In considering a beam of light traveling through space, whether in the form of a flashlight or high-powered laser or dusty sunlight creeping around a curtain's edge, it's easiest and most intuitive to imagine the approaching face of that beam to be flat, like the front of a subway car. And when the beam smashes into some intervening medium, the entire cross-section of the beam, what's more properly known as its wavefront, will arrive at once, exerting the same mechanical pressure (radiation pressure, properly) uniformly across a neat disc. A radiation pancake.
But light is capable of being much more complicated than that. While we might imagine beams of light to be made up of light particles all traveling forward together in a straight line, the situation may in fact be quite different. This is because light can carry angular momentum—as a beam travels forward, its constituent particles might also be swirling and rotating around this forward-traveling axis. In the version of angular momentum known as the orbital angular momentum of light (OAM), the result is that a beam's wavefront may take the shape of a corkscrew or drill bit.
It's easiest to just see:
Twisted light is potentially very useful because it can be used for encoding classical and quantum data, perhaps data being transmitted to and from satellites. But sending beams through free-space is challenging for the simple reason of turbulence—a slight change in the refractive index of air may mean a distorted signal. Transmitting twisted light is an ongoing challenge.
As described in a paper posted to the arXiv pre-print server, Anton Zeilinger and colleagues at the University of Vienna and the Institute for Quantum Optics and Quantum Information have successfully sent a beam of twisted light 143 kilometers between the Canary islands of La Palma and Tenerife, a new record. Moreover, they were able to transmit an OAM-encoded message: "Hello, world."
Some large part of the experiment's success was in using a neural network to identify various modes of twisted light even after they'd been distorted by the intervening atmosphere. At one end, researchers encoded messages by modulating the phase of a green laser beam, which was fired 143 km to a white wall within the Teide Observatory on Tenerife. At the receiving end, researchers took images of the wall-projected beam and then fed them into their neural network. Crucially, this neural network had been trained on images of turbulence-distorted beams, so it was able to strip away this distortion to identify just the message being sent. It was successful about 80 percent of the time.
As for the message, what they actually decoded was "Hello, WorldP." There was an error, but an error that only amounted to one bit out of 72, or about 1 percent error per bit of information. That's not bad. The entire transmission took 271 seconds, which, in terms of speed, puts it at about the same level as smoke signalling, the researchers note, or sending information via neutrinos. The delay largely has to do with the time required to capture quality images of the projected beam at the receiving observatory.
"We don't consider this method as real communication but mere the demonstration of the transmission quality of modes," Zeilinger and co. note. "However, the application of state-of-the-art adaptive optics such as those used in simple and efficient intensity-based methods could further improve the link quality, potentially enabling its application in a multiplexing scheme for classical communication."
"Furthermore, the effective vertical thickness of the atmosphere is 6 kilometers, which is well below our link distance, indicating that earth-to-satellite communication with spatially encoded modes is not limited by atmospheric turbulence."