Quantum Memory Entanglement Over 420 km of Optical Fiber Breaks Fundamental Limit

How the experiment worked

The system consisted of two rubidium atomic-ensemble quantum memories, nicknamed Alice and Bob, connected by spooled optical fiber . A central detection node, Charlie, performed Bell-state measurements on photons arriving from both memories to herald successful entanglement . The team used the DLCZ (Duan-Lukin-Cirac-Zoller) single-photon entanglement scheme, which generates entanglement probabilistically by detecting one emitted photon rather than requiring two-photon coincidence. This dramatically improved the entanglement generation rate at long distances .

Three technical advances that made it possible

Three core innovations enabled the record-breaking distance:

1. Telecommunications-band wavelength conversion — Photons emitted from the rubidium memories were converted to the telecom S-band (around 1.5 μm) via quantum frequency conversion, allowing transmission over fiber with a loss of just ~0.17 dB/km .
2. Active fiber stabilization system — A real-time feedback system compensated for phase fluctuations in the long fiber links, maintaining interference visibility over the full 420 km path . Without this, the phase drift from temperature changes and vibrations would have destroyed the quantum interference.
3. Single-photon DLCZ entanglement protocol — Rather than requiring two-photon coincidence, the scheme relied on detecting a single photon from each memory node, which dramatically improved the entanglement generation rate at long distances .

Surpassing the PLOB bound

A key theoretical result from the experiment is that it surpassed what is known as the PLOB bound (Pirandola–Laurenza–Ottaviani–Banchi) — the fundamental limit on entanglement distribution rate through a lossy channel without quantum repeaters . This bound, derived in 2017, sets the maximum rate at which two remote parties can distribute qubits or ebits over a direct fiber link .

The team demonstrated that their measured memory-memory entangling probability exceeded the PLOB bound for distances greater than approximately 230–320 km of direct fiber . This means that a memory-assisted quantum network can achieve higher entanglement rates than any direct fiber link beyond that crossover distance, proving the advantage of quantum memories as repeater nodes . It is the first experimental demonstration that a memory-based quantum network can beat the fundamental repeaterless limit, validating a key premise of quantum repeater architectures .

Significance for the future quantum internet

This breakthrough bridges the gap from metropolitan-scale quantum networks (tens of km) to intercity-scale distances (hundreds of km). It shows that stored quantum information can be reliably entangled over distances that connect cities, laying a practical foundation for building scalable, memory-based quantum repeaters . These repeaters could one day form the backbone of a global quantum internet, enabling long-distance quantum key distribution, distributed quantum computing, and secure quantum communication .

The work builds on a steady progression from the same team: entanglement over 50 km in 2020 , a three-node metropolitan quantum network in Hefei in 2024 , and further demonstrations of high-fidelity entanglement and device-independent quantum key distribution . The 420 km result now shows that quantum memories can serve as effective repeater nodes, making a global quantum network — once a theoretical vision — a concrete engineering target.