Quantum Entanglement Record Shattered: Chinese Physicists Entangle Quantum Memories Across 420 km of Fiber

Physicists at the University of Science and Technology of China (USTC) have smashed the distance record for fiber-based quantum entanglement between two pieces of matter. The team entangled two atomic quantum memories — clouds of laser-cooled rubidium-87 atoms — across 420 kilometers of optical fiber . That is more than four times the previous best for matter-to-matter entanglement over fiber, and it is the first time a memory-based scheme has beaten a fundamental theoretical ceiling known as the PLOB bound .

Published in Physical Review Letters as an Editors' Suggestion and first posted on arXiv , the experiment is a concrete step from metropolitan-scale quantum networks toward real intercity links.

How the experiment worked

The system was built around two quantum memory nodes, nicknamed Alice and Bob, with a central measurement node (Charlie) in between . Each memory node was a cloud of rubidium-87 atoms cooled and trapped by lasers. To create entanglement, the team used a well-known protocol called the Duan-Lukin-Cirac-Zoller (DLCZ) scheme . In this approach, each memory probabilistically emits a single photon. Detecting that photon at the central node signals that the two memories have become entangled.

The team had to overcome three major obstacles to reach 420 km:

Wavelength conversion. Rubidium memories naturally emit photons at around 780 nm, in the near-infrared. These would be quickly absorbed over long fiber runs. The team converted each photon to the telecom band (around 1550 nm) — the same wavelength used for standard fiber-optic communications — so the signal could travel hundreds of kilometers without being absorbed .

Active stabilization. Over 420 km of fiber, temperature changes, vibrations, and other mechanical effects can destroy the phase relationship needed for single-photon interference. The team deployed a stabilization system that continuously compensated for these fluctuations, preserving coherence across the full link .

Single-photon interference. The core measurement that creates entanglement happens at the central node, where photons from Alice and Bob interfere. Achieving stable interference with single photons over such a long distance was a major technical challenge that the team successfully solved .

Breaking the PLOB bound

Perhaps the most significant finding is that the experiment surpassed the PLOB bound — the theoretical maximum rate at which entanglement can be distributed over a lossy channel without quantum repeaters . The measured memory-memory entangling probability exceeded this limit at distances beyond approximately 230 km, and the advantage continued to grow as the fiber length increased all the way to 420 km . As one report noted, this is the first memory-based demonstration to exceed that fundamental ceiling .

The team also demonstrated that their memory-assisted scheme outperforms direct fiber-based entanglement distribution beyond roughly 320 km . This happens because the memory-based approach scales with the square root of the channel transmittance, while direct transmission scales linearly with transmittance — a fundamental difference that gives memories the edge at long distances .

Why it matters for the quantum internet

It shifts the scale from metropolitan to intercity. Previous fiber-based matter-matter entanglement records were in the 50–100 km range, covering a typical metropolitan area . At 420 km, the experiment crosses into distances that connect cities. As the authors write, "Our experiment provides a testbed for studying applications of quantum networks beyond the scale of a single city" .

It validates memory-assisted quantum networks. The result proves that quantum memories can store and herald entanglement over practical fiber lengths while beating the repeaterless limit . This is a necessary precondition for building scalable quantum repeaters and, ultimately, a full quantum internet .

It aligns with existing telecom infrastructure. The key components — telecom-band wavelength conversion, active fiber stabilization, and cold-atom memories — are all compatible with standard fiber-optic equipment . This suggests a realistic upgrade path from today's quantum key distribution (QKD) networks to a future quantum internet with memory-enabled long-distance entanglement distribution.

Key caveats

While the technical achievement is genuine, it is important to note that this was a laboratory demonstration. The experiment used spooled fiber in a controlled environment, not a field-deployed link . The fiber included a short field segment running from the university lab to a measurement node at Hefei Software Park, but most of the distance was on spools . Real-world deployment will require solving additional engineering challenges, including the stability of field-deployed fiber, integration with switching and routing hardware, and scaling to multiple interconnected nodes .