Hefei Researchers Make the First Metropolitan-Scale Bell Nonlocality Certification in Quantum Computing
In a significant step toward the quantum internet, USTC researchers have created the first metropolitan-scale quantum repeater that satisfies Bell nonlocality criteria. The 14.5-kilometer Hefei experiment represented a significant advancement from lab prototypes to practical, quick quantum networks.
Overcoming the Exponential Loss Limit
The fundamental idea behind the “quantum internet” is the ability of multiple nodes to communicate entanglement. However, quantum signals, which are usually conveyed by single photons, face a formidable challenge: when they pass through conventional optical fibers, they rapidly degrade. Scientists employ quantum repeaters (QRs), which serve as relay stations to announce or confirm entanglement across network segments before switching it to connection endpoints.
Single-Photon Interference (SPI) techniques have been the mainstay of metropolitan-scale demonstrations to far. Although SPI may attain high speeds, it frequently produces low-quality entanglement that fails a Bell test, which is the gold standard for quantum nonlocality, and is infamously vulnerable to phase variations in fiber lines.
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The Innovation: MQR-TM
A novel protocol known as Multiplexed Quantum Repeater based on Time Measurements (MQR-TM) was introduced by the USTC team under the direction of Professors Zong-Quan Zhou, Chuan-Feng Li, and associates. This solution expertly combines two previously opposing advantages: the high speed of SPI and the phase robustness of Two-Photon Interference (TPI).
The researchers stated, “Our architecture supports autonomous quantum node operation without fiber channel phase stabilization,” emphasizing how well-suited it is for integration with the current telecom infrastructure. The system becomes resistant to the “noise” and phase drifts that afflict long-distance fiber lines by employing time-bin entanglement, in which information is stored in the photon arrival time.
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The XingHan 2.0 Hefei Experiment
XingHan 2.0 was the name of the experimental network, which included three nodes. Node B was situated in the Hefei National Laboratory (HFNL), 14.5 km away from Node A, which was situated at USTC. The measuring hub was a central Node C located at a China Unicom office, which was connected to the other nodes by 7.9 km and 9.9 km of installed commercial fiber.
The scientists employed rare-earth-ion doped crystals (151Eu³⁺: Y₂SiO₂) chilled to about 3 Kelvin to function as quantum memory at each repeater node. Quantum information may be stored in these crystals for extended periods of time. Entangled photon pairs were produced using a high-brightness source: a 1537 nm “telecom” photon that was sent to Node C for measurement and a 580 nm “signal” photon that was kept in the local crystal.
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Breaking Records in Speed and Quality
The Hefei trial produced both exceptional speed and unmatched quality.
Bell Nonlocality Certification: The group produced a Bell state with 78.6 ± 2.0% fidelity. With a measured value of S = 2.22 ± 0.06, they saw a 3.7 standard deviation violation of the CHSH-Bell inequality. This shows that the entanglement dispersed throughout the city was genuinely non-local as it exceeds the classical limit of 2.
High Speed Gain: The system stored 1,205 temporal modes concurrently by utilizing temporal multiplexing. This allowed for an Entanglement Distribution Rate (EDR) of 0.94 Hz, which is more than two orders of magnitude higher than previous urban demonstrations.
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A Practical Framework for the Future
This innovation is significant because it is scalable. The MQR-TM protocol avoids the need for active, real-time stabilization of underground fiber cables, which is a costly and technically challenging undertaking over long distances, because it is resilient against phase noise.
Additionally, even more ambitious applications are made possible by the team’s utilization of 151Eu³⁺:Y₂SiO₄ crystals. These particular crystals have demonstrated the ability to store coherent light for up to an hour. Using this approach, the researchers are presently constructing transportable quantum memory.
The authors concluded in the sources that “in the near future, the current architecture could enable heralded entanglement distribution involving transportable QMs.” This might enable a mobile quantum internet by creating a flexible network in which nodes are not restricted by fixed fiber cables.
The Hefei experiment offers the strongest proof to date that a metropolitan quantum backbone is not only a theoretical possibility but a workable reality as quantum technologies advance toward device-independent security and large-scale computer networks.
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