A team of researchers, including Pan Jianwei, Bao Xiaohui, and Zhang Qiang from the University of Science and Technology of China, along with collaborators from various institutions, have achieved a significant milestone in long-distance quantum relay technology. They successfully established quantum entanglement between two cold atomic quantum memories over a staggering 420 kilometers of optical fiber, surpassing the theoretical limit for entanglement distribution without relays, which stands at just over 230 kilometers. This breakthrough paves the way for the construction of intercity quantum networks. The related research was published as an Editor's Suggestion in Physical Review Letters on August 11.

In recent years, this team has consistently achieved breakthroughs. This time, they tackled multiple technical challenges by switching the wavelengths of quantum memories, altering quantum frequency conversion schemes, and developing dual-wavelength three-frequency phase-locking technology. These advancements have significantly extended both the entanglement distance and success rates. The co-first authors of the paper are Luo Xiyu, Wang Chaoyang, and Zheng Mingyang. The research received support from multiple national and local projects.