China pushes quantum memory entanglement 4x farther, reaching 260 miles over fiber

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. 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.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. Quantum memory fix Quantum signals lose strength as they move through optical fiber. That problem becomes increasingly severe as engineers extend the distance between network nodes. 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. 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. Direct transmission becomes increasingly impractical beyond that point. Quantum memories could offer a way around the limitation by breaking longer networks into smaller sections. Keeping those memories synchronized presented another challenge. Temperature changes and mechanical vibrations can interfere with the quantum effects needed to establish entanglement. 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. Researchers also modified the operating wavelength of the quantum memories. The change reduced signal losses during transmission and helped preserve the fragile quantum connection. Lab links to networks 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. The new distance pushes that approach into territory more relevant to wide-area networks. It also demonstrates that quantum memories can maintain entanglement despite the losses and environmental disturbances found in long fiber connections. 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. 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. Pan’s 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. That system used individual rubidium atoms trapped at separate network nodes. The researchers reported that work in Science. The latest experiment does not amount to a functional long-distance quantum internet. It does show that quantum memories can maintain entanglement across distances that pose serious limits for direct fiber transmission. The study was published August 11 in Physical Review Letters. Get the latest in engineering, tech, space & science - delivered daily to your inbox.Aamir is a seasoned tech journalist with experience at Exhibit Magazine, Republic World, and PR Newswire. With a deep love for all things tech and science, he has spent years decoding the latest innovations and exploring how they shape industries, lifestyles, and the future of humanity.