Efforts to build a quantum internet are under way, but turning what was once a lab experiment into a robust, modern communications network is not without its difficulties. Now, two researchers have come up with a way to ensure quantum networks are unhackable, even at scale.
Theoretically, messages sent via quantum networks are secured by the laws of quantum mechanics, because any attempt to intercept information sent between two parties will disturb its fragile quantum state, revealing the eavesdropper. One way to check a quantum line is secure involves solving an equation called a Bell inequality 鈥� if the result exceeds a certain number, there is a limit to how much information a spy can extract without being detected.
But this is only true for messages sent between two points. On a more complex network, the Bell inequality doesn鈥檛 apply. That鈥檚 roughly because quantum hackers can gain a little bit of information from each part of the network and then piece it together. One solution would be to create point-to-point connections between everyone on a quantum network, but that is hardly practical, says at University College London. 鈥淭he naive method of directly extending the previous point-to-point results doesn鈥檛 result in an efficient way to communicate,鈥� he says.
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Lee and his colleague at the University of Oxford have found a better way. They have shown that a more complex version of the Bell inequality can provide the same security assurances for a network as the simpler version does for point-to-point contact. There鈥檚 just one problem: figuring out what this equation looks like is very difficult.
The pair overcome this by using a technique from machine learning called causal inference to study the structure of the network. Essentially, a computer analyses the direction of information flow between the different nodes to figure out its causal structure. For example, if node A is connected to node C via node B, A and C can鈥檛 communicate unless the message goes through B first.
Knowing this structure lets the pair come up with a Bell inequality for any kind of network, which can be used to guarantee its security. 鈥淕iven the network structure, you derive the corresponding equation,鈥� says Lee.
Researchers in the UK are currently building a quantum network between Bristol, London and Cambridge, and Lee hopes his technique will be used to keep it secure. 鈥淵ou can think of our research as the software that will run on this hardware,鈥� he says.
Physical Review Letters
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