Entanglement of two quantum memories via fibres over dozens of kilometres

Abstract

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 progress3,4,5,6,7,8,9,10,11,12, at present the maximal physical separation achieved between two nodes is 1.3 kilometres10, and challenges for longer distances remain. Here we demonstrate entanglement 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. We use cavity enhancement to efficiently create atom–photon entanglement13,14,15 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 of the atomic quantum network, paving the way towards establishing atomic entanglement over many nodes and over much longer distances.

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Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

This work was supported by the National Key R&D Program of China (2017YFA0303902, 2018YFB0504300, 2017YFA0304000), the Anhui Initiative in Quantum Information Technologies, the National Natural Science Foundation of China and the Chinese Academy of Sciences. We thank QuantumCTek for providing the field-deployed fibres.

Author information

X.-H.B. and J.-W.P. conceived the research. Q.Z., X.-H.B. and J.-W.P. designed the experiment. Y.Y., X.-Y.L., B.J., P.-F.S., R.-Z.F., C.-W.Y. and X.-H.B. carried out the experiment with assistance from all other authors. F.M., M.-Y.Z., X.-P.X. and Q.Z. built the QFC module. W.-J.Z., L.-X.Y. and Z.W. fabricated the superconducting nanowire single-photon detectors. Y.Y., Q.Z., X.-H.B. and J.-W.P. analysed the data and wrote the paper with input from all other authors.

Correspondence to Qiang Zhang or Xiao-Hui Bao or Jian-Wei Pan.

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Supplementary information

Supplementary Information

This file contains the following sections: I. General information of experimental setups; II. Phase stabilization; III. Lasers in outdoor application; IV. Analysis on experimental imperfections; V. Entanglement evaluation of Fock state entanglement; and additional references.

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Yu, Y., Ma, F., Luo, X. et al. Entanglement of two quantum memories via fibres over dozens of kilometres. Nature 578, 240–245 (2020). https://doi.org/10.1038/s41586-020-1976-7