Physicists have set a new record for quantum communication by successfully entangling quantum memories over 420 kilometers of optical fiber. This achievement, led by researchers at the University of Science and Technology of China, marks a major step toward practical quantum networks. The distance covered is more than four times greater than previous demonstrations and moves beyond the limits of standard direct photon transmission.
Quantum entanglement involves linking the properties of particles so that measuring one reveals information about the other, regardless of the distance between them. While many previous experiments relied on photons traveling through fiber, this research utilized quantum memories consisting of laser-cooled rubidium atoms. These storage units, nicknamed Alice and Bob, were placed at either end of the fiber link to maintain the connection.
The team overcame significant technical obstacles to reach this distance. First, they converted the light emitted by the atomic memories into telecommunications-compatible wavelengths to reduce signal loss. Second, they implemented a stabilization system to counter environmental noise, such as temperature fluctuations and vibrations, which often disrupt quantum interference. Finally, they used a single-photon scheme that increased the efficiency of the connection.
Most importantly, this experiment confirms that memory-based systems can exceed the performance limits defined by the PLOB bound. Beyond 320 kilometers, the researchers achieved higher entanglement rates than possible with direct transmission alone. This result provides a clear path for constructing quantum repeaters and suggests that large-scale quantum internet architecture is becoming more practical.