Ground stations use lasers to link with the Micius quantum satelliteJin Liwang/Xinhua/Alamy

Two observatories in China have used a quantum communications satellite to send an encrypted message a record-breaking 1200 kilometres – a major step towards building a secure quantum internet.

China launched its Micius quantum satellite in 2016. It produces pairs of photons that are quantum entangled, meaning the measured state of one photon is linked to the measured state of the other, regardless of the distance between them.

Entanglement can’t directly transfer information, because that would mean data is travelling faster than light. But entangled particles can be used to create secret “keys” that enable extraordinarily secure communication.

Artur Ekert at the University of Oxford and his colleagues used Micius to beam entangled photons to observatories 1200 kilometres apart in China, allowing those two observatories to share quantum encrypted data from farther apart than ever before.

The previous record for this kind of communication is just 100 kilometres along a fibre optic cable. “Fibres are good for intermediate distances, for 30 to 50 kilometres or so, but too noisy for longer distances,” says Ekert.

The latest system had an error rate of just 4.5 per cent. This is particularly important in quantum communication, because any attempt at hacking or eavesdropping on the signal to find out the key would cause more errors in the communication. Starting with a low error rate is necessary so that the additional errors caused by eavesdropping are noticeable.

For example, if the satellite were to be hacked, this would be noticed by tests run on the ground when the observatories received the photons. This kind of communication could eventually be used to build a secure, unhackable internet of quantum information. “Entanglement provides almost ultimate security,” says Ekert.

Journal reference: Nature,DOI: 10.1038/s41586-020-2401-y

PHYSICS15 June 2020 By Leah Crane