Quantum networks could eventually connect computers and sensors across cities, but distance remains a major engineering hurdle. Quantum signals weaken rapidly inside optical fiber, making long connections difficult to maintain.<\/p>\n
Researchers in China have now demonstrated a way to keep quantum systems linked across a much longer fiber route. The setup used two quantum memory systems containing laser-cooled rubidium atoms.<\/p>\n
The result could give engineers a useful platform for developing quantum networks beyond metropolitan areas. It also addresses a key limitation that has restricted direct quantum transmission over long fiber connections.<\/p>\n
Quantum memory fix<\/p>\n
Quantum signals lose strength as they move through optical fiber. That problem becomes increasingly severe as engineers extend the distance between network nodes.<\/p>\n
The researchers used clouds of rubidium atoms as quantum memories. These systems can store quantum states instead of forcing information to travel continuously across the entire connection.<\/p>\n
Their experiment maintained entanglement across 260 miles (420 kilometers) of fiber. That result exceeds the roughly 199-mile (320-kilometer) threshold associated with direct transmission through a lossy fiber channel.<\/p>\n
Direct transmission becomes increasingly impractical beyond that point. Quantum memories could offer a way around the limitation by breaking longer networks into smaller sections.<\/p>\n
Keeping those memories synchronized presented another challenge. Temperature changes and mechanical vibrations can interfere with the quantum effects needed to establish entanglement.<\/p>\n
The team developed an active stabilization system to compensate for those fluctuations. It continuously adjusted the setup to keep the quantum states aligned across the fiber link.<\/p>\n
Researchers also modified the operating wavelength of the quantum memories. The change reduced signal losses during transmission and helped preserve the fragile quantum connection.<\/p>\n
Lab links to networks<\/p>\n
The experiment extends several years of work by Pan Jianwei and his colleagues at the University of Science and Technology of China. The group demonstrated quantum-memory entanglement across 31 miles (50 kilometers) of fiber in 2020. In 2024, researchers established a three-node quantum network.<\/p>\n
The new distance pushes that approach into territory more relevant to wide-area networks. It also demonstrates that quantum<\/a> memories can maintain entanglement despite the losses and environmental disturbances found in long fiber connections.<\/p>\n Quantum entanglement creates a shared quantum state between separated systems. Scientists view that capability as a foundation for future quantum communications, computing, and networked sensing.<\/p>\n A larger quantum network could eventually connect quantum processors across cities. Such systems could also allow distributed sensors to coordinate measurements across much larger areas.<\/p>\n Pan\u2019s team has made progress in quantum-secure communications as well. In February, another group led by Pan transmitted secure information across more than 62 miles (100 kilometers) of optical fiber.<\/p>\n That system used individual rubidium atoms trapped at separate network nodes. The researchers reported that work in Science.<\/p>\n The latest experiment does not amount to a functional long-distance quantum internet. It does show that quantum memories can maintain entanglement<\/a> across distances that pose serious limits for direct fiber transmission.<\/p>\n