Receiving signal from quantum communications satellite

Quantum key distribution network established across continents

Image credit: Jin Liwang/Xinhua

Chinese researchers and their collaborators have brought a secure quantum communications network online, successfully conducting a quantum-encrypted video conference between Asia and Europe.

Standard telecommunications signals can cross huge distances, frequently bouncing to satellites in orbit around Earth and back.

These messages tend to be encrypted and decrypted using extremely long strings of numbers known to the sender and receiver: a key. Intercepting and decrypting these messages may take an enormous amount of computational power but is possible, particularly when using supercomputers to crunch through computationally expensive calculations.

This is a particular concern when it comes to sending messages containing sensitive information, relating, for instance, to national security. Quantum encryption has been suggested as the means by which messages may be sent with absolute security.

A pair of people wanting to exchange messages create a quantum key by carrying out measurements on pairs of entangled photons. These are pairs of light particles created at the same instant whose states are tangled together such that no matter the distance between them, changing the state of one causes the state of the other to change instantaneously. If a third party attempts to listen in to the communications, this interception disrupts the photons, and the sender and receiver are immediately alerted to the eavesdropping.

Now, researchers have published a letter in Physical Review Letters describing how quantum key distribution – whereby a key is embedded in entangled photons and sent ahead of an encrypted message – has been achieved over a record-breaking intercontinental distance.

Previously, the longest distance quantum encrypted messages had been successfully sent was 404km.

The researchers achieved this quantum key distribution using Micius, a Chinese satellite launched in August 2016 loaded with quantum optical equipment designed to establish secure quantum communications between ground and space.

The researchers were able to send images of the satellite and physicist Edwin Schrödinger between the Chinese Academy of Sciences in Beijing and its European base in Vienna, separated by 7600km. The Chinese and Austrian researchers were also able to conduct a 75 minute long quantum-encrypted video conference in September 2017.

This could eventually allow absolutely secure messages to be exchanged between senders and receivers thousands of miles apart. This connection is not yet completely watertight yet, however, with the satellite itself still proving a security vulnerability. If the quantum key was read at the source the security of the network could be compromised.

The satellite is operated as part of the ambitious Quantum Experiments at Space Scale project, which could establish a European-Asian quantum-encrypted communications network by 2020 and a global network by 2030, as well as having “enormous prospects” in the defence sector.

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