An alignment laser is beamed from the University of Vienna to ZAMG [Credit: New Journal of Physics]

Twisted light transmitted across roofs of Vienna

Beams of twisted light have been transmitted across the rooftops of Vienna in an experiment that holds promise for high-capacity data transmission.

Researchers 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 (ZAMG) to a receiver 3km away at the University of Vienna, the first time twisted light has been transmitted over a large distance outdoors.

The technology, developed by a team from the University of Vienna and the Austrian Institute for Quantum Optics and Quantum Information, relies on optical vortices, which cause the light to twist like a tornado as it moves along the beam path rather than in a straight line imparting orbital angular momentum (OAM) to the beam.

Previous research has shown that twisting light drastically increases the number of channels that data can be transmitted through – rather than simply using one wavelength of light as a single channel of communication, the light can be theoretically twisted with an infinite number of turns, with each configuration acting as a single communication channel.

OAM beams were exploited by researchers at Boston University last year to send more than a terabit of data a second over optical fibre.

"We have shown for the first time that information can be encoded onto twisted light and sent through a 3km intra-city link with strong turbulences,” said the University of Vienna’s Mario Krenn, co-author of a study published in the New Journal of Physics today.

"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. It is envisaged that this additional degree of freedom could significantly increase data-rates in classical communication."

To date, experiments attempting to send twisted light over free space while at the same time avoiding disturbances from air turbulence have only achieved small distances inside the lab.

In the most recent study, the researchers sent 16 different twisted configurations of a specific wavelength of light to the receiver at the University of Vienna where a camera was used to capture the beams of light.

An artificial neural network was then deployed to reveal the pattern and remove any possible disturbances that may have been caused by air turbulence.

After distinguishing and characterising the 16 different patterns, the researchers then encoded the light with real information – grey-scale images of Wolfgang-Amadeus Mozart, Ludwig Boltzmann and Erwin Schrödinger.

As well as being capable of carrying much more data than traditional light-based communication technology, Krenn and colleagues believe that the OAM of light can be used in quantum communication.

A secret key made from a string of polarised, or ‘spinning’, photons can be passed between two individuals to protect data they want to share with each other as 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" and 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 polarisation, as is common in the most recently proposed quantum experiments," Krenn said.

"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."

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