https://www.cas.cn/syky/202608/t20260817_5118473.shtml
https://journals.aps.org/prl/abstract/10.1103/ccd6-rf1s
The University of Science and Technology of China (USTC), in collaboration with the CAS Shanghai Institute of Microsystem and Information Technology and other institutions, established quantum entanglement between two cold-atom quantum memories over 420 kilometers of optical fiber, surpassing the theoretical limit for entanglement distribution without relays at distances exceeding 230 kilometers. This achievement substantially extends the entanglement distance between material-based quantum bits, laying the foundation for the construction of inter-city scale quantum networks.
Long-distance entanglement between quantum memories serves as the technical basis for building the quantum internet and holds promise for applications such as long-range quantum communication based on quantum relays, distributed quantum computing, and distributed quantum sensing. The USTC team has consistently achieved breakthroughs in this field in recent years. In 2020, the team successfully realized entanglement between dual-node quantum memories via 50 kilometers of optical fiber; in 2024, building on this, they constructed the world’s first metropolitan three-node quantum memory network in Hefei; and subsequently, they achieved high-fidelity dual-node entanglement over 100 kilometers and device-independent quantum key distribution.
The team has conducted years of research on entanglement connection based on single-photon interference, overcoming technical challenges such as phase locking over long fibers, remote phase synchronization of independent lasers, and high-efficiency, low-noise quantum frequency conversion. Extending the entanglement distance to hundreds of kilometers requires further reduction of channel transmission loss and improved long-term stability of single-photon phase locking.
In the experiment, the team switched the quantum memory wavelength from 795 nm to 780 nm and adopted a new quantum frequency conversion scheme, enabling signal photons to match the ultra-low-loss transmission window of optical fibers. Regarding single-photon phase locking, the team developed a dual-wavelength, three-frequency phase-locking technique that combines continuous-wave and time-division multiplexing, significantly enhancing the long-term stability of photon phase locking over ultra-long fibers. By integrating these two new technologies, the team conducted experimental studies on dual-node entanglement across fiber links of various lengths.
The experiments demonstrated that photons emitted from the two cold-atom quantum memories maintained stable single-photon interference over distances up to 420 kilometers, and quantum entanglement between the memories was directly verified. This experiment not only achieved a significant increase in the distance of entanglement between memories but also made a major breakthrough in the success rate of entanglement distribution. While the success rate of direct entanglement distribution is proportional to the transmission efficiency of the entire channel, this experiment utilized single-photon interference to establish entanglement; consequently, the success probability is proportional to the transmission efficiency of only half the channel, allowing it to surpass the theoretical limit for direct entanglement distribution (the PLOB bound) over long distances (exceeding 230 kilometers). These experimental results and the associated technologies pave the way for constructing intercity-scale quantum networks and conducting related application research.