Overview
On August 11, Physical Review Letters published, as an Editors' Suggestion, a study by the team of Pan Jianwei, Bao Xiaohui, and Zhang Qiang at the University of Science and Technology of China (USTC), in collaboration with the Jinan Institute of Quantum Technology and the CAS Shanghai Institute of Microsystem and Information Technology. The work reports the first quantum entanglement between two cold-atom quantum memories across 420 km of optical fiber, achieving entanglement distribution that surpasses the PLOB theoretical bound at distances beyond 230 km.
Progress Timeline
| Year | Distance / Nodes | Team | |---|---|---| | 2020 | 50 km fiber, two nodes | Pan Jianwei's team | | 2024 | Hefei metropolitan three-node quantum memory network (world first) | Pan Jianwei's team | | 2026 | Hundred-km high-fidelity two-node entanglement + device-independent QKD | Pan Jianwei's team | | Aug 2026 | 420 km two-node entanglement, bypassing the PLOB limit | Pan Jianwei's team |
From 50 km to 420 km: an 8.4× distance increase in six years.
Bypassing the PLOB Bound
The PLOB (Pirandola-Laurenza-Ottaviani-Banchi) bound caps the success rate of *direct* entanglement distribution by total channel transmission efficiency — beyond ~230 km, direct distribution cannot exceed it. Instead of approaching the bound, the team bypassed it: by using single-photon interference for entanglement connection, the success probability scales with the transmission efficiency of *half* the channel rather than the whole.
Two Key Technical Breakthroughs
1. Wavelength switching + frequency conversion. The memories originally emitted at 795 nm, which suffers high fiber loss. The team switched to 780 nm and adopted a new quantum frequency conversion scheme, shifting signal photons toward the ultra-low-loss telecom C-band window — while preserving quantum coherence during conversion. 2. Dual-wavelength, triple-frequency phase locking. Combining continuous and intermittent locking techniques, the team greatly improved long-term phase stability over ultra-long fiber, keeping single-photon interference stable even at 420 km.
Significance for Quantum Networks
Practical quantum internet applications — quantum-repeater-based intercity secure communication, distributed quantum computing, and distributed quantum sensing — all require hundred-km-scale entanglement. Bypassing the PLOB bound turns these from theoretical possibilities into engineering goals. This differentiates USTC's quantum-memory-plus-long-fiber route, designed specifically for the quantum internet, from fault-tolerant quantum computing efforts (e.g., QuEra, Quantinuum, Google Willow).
Remaining Limitations and Next Steps
1. Entanglement success rate still falls with distance — at 420 km, the success rate is on the order of one-thousandth; further loss reduction and phase-locking improvements are needed. 2. Memory coherence time (milliseconds to seconds) limits how long entanglement persists; distance × time is the true constraint. 3. Multi-node relaying not yet demonstrated — three- and four-node quantum internet prototypes are the next step.
Planned milestones include three-node quantum repeaters, deployment on carrier-grade fiber (with wavelength-division multiplexing and quantum-classical coexistence), and application-layer integration.
Outlook
At the current pace, projections suggest single links beyond 1,000 km with three-node repeaters by 2027–2028, inter-provincial trunk lines coexisting with classical fiber by 2028–2030, and a mature quantum internet protocol stack by 2030+.
Key data: 420 km fiber entanglement; PLOB bound bypassed beyond 230 km; wavelength 795 → 780 nm; dual-wavelength triple-frequency phase locking; co-first authors Luo Xiyu, Wang Zhaoyang, Zheng Mingyang; PRL, Aug 11, 2026, Editors' Suggestion.
Sources: USTC news release (Aug 13, 2026); Physical Review Letters (Aug 11, 2026); Jinan Institute of Quantum Technology; CAS Shanghai Institute of Microsystem and Information Technology.