Quantum physics: Quantum memories go the distance

The entanglement of two quantum memories over a distance of 50 kilometres is demonstrated in research published in Nature. This distance is much greater than previously reported and may pave the way for long-distance entanglement over multiple nodes, which could aid the development of a quantum internet.

Quantum communication requires the transmission of entangled particles. In the past two decades there has been notable progress in achieving remote entanglement, in which entangled photons are transmitted between nodes along optical fibres or via satellite; however, severe transmission losses limit the success of photon distribution. Moreover, the entanglement of quantum memories — the quantum equivalent of ordinary computer memory — has been limited to a distance of 1.3 kilometres, suggesting that existing systems may not be scalable.

Jian-Wei Pan, Xiao-Hui Bao and colleagues exploit a quantum effect called cavity enhancement — which reduces photon coupling loss during transmission — to create entangled atoms and photons, which they then convert to a frequency that is suitable for telecommunications. They report entanglement between two nodes connected by up to 50 kilometres of optical cables, distances that could connect cities. These results demonstrate that, compared with entangled photons, atom–photon entanglement over a series of nodes may be better suited to the long-distance transmission of quantum entanglement.