Quantum cryptography: On wings of light

“This demonstrates that quantum cryptography can be implemented as an extension to existing systems”

LMU physicists have, for the first time, successfully transmitted a secure quantum code through the atmosphere from an aircraft to a ground station.

Can worldwide communication ever be fully secure? Quantum physicists believe they can provide secret keys using quantum cryptography via satellite. Unlike communication based on classical bits, quantum cryptography employs the quantum states of single light quanta (photons) for the exchange of data. Heisenberg’s uncertainty principle limits the precision with which the position and momentum of a quantum particle can be determined simultaneously, but can also be exploited for secure information transfer. Like its classical counterpart, quantum cryptography requires a shared key with which the parties encode and decode messages. However, quantum mechanical phenomena guarantee the security of quantum key distribution. Because quantum states are fragile, interception of the key by an eavesdropper will alter the behavior properties of the particles, and thus becomes detectable.

This encrypting strategy is already being used by some government agencies and banks. Data are sent either along glass-fiber cables or through the atmosphere. However, optical key distribution via these channels is limited to distances of less than 200 km, due to signal losses along the way. In 2007, LMU physicist Harald Weinfurter and his group successfully transmitted a key over 144 km of free space between ground stations on the islands of Tenerife and La Palma. Distribution of such keys via satellite networks would make secure data transmission possible on a global scale.

Optical data from a mobile transmitter

A team led by Weinfurter and Sebastian Nauerth at the Physics Faculty at LMU Munich, in collaboration with the German Center for Aeronautics and Space Research (DLR), has now succeeded in optically transmitting quantum information between a ground station and a plane in flight. This is the first time that quantum cryptography has been used for communication with a mobile transmitter.

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