The quantum internet is starting small, but growing. Researchers have created a network that lets four users communicate simultaneously through channels secured by the laws of quantum physics, and they say it could easily be scaled up.
S枚ren Wengerowsky at the University of Vienna and his colleagues devised a network that uses quantum key distribution (QKD) to keep messages secure. The general principle of QKD is that two photons are entangled, meaning their quantum properties are linked.
Two users receive a photon each and use them to generate a secure channel. The fragile quantum state means that if any hackers were to try to disrupt the channel, the users would be alerted and abandon it, preventing any eavesdropping.
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QKD systems often rely on the polarisation, or orientation, of a photon, but here the team used the frequency of the photon鈥檚 light, which can be tuned to allow for connections with several other users.
Frequent communication
The team created entangled photons with a laser, then split the resulting light spectrum into 12 separate channels, each with a different frequency. Their four users 鈥� called Alice, Bob, Chloe and Dave for this demonstration 鈥� were connected through fibre optic cables that each held three channels. So, for example, Alice has a secure line to Bob, Chloe and Dave.
These private lines eliminate the need for communications to pass through a central hub, and because the system operates on standard telecommunications equipment, it should be easy to roll out on existing networks, says Wengerowsky.
鈥淚f you want to scale the network up, you just add the hardware required for one of the users,鈥� says Robert Ursin, who was part of the team that built this quantum network. In other words, it鈥檚 easy to just plug a new user right into the network. 鈥淚t鈥檚 much like the classical internet where everyone gets a router.鈥�
Some other types of quantum networks require so-called trusted nodes through which all communications must pass, but which are vulnerable to attack, reducing the benefits of quantum security. Or they may need active switches made of mirrors that direct the information flow like railroad switches. This system doesn鈥檛 use any of those things, which will make it easier to scale up.
鈥淭his is a nice demonstration because it鈥檚 all passive optics, so in that sense it鈥檚 very manageable. I do hope it will help make quantum key distribution more mainstream,鈥� says Thomas Jennewein at the University of Waterloo.
He adds that while it may be simple enough to add another fibre when you introduce a new user, at some point there is a limit to how many times you can split the light spectrum to create additional channels.
鈥淭he entanglement quality could start degrading as more and more channels are combined into one fibre, or the detector system might get overwhelmed at some point,鈥� he says. 鈥淚鈥檓 sure there are ways to fix that or improve it, but none of that was laid out in the paper.鈥�
Ursin says that in the future, they could potentially take advantage of more properties of the photons 鈥� their polarisation, for instance 鈥� to encode more information into the quantum keys to make them more secure.
Nature
