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Testing quantum correlations versus single-particle properties within Leggett’s model and beyond

Nature Physics volume 4, pages 681–685 (2008)Cite this article

Abstract

Quantum theory predicts and experiments confirm that nature can produce correlations between distant events that are non-local in the sense of violating a Bell inequality1. Nevertheless, Bell’s strong sentence ‘Correlations cry out for explanations’ (ref. 2) remains relevant. The maturing of quantum information science and the discovery of the power of non-local correlations, for example for cryptographic key distribution beyond the standard quantum key distribution schemes3,4,5, strengthen Bell’s wish and make it even more timely. In 2003, Leggett proposed an alternative model for non-local correlations6 that he proved to be incompatible with quantum predictions. We present here a new approach to this model, along with new inequalities for testing it. These inequalities can be derived in a very simple way, assuming only the non-negativity of probability distributions; they are also stronger than previously published and experimentally tested Leggett-type inequalities6,7,8,9. The simplest of the new inequalities is experimentally violated. Then we go beyond Leggett’s model, and show that we cannot ascribe even partially defined individual properties to the components of a maximally entangled pair.

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Figure 1: Alice’s (green) and Bob’s (red) measurement settings used to test inequality (4).
Figure 2: Experimental set-up.
Figure 3: Violation of Leggett’s model.
Figure 4: Experimental test of the generalized Leggett inequality for different degrees of purity η.

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Acknowledgements

We thank A. Ekert and C. Simon for comments. C.B., N.B. and N.G. acknowledge financial support from the EU project QAP (IST-FET FP6-015848) and Swiss NCCR Quantum Photonics. A.L.L. and C.K. acknowledge ASTAR for financial support under SERC grant No 052 101 0043. This work is partly supported by the National Research Foundation and Ministry of Education, Singapore.

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Authors and Affiliations

  1. Group of Applied Physics, University of Geneva, 20 Rue de l’Ecole de Médecine, CH-1211 Geneva 4, Switzerland

    Cyril Branciard, Nicolas Brunner & Nicolas Gisin

  2. Centre for Quantum Technologies/Physics Department, National University of Singapore, 117543, Singapore

    Christian Kurtsiefer, Antia Lamas-Linares, Alexander Ling & Valerio Scarani

Authors
  1. Cyril Branciard
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  2. Nicolas Brunner
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Correspondence to Cyril Branciard.

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Branciard, C., Brunner, N., Gisin, N. et al. Testing quantum correlations versus single-particle properties within Leggett’s model and beyond. Nature Phys 4, 681–685 (2008). https://doi.org/10.1038/nphys1020

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