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Received: 26 March 2019
Accepted: 12 November 2019
Published online: 12 February 2020
A quantum internet that connects remote quantum processors1,2 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 progress3-12, at present the maximal physical separation achieved between two nodes is 1.3 kilometres10, and challenges for longer distances remain. Here we demonstrate entanglement oftwo atomic ensembles in one laboratory via photon transmission through city-scale optical fibres. The atomic ensembles function as quantum memories that store quantum states. We use cavity enhancement to efficiently create atom-photon entanglement13-15 and we use quantum frequency conversion16 to shift the atomic wavelength to telecommunications wavelengths. We realize entanglement over 22 kilometres of field-deployed fibres via two-photon interference17,18 and entanglement over 50 kilometres of coiled fibres via single-photon interference19. Our experiment could be extended to nodes physically separated by similar distances, which would thus form a functional segment ofthe atomic quantum network, paving the way towards establishing atomic entanglement over many nodes and over much longer distances.
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Establishing remote entanglement is a central theme in quantum communication1,2,20. So far, entangled photons have been distributed over long distances both in optical fibres21 and in free space with the assistance of satellites22. In spite of this progress, the distribution succeeds only with an extremely low probability owing to severe transmission losses and because photons have to be detected to verify their survival after transmission. Therefore the distribution of entangled photons has not been scalable to longer distances or to multiple nodes20,23. A very promising solution is to prepare separate atom-photon entanglement in two remote nodes and to distribute the photons to a intermediate node for interference17,19. Proper measurement of the photons will project the atoms into a remote entangled state. Although the photons will still undergo transmission losses, the success of remote atomic entanglement will be heralded by the measurement of photons. Therefore, if the atomic states can be stored efficiently for a sufficiently long duration, multiple pairs of heralded atomic entanglement could be further connected efficiently to extend entanglement to longer distances or over multiple quantum nodes through entanglement swapping23, thus making quantum-internet-based applications feasible2,24,25
Towards this...