Secure quantum networks could soon be stretching across Europe, thanks to a new scheme that ups the theoretical limit for transmitting messages using the weird properties of quantum mechanics.
Quantum key distribution (QKD) is a way of exchanging secret encryption keys using photons – particles of light – encoded with certain quantum properties. Done properly, it is impossible to hack, because any attempts at intercepting the key will disrupt the delicate quantum state.
QKD is increasingly being used in the real world but suffers from fundamental limitations: the photons can only be sent so far through optical fibres before they scatter and lose their quantum state, meaning that data rates drop at increasing distance. Now, and his colleagues at Toshiba Research Europe in Cambridge, UK have found a way round this.
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In regular QKD, the sender (known as Alice) randomly transmits photons via one of two paths, which slightly alters a property called their phase. They reach the receiver, Bob, who also randomly alters the photons’ phase by a different amount before measuring them. The pair use the difference between the phases to transmit a series of data bits – 0s and 1s – that establish a secret encryption key.
The team realised they could improve this by having Alice send one lot of photons encoded with her phase while Bob sends the other encoded with his phase. The photons meet in the middle, where they are measured by a third party, Charlie. “It means you can have effectively twice the distance and achieve the same key rate,” says Shields. A higher key rate, or data rate, means more data can be sent in less time.
Blind trust
Crucially, Charlie doesn’t know the phases used by Alice and Bob and so isn’t able to copy their encryption key. That’s in contrast to other methods of extending QKD over large distances, which involve “trusted nodes” that rebroadcast the signal – but can also eavesdrop on it. Theoretical devices called quantum repeaters would do away with this requirement, but require technology we don’t yet have.
Shield’s team say their scheme is practical with current tech and should make it possible to send data at around 10 bits per second at distances of up to 550 kilometres over standard optical fibre. This is far better than thought possible under previous theoretical limits – the best experiment so far was able to transmit 1 bit per hour over 400 kilometres of specially designed ultralow-loss fibre. Shield’s say they are now conducting their own QKD experiment to see if their techniques works as promised.
Such distances should just about make it possible to build secure networks between European capitals – London to Paris is around 350 kilometres as the crow flies, but fibre is unlikely to lie in a straight line, so the distance required will be longer. “Hopefully it would still be possible to go London to Paris,” says Shields. “We want to create long-distance links and put this into real-world use.”
“This is a conceptual breakthrough in quantum communication,” says  at the University of Toronto, Canada, but he points out that the team still need to prove that their scheme retains the quantum security properties of existing QKD, which will require more theoretical development. “The work is stunning, but only suggestive,” he says.
Nature
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