Experimental demonstration of non-bilocality with truly independent sources and strict locality constraints


The ongoing interest in creating a secure global quantum network culminated recently in the demonstration of transcontinental quantum communication1. There is a pressing need to examine the properties attached to a quantum network architecture from multiple perspectives, including physics foundations2, communication security3, the efficient use of resources and innovative technological applications4,5. Here, we present an experimental realization of a five-node quantum network, in which quantum sources at two nodes deliver entangled photon pairs to three measurement nodes. With relevant events between five nodes separated space-like, we demonstrate violation of the Bell inequality and bilocal inequality6, with the locality, measurement independence and quantum source independence loopholes closed simultaneously in a quantum network. This experimental realization may be valuable for the design and implementation of future quantum networks.

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Fig. 1: Space–time diagram of the simplest quantum network.
Fig. 2: Schematics for testing the bilocality in a network.
Fig. 3: Space–time configuration of relevant events in each experimental trial.
Fig. 4: \({\cal{B}}_{13}\) and \(\cal{S}\) versus noise parameter p.

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.


  1. 1.

    Liao, S.-K. et al. Satellite-relayed intercontinental quantum network. Phys. Rev. Lett. 120, 030501 (2018).

    ADS  Article  Google Scholar 

  2. 2.

    Brunner, N., Cavalcanti, D., Pironio, S., Scarani, V. & Wehner, S. Bell nonlocality. Rev. Mod. Phys. 86, 419–478 (2014).

    ADS  Article  Google Scholar 

  3. 3.

    Vazirani, U. & Vidick, T. Fully device-independent quantum key distribution. Phys. Rev. Lett. 113, 140501 (2014).

    ADS  Article  Google Scholar 

  4. 4.

    Colbeck, R. A. Quantum and Relativistic Protocols for Secure Multi-party Computation. PhD thesis, Univ. Cambridge (2007).

  5. 5.

    Lee, C. M. & Hoban, M. J. Towards device-independent information processing on general quantum networks. Phys. Rev. Lett. 120, 020504 (2018).

    ADS  MathSciNet  Article  Google Scholar 

  6. 6.

    Branciard, C., Gisin, N. & Pironio, S. Characterizing the nonlocal correlations created via entanglement swapping. Phys. Rev. Lett. 104, 170401 (2010).

    ADS  Article  Google Scholar 

  7. 7.

    Einstein, A., Podolsky, B. & Rosen, N. Can quantum-mechanical description of physical reality be considered complete? Phys. Rev. 47, 777–780 (1935).

    ADS  Article  Google Scholar 

  8. 8.

    Bell, J. S. On the Einstein–Podolsky–Rosen paradox. Physics 1, 195–200 (1964).

    MathSciNet  Article  Google Scholar 

  9. 9.

    Hensen, B. et al. Loophole-free Bell inequality violation using electron spins separated by 1.3 kilometres. Nature 526, 682–686 (2015).

    ADS  Article  Google Scholar 

  10. 10.

    Giustina, M. et al. Significant-loophole-free test of Bell’s theorem with entangled photons. Phys. Rev. Lett. 115, 250401 (2015).

    ADS  Article  Google Scholar 

  11. 11.

    Shalm, L. K. et al. Strong loophole-free test of local realism. Phys. Rev. Lett. 115, 250402 (2015).

    ADS  Article  Google Scholar 

  12. 12.

    Rosenfeld, W. et al. Event-ready Bell test using entangled atoms simultaneously closing detection and locality loopholes. Phys. Rev. Lett. 119, 010402 (2017).

    ADS  Article  Google Scholar 

  13. 13.

    Chaves, R., Kueng, R., Brask, J. B. & Gross, D. Unifying framework for relaxations of the causal assumptions in Bell’s theorem. Phys. Rev. Lett. 114, 140403 (2015).

    ADS  Article  Google Scholar 

  14. 14.

    Chaves, R. Polynomial Bell inequalities. Phys. Rev. Lett. 116, 010402 (2016).

    ADS  MathSciNet  Article  Google Scholar 

  15. 15.

    Rosset, D. et al. Nonlinear Bell inequalities tailored for quantum networks. Phys. Rev. Lett. 116, 010403 (2016).

    ADS  MathSciNet  Article  Google Scholar 

  16. 16.

    Fritz, T. Beyond Bell’s theorem II: scenarios with arbitrary causal structure. Commun. Math. Phys. 341, 391–434 (2016).

    ADS  MathSciNet  Article  Google Scholar 

  17. 17.

    Żukowski, M., Zeilinger, A., Horne, M. & Ekert, A. ‘Event-ready-detectors’ Bell experiment via entanglement swapping. Phys. Rev. Lett. 71, 4287–4290 (1993).

