Abstract
To make precise the sense in which nature fails to respect classical physics, one requires a formal notion of classicality. Ideally, such a notion should be defined operationally, so that it can be subject to direct experimental test, and it should be applicable in a wide variety of experimental scenarios so that it can cover the breadth of phenomena thought to defy classical understanding. Bell's notion of local causality fulfils the first criterion but not the second. The notion of noncontextuality fulfils the second criterion, but it is a long-standing question whether it can be made to fulfil the first. Previous attempts to test noncontextuality have all assumed idealizations that real experiments cannot achieve, namely noiseless measurements and exact operational equivalences. Here we show how to devise tests that are free of these idealizations. We perform a photonic implementation of one such test, ruling out noncontextual models with high confidence.
Highlights
To make precise the sense in which nature fails to respect classical physics, one requires a formal notion of classicality
The most successful attempt, due to Bell[1], shows a conflict between quantum theory and a feature of classical relativistic theories termed local causality, which asserts that no causal influences propagate faster than light
A leading candidate for a notion of nonclassicality with a broader scope is the failure of quantum theory to admit of a noncontextual model, as proven by Kochen and Specker[2]
Summary
To make precise the sense in which nature fails to respect classical physics, one requires a formal notion of classicality. The most successful attempt, due to Bell[1], shows a conflict between quantum theory and a feature of classical relativistic theories termed local causality, which asserts that no causal influences propagate faster than light. The latter assumption can only be tested for scenarios wherein there are two or more systems that are space-like separated. Previous work on testing noncontextuality—including the only experiment to have circumvented the problem of noisy measurements (by focusing on preparations)5—has failed to provide a satisfactory account of how the deviation from strict operational equivalence should be accounted for in the interpretation of the results.
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