By IQT News posted 04 Oct 2021

(Phys.org) Teng-Yun Chen and a research team in quantum physics, quantum information and interdisciplinary information sciences in China, presented a field operation of a quantum metropolitan area network with 46 nodes. They realized diverse topological structures and ran the network for 31 months via standard equipment. They then realized QKD pairing and key management for secure communications including real-time voice telephone, text messaging and file transmission with one-time pad encryption to support 11 pairs of users to make simultaneous audio calls. The technique can be combined with an intercity quantum backbone and via ground-satellite links to form a global quantum network. Their report is now published on Science Advances.
Chen’s constructed a 46-node quantum metropolitan-area network throughout the city of Hefei. Quantum key distribution (QKD) ultimately aims to construct a global quantum network where communication traffics have information-theoretic security guarantees.
Chen’s 46-node quantum metropolitan-area network connected  40 user nodes, three trusted relays and three optical switches, throughout Hefei. The network covered the entire urban area and connected several organizations within the city districts including governments, banks, hospitals, and research universities. They first reviewed the basic topological structures in a network where the most robust method used a fully connected topology where each user was directly connected to every other user in the network. The type of network did not require the users to trust one another.
Chen et al. developed a key management process to allow users to generate keys in high priority. To accomplish this, they designed a switching strategy based on the number of keys stored in the local memories for the users. They then connected a 16-port optical switch to 16 users to obtain a total of 120 possible key-pairing schemes by which two users could be connected for the QKD process for a switching time ranging from 10 to 60 minutes.
In this way, Teng-Yun Chen and colleagues developed a practical and large-scale metropolitan quantum key distribution (QKD) network with commercial QKD products for practical use in Hefei, China. The team could scale the quantum network by adding more users and relays to connect to the Shanghai-Beijing backbone as a national network.

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