Article | Published:

An experimental test of non-local realism

Nature volume 446, pages 871875 (19 April 2007) | Download Citation


  • A Corrigendum to this article was published on 13 September 2007


Most working scientists hold fast to the concept of ‘realism’—a viewpoint according to which an external reality exists independent of observation. But quantum physics has shattered some of our cornerstone beliefs. According to Bell’s theorem, any theory that is based on the joint assumption of realism and locality (meaning that local events cannot be affected by actions in space-like separated regions) is at variance with certain quantum predictions. Experiments with entangled pairs of particles have amply confirmed these quantum predictions, thus rendering local realistic theories untenable. Maintaining realism as a fundamental concept would therefore necessitate the introduction of ‘spooky’ actions that defy locality. Here we show by both theory and experiment that a broad and rather reasonable class of such non-local realistic theories is incompatible with experimentally observable quantum correlations. In the experiment, we measure previously untested correlations between two entangled photons, and show that these correlations violate an inequality proposed by Leggett for non-local realistic theories. Our result suggests that giving up the concept of locality is not sufficient to be consistent with quantum experiments, unless certain intuitive features of realism are abandoned.

Access optionsAccess options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.


  1. 1.

    Die gegenwärtige Situation in der Quantenmechanik. Naturwissenschaften 23, 807–812; 823–828. 844–849 (1935)

  2. 2.

    , , & Proposed experiment to test local hidden-variable theories. Phys. Rev. Lett. 23, 880–884 (1969)

  3. 3.

    Nonlocal hidden-variable theories and quantum mechanics: An incompatibility theorem. Found. Phys. 33, 1469–1493 (2003)

  4. 4.

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

  5. 5.

    to Erwin Schrödinger, 19 June 1935 (Albert Einstein Archives, Jewish National and University Library, The Hebrew University of Jerusalem).

  6. 6.

    in Albert Einstein: Philosopher-Scientist Vol. 7 (ed. Schilpp, P. A.) 201–241 (Library of Living Philosophers, Evanston, 1949)

  7. 7.

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

  8. 8.

    & Experimental consequences of objective local theories. Phys. Rev. D 10, 526–535 (1974)

  9. 9.

    in Quantum [Un]speakables: From Bell to Quantum Information (eds Bertlmann, R. A. & Zeilinger, A.) 61–98 (Springer, Heidelberg, 2002)

  10. 10.

    , & in Bell’s Theorem, Quantum Theory, and Conceptions of the Universe (ed. Kafatos, M.) 69–72 (Kluwer, Dordrecht, 1989)

  11. 11.

    , , & Bell’s theorem without inequalities. Am. J. Phys. 58, 1131–1143 (1990)

  12. 12.

    & Experimental test of local hidden-variable theories. Phys. Rev. Lett. 28, 938–941 (1972)

  13. 13.

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

  14. 14.

    , & Experimental test of Bell’s inequalities using time-varying analyzers. Phys. Rev. Lett. 49, 1804–1807 (1982)

  15. 15.

    , , , & Violation of Bell’s inequality under strict Einstein locality conditions. Phys. Rev. Lett. 81, 5039–5043 (1998)

  16. 16.

    et al. Experimental violation of a Bell’s inequality with efficient detection. Nature 409, 791–794 (2001)

  17. 17.

    , , , & Experimental test of quantum nonlocality in three-photon Greenberger-Horne-Zeilinger entanglement. Nature 403, 515–519 (2000)

  18. 18.

    Testing Bell’s inequalities with periodic switching. Phys. Lett. A 118, 1–2 (1986)

  19. 19.

    Bell’s inequality test: more ideal than ever. Nature 398, 189–190 (1999)

  20. 20.

    Count them all. Nature 409, 774–775 (2001)

  21. 21.

    & Principles of Optics: Electromagnetic Theory of Propagation, Interference and Diffraction of Light 2nd edn (Pergamon, Oxford, 1964)

  22. 22.

    & Bell inequalities with auxiliary communication. Phys. Rev. Lett. 90, 157904 (2003)

  23. 23.

    The problems in quantum foundations in the light of gauge theories. Found. Phys. 16, 361–377 (1986)

  24. 24.

    A suggested interpretation of the quantum theory in terms of “hidden” variables. I and II. Phys. Rev. 85, 166–193 (1952)

  25. 25.

    , & A causal account of non-local Einstein-Podolsky-Rosen spin correlations. J. Phys. Math. Gen. 20, 4717–4732 (1987)

  26. 26.

    & The angular correlation of scattered annihilation radiation. Phys. Rev. 77, 136 (1950)

  27. 27.

    & Polarization correlation of photons emitted in an atomic cascade. Phys. Rev. Lett. 18, 575–577 (1967)

  28. 28.

    & Bell’s theorem. Experimental tests and implications. Rep. Prog. Phys. 41, 1881–1927 (1978)

  29. 29.

    Free variables and local causality. Dialectica 39, 103–106 (1985)

Download references


We are grateful to A. J. Leggett for stimulating this work and for discussions. We also thank D. Greenberger, M. A. Horne, T. Jennewein, J. Kofler, S. Malin and A. Shimony for their comments. T.P. is grateful for the hospitality of the IQOQI, Vienna. M.A. thanks L. Gohlike for his hospitality at the Seven Pines VIII. We acknowledge support from the Austrian Science Fund (FWF), the European Commission, the Austrian Exchange Service (ÖAD), the Foundation for Polish Science (FNP), the Polish Ministry of Higher Education and Science, the City of Vienna and the Foundational Questions Institute (FQXi).

Author information


  1. Faculty of Physics, University of Vienna, Boltzmanngasse 5, A-1090 Vienna, Austria

    • Simon Gröblacher
    • , Rainer Kaltenbaek
    • , Časlav Brukner
    • , Marek Żukowski
    • , Markus Aspelmeyer
    •  & Anton Zeilinger
  2. Institute for Quantum Optics and Quantum Information (IQOQI), Austrian Academy of Sciences, Boltzmanngasse 3, A-1090 Vienna, Austria

    • Simon Gröblacher
    • , Časlav Brukner
    • , Markus Aspelmeyer
    •  & Anton Zeilinger
  3. Institute of Theoretical Physics and Astrophysics, University of Gdansk, ul. Wita Stwosza 57, PL-08-952 Gdansk, Poland

    • Tomasz Paterek
    •  & Marek Żukowski
  4. The Erwin Schrödinger International Institute for Mathematical Physics (ESI), Boltzmanngasse 9, A-1090 Vienna, Austria

    • Tomasz Paterek


  1. Search for Simon Gröblacher in:

  2. Search for Tomasz Paterek in:

  3. Search for Rainer Kaltenbaek in:

  4. Search for Časlav Brukner in:

  5. Search for Marek Żukowski in:

  6. Search for Markus Aspelmeyer in:

  7. Search for Anton Zeilinger in:

Competing interests

Reprints and permissions information is available at The authors declare no competing financial interests.

Corresponding authors

Correspondence to Markus Aspelmeyer or Anton Zeilinger.

Supplementary information

PDF files

  1. 1.

    Supplementary Equations

    This file contains Supplementary Equations. The authors give an explicit example of a nonlocal hidden-variable model that fulfils all assumptions of the considered class. It is shown that the model can explain existing Bell experiments. In the second part the authors provide a detailed derivation of the inequality for the class of theories.

About this article

Publication history





Further reading


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.