Chinese Scientists Entangle Quantum Memories Across 420 km of Fiber, Breaking Theoretical Limit
A team of Chinese researchers has demonstrated quantum entanglement between two cold-atom quantum memory nodes separated by 420 kilometres of fibre — an eightfold increase over their previous record and the first memory-based demonstration to exceed a fundamental theoretical ceiling for long-distance entanglement distribution.
The result, published in Physical Review Letters in July 2026 as an Editors’ Suggestion, was produced by a joint team from the University of Science and Technology of China (USTC), led by Pan Jianwei, Bao Xiaohui, and Zhang Qiang, together with collaborators at the Jinan Institute of Quantum Technology and the Shanghai Institute of Microsystem and Information Technology under the Chinese Academy of Sciences.
According to 科学网 (ScienceNet.cn), which carried the official announcement from USTC, the experiment achieved entanglement between two laser-cooled atomic ensemble memories via single-photon interference across a total fibre length of 420 km. The experimental setup placed both memory nodes inside USTC’s Hefei laboratory, with a 10.1 km section of deployed commercial field fibre connecting them to a middle relay node at Hefei Software Park; the remaining distance was completed using spooled ultra-low-loss fibre of the same grade as deployed telecommunications infrastructure, with a loss of 0.17 dB/km at the signal wavelength — a standard approach for distance validation experiments in the field.
Two engineering challenges, both solved independently by the team, enabled the leap from 50 km (their 2020 record, published in Nature) to 420 km. The first was quantum frequency conversion: the write-out photons emitted by the rubidium atom ensembles at 780 nm were shifted to 1522 nm in the telecom S-band, dramatically reducing fibre attenuation from 3.5 dB/km to around 0.17 dB/km. The second was phase stabilisation: at such distances, even tiny fluctuations in fibre temperature or vibration disrupt the single-photon interference on which the entanglement protocol depends. The team developed a dual-band phase-locking scheme combining continuous wide-detuning locking to suppress high-frequency noise and intermittent locking to correct slow, low-frequency drift, achieving overall phase stability of approximately 7 degrees across the full 420 km.
The scientific significance extends beyond the distance record. The experiment employs the Duan-Lukin-Cirac-Zoller (DLCZ) entanglement scheme, in which a successful entanglement event is heralded by detecting only a single photon — meaning the probability of success scales with the square root of channel transmittance, rather than the transmittance itself. For direct transmission of entangled photons, the success rate is bounded by what physicists call the PLOB bound, a theoretical maximum set by information theory. As 量科网 reported, the USTC experiment crosses above that bound beyond 230 km, demonstrating a rate advantage that grows with distance and cannot be replicated without memory-based nodes.
The result is the latest in a series of milestones from Pan’s group: a three-node city-scale quantum storage network in Hefei in 2024 (published in Nature), and high-fidelity dual-node entanglement combined with device-independent quantum key distribution over 100 km in early 2026 (published in Science). Chinese technology media, including 网易科技, have contextualised the 420 km result within a broader geopolitical backdrop: in the same month the paper appeared, the United States had issued executive orders aimed at restricting allied-country exports of quantum hardware to China, and tightened controls on dilution refrigerators — the specialised cooling equipment used in superconducting quantum computers. That category of equipment was first placed on export control lists in 2022; by 2024, imports to China had fallen to zero. In June 2026, the Chinese Academy of Sciences and OriginQ (国盾量子) announced the completion of a domestically developed, all-domestic-component dilution refrigerator capable of reaching below 6 mK, according to a Xinhua/CAS report.
The 420 km fibre result is relevant to quantum communication rather than superconducting quantum computing, and does not depend on dilution refrigerators. But the juxtaposition — tightening hardware controls on one track and a record-setting communication milestone on the other — defines how China’s state science media is framing the country’s quantum roadmap in mid-2026.
No substantive coverage of the 420 km entanglement result was found in major English-language news outlets at the time of publication.
Sources
科学网 (ScienceNet.cn) — “中国科大实现量子存储器间纠缠距离420公里” (Chinese, China)
量科网 (QTC.com.cn) — “中国科大将量子存储器间纠缠距离提升至420公里” (Chinese, China)
网易科技 (163.com) — “美国严密封锁量子硬件！中国：你封你的，我420公里量子链路铺通” (Chinese, China)
中国科学院 / Xinhua — “国产单核心大冷量稀释制冷机成功下线” (Chinese, China)
Physical Review Letters — “Entangling quantum memories through a 420 km long fiber” (English, Academic, accepted June 9, 2026)