Secure quantum communications network is the largest of its kind

By Chris Stokel-Walker

A more secure internet could be around the corner thanks to the reported largest-ever quantum network of its kind.

Quantum communication systems are more secure than regular networks, because they rely on the quantum properties of photons, rather than computer code that can be cracked. But building one is expensive. “It just doesn’t scale well,” says Siddarth Joshi at the University of Bristol, UK.

Now Joshi and his colleagues have developed a kind of quantum network using a method called multiplexing entanglement. Entanglement is a quantum property that sees two objects, such as a pair of photons, linked so that measuring one of them instantly influences the measured state of the other, regardless of the distance between them. This can be used to generate a secure encryption key.

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Rather than connecting users one-to-one, which is costly when dealing with a large network, multiplexing entanglement splits photons from a single laser according to their wavelength. Each wavelength can hold a data stream, meaning the system could support between 50 and 100 users with existing hardware, says Joshi.

The team tested the system with eight users on existing optical fibres in Bristol. More than 100 people could potentially use the system, says Joshi, because not everyone needs to be connected all the time

The team says its test system is the world’s largest entanglement-based network, in terms of number of users, and is different from larger networks, such as one in China, that use a method called “trusted nodes”. There, a message is passed along a line of users, meaning any legitimate user on the network can listen in.

“The Chinese network is addressing how to build a 2000-kilometre link between Beijing and Shanghai,” says Joshi. “We’re addressing how to connect everyone together.”

That step away from trusted nodes is a major development, says Harun Šiljak at Trinity College Dublin, Ireland. The Bristol development is “an important step towards the quantum internet idea of unprecedented security and privacy”, he says.

But Šiljak cautions that this is a proof of concept and has questions about keeping the error rates of photon recognition in realistic bounds when operating a larger network. If error rates exceed a certain percentage, it may not be possible to produce secure encryption keys.

Joshi believes that risk is manageable, and the method can be used to connect millions of devices together. “What I am trying to set out to do is to build the quantum internet,” he says.

Journal reference: Science Advances, DOI: 10.1126/sciadv.aba0959