Heralded quantum entanglement between two crystals


Quantum networks must have the crucial ability to entangle quantum nodes1. A prominent example is the quantum repeater2,3,4, which allows the distance barrier of direct transmission of single photons to be overcome, provided remote quantum memories can be entangled in a heralded fashion. Here, we report the observation of heralded entanglement between two ensembles of rare-earth ions doped into separate crystals. A heralded single photon is sent through a 50/50 beamsplitter, creating a single-photon entangled state delocalized between two spatial modes. The quantum state of each mode is subsequently mapped onto a crystal, leading to an entangled state consisting of a single collective excitation delocalized between two crystals. This entanglement is revealed by mapping it back to optical modes and by estimating the concurrence of the retrieved light state5. Our results highlight the potential of crystals doped with rare-earth ions for entangled quantum nodes and bring quantum networks based on solid-state resources one step closer.

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Figure 1: Experimental set-up.
Figure 2: Results.


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The authors thank H. de Riedmatten, P. Sekatski and J. Laurat for stimulating discussions, and A. Korneev for help with the superconducting detector. This work was supported by the Swiss National Centres of Competence in Research (NCCR) project ‘Quantum Science Technology (QSIT)’, the Science and Technology Cooperation Program Switzerland–Russia, the European Union FP7 project 247743 ‘Quantum repeaters for long distance fibre-based quantum communication (QUREP)’ and the European Research Council Advanced Grant ‘Quantum correlations (QORE)’. F.B. was supported in part by le Fond Québécois de la Recherche sur la Nature et les Technologies.

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All authors conceived the experiment. I.U., C.C. and F.B. performed the measurements. I.U., C.C., F.B., N.S. and M.A. analysed the data. All authors contributed to the writing of the manuscript. I.U., C.C. and F.B. contributed equally to this work.

Corresponding author

Correspondence to Mikael Afzelius.

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The authors declare no competing financial interests.

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Usmani, I., Clausen, C., Bussières, F. et al. Heralded quantum entanglement between two crystals. Nature Photon 6, 234–237 (2012). https://doi.org/10.1038/nphoton.2012.34

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