200-km multi-user fully connected quantum entanglement distribution network in noisy environments.

Long-distance entanglement distribution is a fundamental operation for achieving a large-scale and scalable fully connected quantum communication network. However, current entanglement distribution methods cannot simultaneously meet the requirements of increasing user numbers and distribution distances. Here, we develop a three-user fully connected entangled distribution quantum network in the experiment. We demonstrate that the self-made periodically poled lithium niobate (PPLN) waveguide can achieve a fidelity of over 96% for the entangled state shared between users in the network. The results show that by introducing additional noise, the fidelity of the entangled state shared between any two users in the network is still greater than 85% after completing entanglement distribution over 200 km, which is far higher than the fidelity without adding noise. Our work provides a new, to the best of our knowledge, approach and experimental basis for building a large-scale and scalable fully connected quantum communication network over long distances.