Chinese Researchers Extend Quantum Memory Entanglement to 420 Kilometers

A research team led by the University of Science and Technology of China has successfully demonstrated quantum entanglement between two cold-atom quantum memories across 420 kilometers of optical fiber. Published in Physical Review Letters, the experiment also surpassed the theoretical limit for direct unrepeatered quantum entanglement distribution at distances exceeding 230 kilometers. By adapting operating wavelengths to minimize fiber transmission loss and implementing dual-wavelength phase locking, the breakthrough provides critical technical groundwork for constructing intercity-scale quantum networks.

Long-distance quantum memory entanglement is a cornerstone for realizing practical quantum repeaters, which are necessary to scale quantum communication, distributed quantum computing, and quantum sensing beyond the physical loss limits of standard optical fiber.

A Chinese research group has achieved quantum entanglement between two cold-atom quantum memories connected by 420 kilometers of optical fiber, setting a new benchmark for distance between matter qubits, according to reports from domestic media. The experiment was conducted by researchers from the University of Science and Technology of China (USTC), including Pan Jianwei, Bao Xiaohui, and Zhang Qiang, in collaboration with the Jinan Institute of Quantum Technology and other partner institutions. Their findings were recently published as an Editors' Suggestion in the journal Physical Review Letters. Establishing remote entanglement between stationary quantum memories is essential for building scalable quantum repeaters, which serve as foundational components for long-range quantum communication networks, distributed quantum computation, and quantum sensing. A central challenge in scaling these systems is maintaining high entanglement generation rates while mitigating channel losses and phase instability across extended fiber links. To overcome these constraints, the team introduced several technical modifications to reduce transmission loss and stabilize the optical paths. They shifted the operating wavelength of the cold-atom quantum memories from 795 nanometers to 780 nanometers and updated their quantum frequency conversion architecture to align emitted signal photons with the ultra-low-loss transmission window of standard telecom fibers. In addition, the researchers developed a dual-wavelength, three-frequency phase-locking technique combining continuous and time-multiplexed mechanisms, significantly improving long-term single-photon phase stability over ultralong fiber lines. Using these methods, the team demonstrated stable single-photon interference from the emitted photons of two separate cold-atom memories across link lengths reaching up to 420 kilometers, directly confirming quantum entanglement between the nodes. The researchers noted that at distances beyond 230 kilometers, the entanglement distribution efficiency surpassed the theoretical upper boundary for direct, repeaterless entanglement transmission. According to the researchers, these results demonstrate the technical feasibility of constructing intercity-scale quantum memory networks and provide a path forward for implementing multi-stage quantum repeater protocols in practical communication environments.