A 300-km fully-connected quantum secure direct communication network

Transmission distance and number of users limit the realization of large-scale scalable quantum communication networks, as existing quantum network construction techniques struggle to address these two important factors simultaneously. In this paper, we propose a long-distance large-scale and scalable fully-connected quantum secure direct communication (QSDC) network, which employs a double-pumped structure and the introduction of extra noise to successfully realize QSDC over 300 km between four users in the network in pairs. The results demonstrate that the fidelity of the entangled state shared between users following communication remains above 85%. The results of our research demonstrate the viability of this quantum communication network, offering a novel foundation for the future realization of long-distance large-scale quantum communication.