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  • Quantum Code Sent from Aircraft to Ground Station
Apr 2013
MUNICH, April 3, 2013 — The first secure quantum code transmitted using quantum cryptography from a plane in flight to a ground station has provided an important step toward creating a secure laser-based global communications network.

The optical transmission was made by Ludwig Maximilians University of Munich (LMU) physicists using a laser-based wireless communications system from the German Center for Aeronautics and Space Research (DLR).

“This demonstrates that quantum cryptography can be implemented as an extension to existing systems,” said LMU physicist Sebastian Nauerth.

The Dornier 228 aircraft used in the LMU experiments to transmit a secure quantum code through the atmosphere from an aircraft to a ground station. The glass dome that houses the telescope can be seen on the base of the fuselage. Images courtesy of LMU.

Unlike communication based on classical bits, quantum cryptography uses the quantum states of single photons for data exchange. Quantum physicists believe they can provide secret encrypted keys via satellite because quantum states are fragile; interception of the key by an eavesdropper will alter the behavior properties of the particles, and thus becomes detectable.

The encryption strategy is already in use by some government agencies and banks, with data being sent either along glass fiber cables or through the atmosphere, but optical key distribution via these channels is limited to distances of less than 200 km because of signal losses along the way.

The mobile telescope on the aircraft automatically tracks the position of the ground-based receiver. The incoming and outgoing light signals are directed through a tunnel at the base of the aircraft.

In 2007, LMU physicist Harald Weinfurter and colleagues transmitted a key over 144 km of free space between ground stations on the islands of Tenerife and La Palma.

In the latest experiment, single photons were sent from an aircraft to the receiver on the ground. The challenge was to ensure that the photons could be precisely directed at the telescope on the ground despite the impact of mechanical vibrations and air turbulence.

The ground-based telescope, with cameras and laser light sources for automatic precision positioning. To reduce the level of stray light, the normally open structure was lined with black fabric.

“With the aid of rapidly movable mirrors, a targeting precision of less than 3 m over a distance of 20 km was achieved,” said Florian Moll, project leader at the DLR’s Institute for Communication and Navigation. With this level of accuracy, William Tell could have hit the apple on his son’s head even from a distance of 500 m.

With respect to the rate of signal loss and the effects of air turbulence, the conditions encountered during the experiment were comparable to those expected for transmission via satellite. The same holds for the angular velocity of the aircraft.

The findings were reported in Nature Photonics (doi: 10.1038/nphoton.2013.46). 

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