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Shifting Modes Enable Free-Space Optical Communication
Nov 2014
VIENNA, Nov. 12, 2014 — Twisted beams of light have been used to transmit images through the air over 3 km, demonstrating significant data-carrying capacity of light in free-space communications.

A team from the Institute for Quantum Optics and Quantum Information (IQOQI), in collaboration with the University of Vienna, performed this feat by adjusting the spatial modulation of a green laser beam. Sixteen laser modes were used to encode real information, including grayscale images of Wolfgang Amadeus Mozart, Ludwig Boltzmann and Erwin Schrodinger.

Previously, this was possible only over short distances in laboratory settings, according to the researchers. They employed an artificial neural network to reveal the patterns and correct for disturbances caused by air turbulence.

Earlier studies have found that if a light beam of a certain wavelength is twisted into a corkscrew shape, the number of channels through which data can be transmitted is drastically increased; theoretically, the light can be twisted with an infinite number of turns, with each configuration acting as a single communication channel.

This characteristic — orbital angular momentum (OAM) — in the past has been used to transmit up to 2.5 Tb/s via optical fibers. However, traditional optical fibers are not always available or suitable for every type of light-based communication, such as Earth-to-satellite communications.

Twisted light waves were sent over 3 km across Vienna.
The twisted light waves were sent over 3 km across Vienna, from the Central Institute for Meteorology and Geodynamics to the Institute for Quantum Optics and Quantum Information. Courtesy of the New Journal of Physics/IOP Publishing.

“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,” said Mario Krenn, a doctoral student at IQOQI. “It is envisaged that this additional degree of freedom could significantly increase data rates in classical communication.”

OAM could be used in quantum encryption, as well — a secret key made from a string of polarized photons would pass between two people to protect the data being shared between them. The spinning of the photons would prevent outside parties from intercepting that key.

“Quantum communication could profit greatly from the almost infinite number of OAM states,” Krenn said. “Each single photon can carry an OAM number, thus carrying more information than just one spin, or polarization, as is common in the most recently proposed quantum experiments. A higher information density could make the secret key more robust against several side-channel attacks by eavesdroppers.”

The research was published in New Journal of Physics (doi: 10.1088/1367-2630/16/11/113028).

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