the light was twisted.
In this case, the researchers made 16 different beam structures and used them to encode 16 different levels of grey. Then, pixel by pixel, they sent the encoded portraits along the laser beam. It recreated the images with an error rate of just 1.7%.
Eventually, the technology could be used for more effective and robust Earth-to-satellite communications.
Images: University of Vienna