A secure quantum link has been created over a distance of 511 kilometres between two Chinese cities by using a relay in the centre that doesn’t need to be trusted. This may help extend secure quantum networks.
When a couple of photons are quantum entangled, you can quickly deduce the state of 1 by measuring the other, whatever the distance separating them. It is the basis of quantum encryption – using entangled particles to create secure keys and make certain that messages are secret.
Previous research has created entangled pairs of photons and transmitted someone to a receiver, creating a web link that can set up a quantum key. But Qiang Zhang at the University of Science and Technology of China and his colleagues have extended the maximum distance of a quantum key distribution link through a cable through the use of an intermediate step that doesn’t browse the data, but only checks if it matches what was sent by the other end.
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Lasers at both ends of a fibre-optic cable send photons towards one another. These particles of light are in random phases, the pattern of peaks and troughs within their movement. When a couple of photons with matching phase meet in the centre hub, the system alerts both the sender and the receiver with a traditional data link.
Because each end knows what it transmitted and whether it matched the phase of the other, they are able to exchange a quantum key that works extremely well to encrypt data sent over traditional networks. Crucially, the central hub doesn’t know what was sent, only if the two signals matched.
Read more: Quantum internet signals beamed between drones a kilometre apart
A recently available experiment by Toshiba Europe in Cambridge, UK, demonstrated a web link of 600 kilometres using the same technology, but the apparatus was all housed within a lab. The Chinese team used a fibre-optic connection 511 kilometres long strung between the cities of Jinan and Qingdao, with a central receiver based between in Mazhan.
Zhang says there exists a healthy competition between the two labs to increase each other’s distance records. “In the lab, you have an air conditioning equipment, but in the field when the temperature changes you will learn the photon phase drift off,” he says.
“To turn a thing that works in a lab into a thing that works in the field, I think they do a good job,” says Peter Kruger at the University of Sussex, UK. “In the lab, nobody’s permitted to talk since it ruins the experiment and evidently in the field you can’t control that. Single photons over a huge selection of kilometres is very remarkable.”
Journal reference: Nature Photonics , DOI: 10.1038/s41566-021-00828-5
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