The most advanced quantum communication network yet sprawls through the city of Hefei, China. Comprising three quantum devices and a central server, it is as close to an unhackable quantum internet as we have gotten yet.
Built by at the University of Science and Technology of China and his colleagues, the network shares some features with conventional communication networks, such as transmitting information through optical fibres. But the light travelling in the quantum network comes in chunks whose quantum states encode the information and have properties not found in conventional light signals. Specifically, the property of quantum entanglement can be leveraged to make it nearly impossible to surreptitiously copy or alter the information, making the whole network exceptionally secure.
Similar quantum networks have been built before, but this one stands out because of how many potential users could connect their own quantum processor to it and because of the kind of devices, like quantum memories, that it includes.
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In the centre of the network is a server that is connected to three users, each of which has a processor and a quantum memory. The users can use these devices to encode information into photons, or units of light, and send them to the server. The server can manipulate those photons 鈥� for example, by entangling them 鈥� and then send them back to the users.
Each user鈥檚 quantum memory is made from extremely cold rubidium atoms, which are controlled with lasers, and information can be encoded into the atoms鈥� quantum state. That is important because if a photon sent to the server gets lost or corrupted along the way, these can still hold on to the information that the photon was meant to carry.
Within the network, the team successfully entangled photons from two distant nodes, which is a necessary first step for the two to securely share information. The network could also sustain entanglement between more than one pair of users simultaneously, facilitating multiple secure quantum chats at once.
at the New York-based quantum communication start-up Qunnect says that combining devices like processors, fibres and memories has previously been too complicated because the properties of the photons must be tweaked and readjusted at many points in the network, so there are numerous possible points of failure. Past experiments pointed at promising steps towards building a network, but this is a true 鈥渘etworking mega experiment鈥�, he says.
鈥淭his is a very interesting milestone on the road to the quantum internet,鈥� says at the University of Bristol in the UK. He says that quantum memories are the most important new component since they have not been implemented in a network of this scale before. They are widely considered to be a crucial element for making quantum communication networks large enough to connect between different cities.
However, Joshi says that the rate at which information can be shared throughout the quantum network in Hefei still needs to be improved for the network to become useful. Currently, it can transmit roughly one bit per second, or less than a thousandth of what dial-up modems from decades ago could.
鈥淭he main challenge in making networks like this cover even longer distance and have even more nodes is in improving the quantum memories,鈥� says at the University of Chicago. Both Joshi and Zhong say that improving the memories鈥� storage times would enable the network to transmit more information faster.
arXiv
