Abstract
Remarkable progress is being made in experiments that highlight the distinctive predictions of quantum mechanics. The Leggett-Garg inequality was devised to test for macrorealism (Leggett and Garg in Phys. Rev. Lett. 54:857–860, 1985). Various experiments have been performed, including one with non-invasive measurements in the kind of way that was originally envisaged, using spins in phosphorous impurities in silicon (Knee et al. in Nat. Commun. 3:606, 2012). This has led to fresh understanding of what kind of realism is excluded by the result. The quantum three-box paradox (Aharonov and Vaidman in J. Phys. A, Math. Gen. 24:2315–2328, 1991) provides a further test, which can be re-expressed in terms of the Leggett-Garg inequality. This has been experimentally implemented with projective measurements using an NV− centre in diamond, yielding results 7.8 standard deviations beyond a classical bound (George et al. in Proc. Natl. Acad. Sci. USA 110:3777–3781, 2013). [Editor’s note: for a video of the talk given by Prof. Briggs at the Aharonov-80 conference in 2012 at Chapman University, see quantum.chapman.edu/talk-18.]
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Acknowledgements
I gladly acknowledge my co-authors of [3] and also [16] for the experiments and results and their interpretation, and for many stimulating discussions about the further implications. Those papers should be taken as definitive in the event of any inadvertent discrepancy, though I take responsibility for additional views which go beyond the papers. I thank Richard George for helpful comments on the manuscript, and the John Templeton Foundation, together with the other agencies acknowledged in the papers, for funding the research.
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Briggs, G.A.D. (2014). Experimental Implementations of Quantum Paradoxes. In: Struppa, D., Tollaksen, J. (eds) Quantum Theory: A Two-Time Success Story. Springer, Milano. https://doi.org/10.1007/978-88-470-5217-8_24
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