Chinese scientists have transmitted quantum computer memory between two entangled clouds of atoms over 50 kilometers. They developed an efficient atom-photon entanglement source that is suitable for low-loss transmission in fibers. This means a quantum internet could span a large city network.
They entangled two clouds of a billion atoms then extracted a photon from each cloud.
A quantum internet that connects remote quantum processors should enable a number of revolutionary applications such as distributed quantum computing. Its realization will rely on entanglement of remote quantum memories over long distances. Despite enormous progress, at present the maximal physical separation achieved between two nodes is 1.3 kilometres, and challenges for longer distances remain. Here we demonstrate entanglement of two atomic ensembles in one laboratory via photon transmission through city-scale optical fibres. The atomic ensembles function as quantum memories that store quantum states. They used cavity enhancement to efficiently create atom–photon entanglement and we use quantum frequency conversion to shift the atomic wavelength to telecommunications wavelengths. They realized entanglement over 22 kilometers of field-deployed fibers via two-photon interference and entanglement over 50 kilometers of coiled fibers via single-photon interference. The experiment could be extended to nodes physically separated by similar distances, which would thus form a functional segment of the atomic quantum network, paving the way towards establishing atomic entanglement over many nodes and over much longer distances.