Twisted-light waves transmitted in air over a 3-kilometer path

Could significantly increase data rates for non-fiber communications and improve encryption of quantum communications
November 13, 2014

A 3-kilometer free-space experiment performed in Vienna, comparing transmitted and received modulation patterns allows engineers to characterize the atmospheric stability of transmission modes (credit: Vienna Center for Quantum Science and Technology and Google)

A group of researchers from Austria have sent twisted beams of light across the rooftops of Vienna — the first time that twisted light has been transmitted over a large distance outdoors.

Twisted light allows for transmitting a huge amount of data by twisting the light into a corkscrew shape, so that the rotation allows for encoding data on additional channels, for both classical and quantum communications. The light can be theoretically twisted with an infinite number of turns, with each configuration acting as a single communication channel.

Twisted-light transmission at 2.56 terabits per second over a fiber-optic cable (credit: USC/Nature Photonics)

KurzweilAI has covered a number of experiments using twisted light over fiber-optic links, some showing that twisted-light can be used to transmit 2.5 terabits of data per second.

Twisted light by satellite

But optical fibers are not always suitable, or available, for certain types of communication where light is used — such as Earth-to-satellite communications — so researchers have been trying to send twisted light over free space while avoiding disturbances from air turbulence. So far, this has only been achieved over small distances in the lab.

The results of the new experiment were published Wednesday November 12 in New Journal of Physics.

In the current study, the researchers, from the University of Vienna and the Institute for Quantum Optics and Quantum Information, used a green laser beam to send twisted light through a lens on top of a radar tower at the Central Institute for Meteorology and Geodynamics in Vienna.

The researchers sent 16 different twisted configurations of a specific wavelength of light to a receiver 3 km away at the University of Vienna. A camera was used to capture the beams of light and an artificial neural network was deployed to reveal the pattern and remove any possible disturbances that may have been caused by air turbulence.

The researchers encoded the light with real information — grey-scale images of Wolfgang-Amadeus Mozart, Ludwig Boltzmann. and Erwin Schrödinger. “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,” said study co-author Mario Krenn.

“The [orbital angular momentum (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.”

Encrypted quantum communication

Krenn and his co-authors also believe that the OAM of light can be used in quantum communication experiments, using a secret key made from a string of polarized (“spinning”) photons and passed between two individuals to protect data they want to share.

The laws of physics dictate that any attempt by an eavesdropper to intercept the key and try and measure the “spin” of the photons will inherently alter the spin and thus destroy the secret key. This type of quantum communication has been labelled as “unbreakable.” Krenn believes that the use of the OAM of light can make secret keys even tougher to crack.

“Quantum communication could profit greatly from the almost infinite number of OAM states. 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,” Krenn continued.

“A higher information density could make the secret key more robust against several side-channel attacks by eavesdroppers, which is, of course, a serious problem as we have seen in recent months.”

New Journal of Physics | Communication with spatially modulated light through turbulent air across Vienna

Abstract of Communication with spatially modulated Light through turbulent Air across Vienna

The transverse spatial modes of light offer a large state-space with interesting physical properties [1,2]. For exploiting it in future longdistance experiments, spatial modes will have to be transmitted over turbulent free-space links. Numerous recent lab-scale experiments have found significant degradation in the mode quality after transmission through simulated turbulence and consecutive coherent detection [3-7]. Here we experimentally analyze the transmission of one prominent class of spatial modes, the orbital-angular momentum (OAM) modes, through 3 km of strong turbulence over the city of Vienna. Instead of performing a coherent phase-dependent measurement, we employ an incoherent detection scheme which relies on the unambiguous intensity patterns of the different spatial modes. We use a pattern recognition algorithm (an artificial neural network) to identify the characteristic mode pattern displayed on a screen at the receiver. We were able to distinguish between 16 different OAM mode superpositions with only ~1.7% error, and use them to encode and transmit small grey-scale images. Moreover, we found that the relative phase of the superposition modes is not affected by the atmosphere, establishing the feasibility for performing long-distance quantum experiments with the OAM of photons. Our detection method works for other classes of spatial modes with unambiguous intensity patterns as well, and can further be improved by modern techniques of
pattern recognition.