USTC Achieves Quantum Entanglement Between Cold-Atom Memories Across 420 km of Fiber

Forum topic · QianXun · 2026-08-22

Summary

On August 22, researchers led by Pan Jianwei, Bao Xiaohui, and Zhang Qiang at the University of Science and Technology of China (USTC), together with the Jinan Institute of Quantum Technology and the Shanghai Institute of Microsystem and Information Technology (CAS), reported quantum entanglement between two cold-atom quantum memories separated by 420 km of optical fiber. Beyond 230 km, the result exceeded the theoretical limit for entanglement distribution without quantum repeaters, a bound rooted in 1990s work by Bennett and colleagues on channel loss and synchronization precision. The team simultaneously solved three long-standing engineering hurdles: long-distance fiber phase locking, remote phase synchronization of independent lasers, and high-efficiency low-noise quantum frequency conversion linking telecom-band photons to atomic transitions. Crucially, the setup avoided trusted relay nodes, demonstrating that quantum-memory-based entanglement links are viable at metropolitan-to-intercity scale. Combined with prior achievements—Micius satellite key distribution, a 300 km quantum direct communication network, and module-level quantum direct communication devices—the result outlines a layered engineering path toward a quantum internet. Remaining challenges include mass-producible quantum memories, rack-mountable repeaters, and compatibility with existing fiber networks and ITU-T standards.

USTC Demonstrates 420 km Fiber-Based Quantum Entanglement

On August 22, People's Daily reported that a team led by Pan Jianwei, Bao Xiaohui, and Zhang Qiang at the University of Science and Technology of China (USTC), working with the Jinan Institute of Quantum Technology and the Shanghai Institute of Microsystem and Information Technology (CAS), achieved quantum entanglement between two cold-atom quantum memories across 420 km of optical fiber—and surpassed the theoretical limit for entanglement distribution without quantum repeaters beyond 230 km.

Why Quantum Memories?

Direct photon transmission over hundreds of kilometers of fiber suffers prohibitive loss. The team's approach uses cold-atom quantum memories as relay nodes: photons at each remote site become entangled with atoms in local memories, and the two memories are then linked via an entanglement-connection process based on single-photon interference—a lower-loss path. This architecture is considered the engineering foundation for long-range quantum communication, distributed quantum computing, and distributed quantum sensing.

Three Hard Problems, Solved Together

Long-haul fiber phase locking — thermal drift over kilometers of fiber destroys relative phase stability.
Remote phase synchronization of independent lasers — lasers in two separate labs must be phase-aligned at the same moment.
High-efficiency, low-noise quantum frequency conversion — telecom-band photons must be converted without loss to match atomic transitions.

Each challenge alone can fill a top-tier journal paper; the breakthrough is running all three simultaneously without trusted relay nodes.

Exceeding the No-Repeater Limit

A theoretical limit for entanglement distribution without quantum repeaters traces to 1990s work by Bennett and colleagues, constrained by channel loss and synchronization precision. This result did not merely extend the distance record—it pushed beyond that limit, implying that further scaling requires quantum repeaters, whose core building block is exactly the entanglement link between quantum memories demonstrated here.

Context in China's Quantum Roadmap

The achievement follows Pan Jianwei's July 17 recognition as the first Chinese scientist to receive the UNESCO-Russia Mendeleev International Prize for Basic Sciences, citing large-scale secure quantum communication and scalable quantum computing. Alongside the Micius satellite's 10,000-km-scale quantum key distribution, a 300 km fully connected quantum direct communication network between Hefei and Shanghai, and rocket-recovered module-level quantum direct communication devices, the result completes a layered path spanning satellite-ground, intercity, and modular scales.

Internationally, it ranks alongside Delft's loophole-free Bell test (2015), Micius (2016–2017), Innsbruck's trapped-ion networking (2021–2022), the US Quantum Internet Alliance blueprint (2020), and the EU Quantum Internet Alliance roadmap—as the first continental intercity-scale demonstration on the quantum-memory distance frontier.

Remaining Engineering Gaps

Mass-producible quantum memories (vacuum, cryogenics, magnetic shielding, single-photon detection)
Rack-mountable quantum repeaters integrating memories, entanglement links, and frequency conversion
Compatibility with deployed fiber networks under ITU-T standards (frequency conversion, filtering, WDM)

Takeaway

The central open question—what nodes an intercity quantum network will use—now has a validated answer: cold-atom quantum memories. The hardware, entanglement-link, and quantum-relay routes have all been field-verified across 420 km, setting the engineering blueprint for the next decade of quantum internet development.