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Experimental demonstration of a BDCZ quantum repeater node

Abstract

Quantum communication is a method that offers efficient and secure ways for the exchange of information in a network. Large-scale quantum communication1,2,3,4 (of the order of 100 km) has been achieved; however, serious problems occur beyond this distance scale, mainly due to inevitable photon loss in the transmission channel. Quantum communication eventually fails5 when the probability of a dark count in the photon detectors becomes comparable to the probability that a photon is correctly detected. To overcome this problem, Briegel, Dür, Cirac and Zoller (BDCZ) introduced the concept of quantum repeaters6, combining entanglement swapping7 and quantum memory to efficiently extend the achievable distances. Although entanglement swapping has been experimentally demonstrated8, the implementation of BDCZ quantum repeaters has proved challenging owing to the difficulty of integrating a quantum memory. Here we realize entanglement swapping with storage and retrieval of light, a building block of the BDCZ quantum repeater. We follow a scheme9,10 that incorporates the strategy of BDCZ with atomic quantum memories11. Two atomic ensembles, each originally entangled with a single emitted photon, are projected into an entangled state by performing a joint Bell state measurement on the two single photons after they have passed through a 300-m fibre-based communication channel. The entanglement is stored in the atomic ensembles and later verified by converting the atomic excitations into photons. Our method is intrinsically phase insensitive and establishes the essential element needed to realize quantum repeaters with stationary atomic qubits as quantum memories and flying photonic qubits as quantum messengers.

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Figure 1: The experimental scheme for entanglement swapping.
Figure 2: Correlation functions of a CHSH-type Bell’s inequality with a storage time δ t s = 500 ns.
Figure 3: Visibility of the atom–atom entanglement as a function of the storage time with 6-m fibre connection.
Figure 4: Polarization analysis of photons 1 and 4 when the connection channel is a 300-m fibre.

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Acknowledgements

We thank W. Dür for discussions. This work was supported by the Deutsche Forschungsgemeinschaft, the Alexander von Humboldt Foundation, and the European Commission through the Marie Curie Excellence Grant and the ERC Grant. This work was also supported by the National Fundamental Research Program (grant 2006CB921900), the CAS and the NNSFC.

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Correspondence to Yu-Ao Chen or Jian-Wei Pan.

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This file contains Supplementary Figures and Legends 1-3, Supplementary Methods, Supplementary Discussion, and Supplementary Notes. The Supplementary Methods and Discussion show that the quality of atom-photon entanglement, the method of phase stability, the estimation of the precision of local operations and how to realize long-distance quantum communication. (PDF 411 kb)

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Yuan, ZS., Chen, YA., Zhao, B. et al. Experimental demonstration of a BDCZ quantum repeater node. Nature 454, 1098–1101 (2008). https://doi.org/10.1038/nature07241

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