Physicists have achieved a new distance milestone for quantum entanglement, sending entangled states across 420 kilometers of optical fiber. The result highlights the growing challenge of preserving fragile quantum connections over long distances.

The experiment involved quantum memories, which must retain the relationship between entangled states while signals travel through the fiber. As the distance increases, maintaining that connection becomes increasingly difficult.

The milestone is relevant to the development of long-distance quantum communication and future quantum networks. Extending reliable entanglement over fiber could help researchers explore ways to connect quantum systems across much larger distances.