The quantum network represents a significant platform for the verification of the fundamental principles of quantum mechanics and the realization of quantum communication and computation. However, 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.

Here, we propose a long-distance large-scale and scalable fully-connected quantum secure direct communication (QSDC) network, which employs a doublepumped structure and the introduction of extra noise to successfully realize QSDC over 300 km between four users in the network in pairs. Our work offers a novel foundation for the future realization of long-distance large-scale quantum communication.

Figure 1. The experimental setup of fully-connected QSDC network.

As shown in Figure 1, this network utilizes a double-pumped structure of CH32 and CH33 which can correlate photons from one channel with photons from multiple channels. Furthermore, the incorporation of an appropriate degree of additional noise can markedly increase the performance of quantum communication.

Figure 2 shows that the network transmits the four bits with greater than 85% fidelity for each channel for 300km of fiber transmission (loss about 60dB). These results indicate that each user in the network only requires the operation of two channels of photons to facilitate two-by-two communication. Our work realizes a four-user fully-connected QSDC network with a transmission rate of 300km@104bit@1h which verifies the feasibility of the network.

Figure 2. The fidelity of polarization entanglement for four different coded messages after transmission.

This work is published in “Yilin Yang, Yuanhua Li, Hao Li, Chennan Wu, Yuanlin Zheng, and Xianfeng Chen, A 300-km fully-connected quantum secure direct communication network, Science Bulletin, 70 (2025) 1445-1451.”

Link: https://www.sciencedirect.com/science/article/pii/S209592732500204X