USTC Quantum Memories Reach 420km Fibre Entanglement
USTC researchers entangled two quantum memories across a 420km optical-fibre link, using rubidium atom memories, telecom-compatible wavelengths and stabilisation to move quantum-network research beyond metropolitan-scale demonstrations.
DigiconAsia reported that University of Science and Technology of China researchers entangled two quantum memories across a 420km optical-fibre link, extending matter-to-matter entanglement beyond earlier fibre experiments.
The result is a laboratory milestone for quantum networking because stationary memories, rather than only travelling photons, must be linked reliably before long-distance quantum repeaters can become practical.
Rubidium Memories Linked Across 420km
The study was led by Xi-Yu Luo, Chao-Yang Wang and Ming-Yang Zheng.
The experiment used laser-cooled ensembles of rubidium atoms as quantum memories at two endpoints referred to as Alice and Bob.
A midpoint station called Charlie verified the entanglement.
Photons emitted from both memory nodes travelled towards that station, where their interference pattern was measured; the expected pattern confirmed that the distant memories had become entangled.
Telecom Wavelengths Reduced Fibre Loss
Several technical steps made the long link possible.
The team converted photons emitted by the memories into wavelengths compatible with telecommunications infrastructure, lowering signal loss over long distances.
The researchers also used stabilisation that continuously compensated for environmental disturbances, including temperature variation and mechanical vibration.
Those effects can disrupt fragile quantum states before a useful link is established.
A single-photon entanglement protocol further improved efficiency because only one photon had to traverse the fibre link successfully, rather than two.
That design helped the system operate across a distance more than four times longer than earlier fibre-based matter-to-matter entanglement work cited by the source.
320km Threshold Shows Repeater Value
The experiment also surpassed the Pirandola-Laurenza-Ottaviani-Banchi bound, a 2017 theoretical limit for transmitting quantum information through a lossy channel without repeaters.
Beyond 320km, the system generated entanglement at a rate higher than direct optical transmission alone could achieve.
Earlier experiments had demonstrated about 50km in 2020 and roughly 20km in urban environments in 2023.
The 420km result moves the research from metropolitan demonstrations towards intercity quantum-network tests.
The source did not list commercial deployment dates, field-trial operators, hardware cost, uptime results, security certification or a plan for linking multiple memory nodes into a production quantum network.
