Entangled photons of light can be used to transmit information in an absolutely secure way.

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PARIS: Quantum communication has been sent across a record 144 km using a process that could one day be used to send messages across space with absolute secrecy, scientists report in Nature Physics.

The procedure exploits the quantum properties of particles, such as photons, to become entangled. When entangled, they behave like psychic twins. Even if they are separated by a vast distance, a disturbance to one affects the other - an oddity called entanglement, which Albert Einstein himself lyrically dubbed "spooky interaction".

This method "could be an important step towards future satellite-based quantum-communication networks," Nature Physics said.

In the experiment, polarisation-entangled photon pairs were generated on Roque de los Muchachos mountain on the island of La Palma in Spain's Canary Islands.

While one photon stayed in La Palma, its twin was sent over an optical free-space link to Tenerife, about 144 kilometres away. There the Optical Ground Station of the European Space Agency acted as a receiver.

This was a marked improvement over the 13 kilometre distance achieved by earlier researchers in a similar experiment.

Spooky security

The researchers from Austria, Britain, Germany, Italy and the Netherlands were able to generate a quantum cryptographic key over 144 kilometres. This key is completely secure, because any attempt to 'read' the key between the source and the destination will result in the quantum information becoming corrupted, rendering unintelligible. Further, there is no known way to copy quantum information, so the security is guaranteed between the two end points in the communication.

Despite the counter-intuitive nature of quantum entanglement, quantum communication still doesn't allow information to travel faster than the speed of light. However, it does open up new possibilities in encryption and communication.

Researchers said the experiment "is an essential first step towards future satellite-based quantum communication and experiments on quantum physics in space."