The amount of data that a light beam can transmit depends on its frequency range, or bandwidth—the wider it is, the more data you can cram into the beam. By manipulating the polarization of photons (a quantum property known as intrinsic spin angular momentum), which can be either horizontal or vertical, you can double the amount of information transmitted by a beam. There is another quantum property of photons, called orbital angular momentum (AOM), which can, in principle, have a ground state and an infinite number of values. Each of these values is associated with an integer indicating its helicity (represented by the integer l). In the ground state, where l=0, the wave front of the waves is planar. For all the other states, the wave front rotates along a “twisted” shape reminiscent of fusilli pasta (a helix); the amount of twist increases with increasing l.
In 2001, Anton Zeilinger, a quantum physicist at the University of Vienna, proposed the idea of using the orbital angular momentum of photons to increase the amount of data that can be transmitted by light beams. Several experiments in laboratories confirmed that data could be transmitted via twisted light beams, but the question of whether the quantum states of light photons would survive turbulence in air over long distances remained an open question.
In 2012, a Swedish-Italian research group successfully transmitted twisted microwaves over 450 meters of free space. But microwaves, although they consist of photons, are impervious to air turbulence. Therefore, Zeilinger and his research colleagues in Vienna decided to put the influence of turbulence—especially the turbulence you find above big cities—to the test.
Earlier this month, they reported in the New Journal of Physics the successful transmission of OAM modes via laser beam through open space over a distance of 3 kilometers. To make the experiment possible, they had to restrict the light beam to 16 OAM modes. “In principle, each single photon can have an unbounded number of different OAM values. We did our experiment with a laser, so we used a lot of photons, and so we are very far away from the single-photon regime,” explains Mario Krenn, a physicist at the University of Vienna and lead author of the New Journal of Physics paper.
The researchers transmitted the beam from the radar tower of the Central Institute for Meteorology and Geodynamics to the rooftop of their own institute building. To create the OAM modes, they reflected the light of a laser off a so-called spatial light modulator. “It is a usual pixel display with about one thousand by one thousand pixels where you can change the reflective index of each pixel,” says Krenn. “This allows us to introduce phase changes of 0 and 2π [pi] for each pixel. When we direct the laser beam at this spatial structure, the laser light undergoes these phase changes and develops these specific intensity energy patterns that we show in the video,” says Krenn.
A high quality lens (the first trials with a lower quality lens killed the OAM modes right away) focuses the reflected light into a 6-centimeter-wide beam that projects the light patterns on a screen 3 km away. A CCD camera recorded each of the 16 different patterns that resulted from the various OAM modes.
To the relief of the researchers, turbulence did not affect the OAM modes significantly, although it caused the patterns’ position on the screen to move. “Some calculations suggested that even after one kilometer, you might have a very big problem because of turbulence,” Krenn recalled. “With our method, we show that up to three kilometers of atmosphere does not destroy the OAM modes. [Because] the effective atmosphere is roughly six km thick, [transmitting through it] will not be a problem," says Krenn. “This result has interesting implications for future ground-satellite communication.”
To ascertain the quality of the transmission, the researchers transmitted small grey-scale images of three famous Viennese: Wolfgang Amadeus Mozart; Ludwig Boltzmann; and of course, Erwin Schrödinger. To recognize the patterns and associate them with the 16 OAM states, the researchers used an artificial neural network. “We recorded some test samples [about 450] of different modes, says Krenn. “This was the input to the network, a so-called unsupervised network. You don't have to tell it how it should learn, you supply the input and it characterizes the different structures by itself," says Krenn.