Moving plane exchanges quantum keys with Earth

AN AEROPLANE has beamed quantum encryption keys to a station on the ground, paving the way for an ultra-secure global communications network.

Quantum key distribution (QKD) uses photons polarised in two different ways to encode the 0s and 1s of an encryption key. The laws of quantum mechanics ensure the transmission is secure, as any attempt to intercept the key disturbs the polarisation - a tip-off to the receiver that the key has been seen and should be discarded.

Quantum keys had previously been exchanged between two land-based stations. Now Sebastian Nauerth at the Ludwig Maximilian University of Munich, Germany, and colleagues have extended the feat to an aircraft, a stepping stone to QKD via satellite, which could allow secure messages to be transmitted around the world.

Flying at a height of 20 kilometres and a speed of nearly 300 km per hour, the challenge was to tightly align the infrared laser pulses transmitted by the aircraft with the receiving station on the ground. Any deviation would limit the number of photons reaching the target.

The researchers kept the laser on track using moving mirrors both in the aircraft and on the ground. Performing the experiment shortly after sunset avoided interference from sunlight. The transmission lasted for 10 minutes, amounting to a key long enough to encrypt 10 kilobytes of data. The team presented the work at the QCrypt conference in Singapore on 12 September.

"It is a very challenging experiment," says John Rarity, an optical engineer at the University of Bristol, UK, who was not involved in the work. He adds that aligning the laser with a satellite will be even more difficult. In the meantime, he suggests that solar-powered planes, which can stay aloft for weeks at a time, could enable QKD to be used in military surveillance, for example.

Back on the ground, researchers at the European Space Agency's Optical Ground Station in Spain's Canary Islands broke the distance record for quantum teleportation, which uses quantum entanglement to make message transmission more secure (Nature, doi.org/jbs). They teleported quantum states over 143 km, breaking a record of 97 km reported earlier this year.

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