    ADS  Article  Google Scholar 

  18. 18.

    Aspect, A., Grangier, P. & Roger, G. Experimental tests of realistic local theories via Bell’s theorem. Phys. Rev. Lett. 47, 460–463 (1981).

    ADS  Article  Google Scholar 

  19. 19.

    Carvacho, G. et al. Experimental violation of local causality in a quantum network. Nat. Commun. 8, 14775 (2017).

    ADS  Article  Google Scholar 

  20. 20.

    Saunders, D. J., Bennet, A. J., Branciard, C. & Pryde, G. J. Experimental demonstration of nonbilocal quantum correlations. Sci. Adv. 3, e1602743 (2017).

    ADS  Article  Google Scholar 

  21. 21.

    Andreoli, F. et al. Experimental bilocality violation without shared reference frames. Phys. Rev. A 95, 062315 (2017).

    ADS  Article  Google Scholar 

  22. 22.

    Weinfurter, H. Experimental Bell-state analysis. Europhys. Lett. 25, 559–564 (1994).

    ADS  Article  Google Scholar 

  23. 23.

    Clauser, J. F., Horne, M. A., Shimony, A. & Holt, R. A. Proposed experiment to test local hidden-variable theories. Phys. Rev. Lett. 23, 880–884 (1969).

    ADS  Article  Google Scholar 

  24. 24.

    Sun, Q.-C. et al. Quantum teleportation with independent sources and prior entanglement distribution over a network. Nat. Photon. 10, 671–675 (2016).

    ADS  Article  Google Scholar 

  25. 25.

    Abellán, C., Amaya, W., Mitrani, D., Pruneri, V. & Mitchell, M. W. Generation of fresh and pure random numbers for loophole-free Bell tests. Phys. Rev. Lett. 115, 250403 (2015).

    ADS  Article  Google Scholar 

  26. 26.

    Hong, C., Ou, Z. & Mandel, L. Measurement of subpicosecond time intervals between two photons by interference. Phys. Rev. Lett. 59, 2044–2046 (1987).

    ADS  Article  Google Scholar 

  27. 27.

    Handsteiner, J. et al. Cosmic Bell test: measurement settings from Milky Way stars. Phys. Rev. Lett. 118, 060401 (2017).

    ADS  Article  Google Scholar 

  28. 28.

    Li, M.-H. et al. Test of local realism into the past without detection and locality loopholes. Phys. Rev. Lett. 121, 080404 (2018).

    ADS  Article  Google Scholar 

  29. 29.

    Tavakoli, A., Skrzypczyk, P., Cavalcanti, D. & Acín, A. Nonlocal correlations in the star-network configuration. Phys. Rev. A 90, 062109 (2014).

    ADS  Article  Google Scholar 

  30. 30.

    Andreoli, F., Carvacho, G., Santodonato, L., Chaves, R. & Sciarrino, F. Maximal qubit violation of n-locality inequalities in a star-shaped quantum network. New J. Phys. 19, 113020 (2017).

    ADS  Article  Google Scholar 

  31. 31.

    Gisin, N. The elegant joint quantum measurement and some conjectures about n-locality in the triangle and other configurations. Preprint at http://arxiv.org/abs/1708.05556 (2017).

  32. 32.

    Branciard, C., Rosset, D., Gisin, N. & Pironio, S. Bilocal versus nonbilocal correlations in entanglement-swapping experiments. Phys. Rev. A 85, 032119 (2012).

    ADS  Article  Google Scholar 

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We thank Y. Liu, Y. Li and Y.-Q. Nie for enlightening discussions, Y.-L. Mao for assistance, K.-X. Yang for help with the aerial photographs and Quantum Ctek for providing the components used in the QRNGs. This work was supported by the National Key R&D Program of China (2017YFA0303900, 2017YFA0304000), the National Natural Science Foundation of China and the Chinese Academy of Sciences.

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Q.-C.S., Q.Z., J.F. and J.-W.P. conceived and designed the experiments. B.B. and J.Z. built the QRNGs. W.Z., H.L., L.Y. and Z.W. fabricated the SNSPDs. Q.-C.S. and Y.-F.J. built the experimental network and carried out the experiment. X.J. and X.C. provided experimental assistance. Q.-C.S. and Y.-F.J. analysed the data. Q.-C.S., Q.Z., J.F. and J.-W.P. wrote the manuscript, with input from all authors.

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Correspondence to Qiang Zhang or Jingyun Fan or Jian-Wei Pan.

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Sun, QC., Jiang, YF., Bai, B. et al. Experimental demonstration of non-bilocality with truly independent sources and strict locality constraints. Nat. Photonics 13, 687–691 (2019). https://doi.org/10.1038/s41566-019-0502-7

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