For instance, it could pose a problem for international spying efforts such as the Prism Project or the “Five Eyes” alliance. That is because, with the particles entangled, any eavesdropping attempt would inevitably generate a physical disturbance that would alert the sender or receiver.

It would also improve the precision of measurements and satellite navigation because clocks could be more finely synchronised, with computers around the world working simultaneously on the task.

And it could even change our understanding of the universe, with sensors – linked by the quantum network – potentially able to detect gravitational waves from stars colliding in distant galaxies.

China is generally regarded as a world leader on quantum communication technology – it developed the first quantum satellite and has the longest quantum key distribution network in operation.

Such networks make use of just a small number of entangled particles to code messages for extra security, whereas a future quantum internet would send information entirely using quantum mechanics such as entanglement.

In the experiment, the research team – led by Pan Jianwei, a professor at the University of Science and Technology of China in Hefei, Anhui province – sent a powerful laser beam into a cluster of rubidium atoms.

The laser – stronger than any used in previous experiments – entangled some of the metallic atoms with photons, or particles of light. Those photons then bounced off the atoms and passed through the optical fibre to reach more atoms at the other end, which is when the rubidium atoms at both ends became entangled.

The idea of building a quantum internet with entangled atoms as repeaters – which relay the quantum message from one stop to the next – is not new, but until now the longest distance the photons had travelled was just over 1km.

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Optical fibre, the backbone of modern telecommunications, is not a friendly medium for entangled light particles. According to some estimates, if 100 quadrillion entangled photons travelled through optical fibre, only one of them would survive after 50km.

While there are quantum key distribution networks already in use, they do not transmit information via entangled particles.

The 1,100km (684-mile) optical fibre network that runs between Beijing and Shanghai, for example, can transmit a small number of entangled photons that are used as encryption keys to protect data – but those entangled particles cannot send information.

Information on a future quantum internet would be carried by entangled photons. For now though, this type of quantum communication technology is severely limited by bandwidth.

Pan’s team was able to boost the survival rate of photons travelling along 50km of optical fibre from one out of 100 quadrillion to one out of 100. They did that by developing a crystal made from niobium, lithium and oxygen that can convert a beam of entangled light particles into a frequency commonly used in commercial communications.

And by increasing the number of atomic repeaters, the researchers say they might be able to build a city-scale quantum internet.

“We are working hard towards that goal,” said a researcher involved in the study, who requested anonymity because he had not obtained official approval to speak to the media.

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A quantum internet would also have a wide range of military applications. For instance, it could significantly improve the accuracy of GPS and give missiles unprecedented precision.

American researchers financed by the US Army conducted an entanglement experiment using calcium atoms in September, but the transmission efficiency was only about 1 per cent of that reported by Chinese scientists in similar tests.

The researcher said other countries were catching up with China on quantum communication technology, “especially the Netherlands”.