**6. Open issues in quantum communication networks**

In recent years, the experimental research [15, 20, 38, 39] of quantum entanglement has mainly focused on how to expand the entanglement distribution distance, and the construction of quantum communication networks is mostly based on simple network topologies, mainly point-to-point network topologies, and a small number of star or bus network topology containing several nodes. There is little research on quantum communication networks under complex network structures, and research work should also be concentrated in the field of network security and quantum state transmission. Few works [40] have studied entanglement distribution from the level of quantum communication networks, so it is urgent and challenging to realize the connectivity of quantum communication network, repeating, switching, and routing quantum communication network and multi-layer quantum communication network.

• Connectivity of quantum communication network. How to deploy entangled particles and the location of distribution nodes play a vital role in the connectivity of the network. However, most of the current quantum communication network construction work is based on simple network topologies, mainly point-to-point network topologies, and a small number of star or bus network topologies that contain several nodes. There is little research on quantum communication networks under complex network structures, and research work is mainly focused on the field of network security and quantum state transmission, and the research on how to improve network connectivity is still insufficient.

