An experimental test of non-local realism (0704.2529v2)

Abstract: 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.

• The paper demonstrates that non-local hidden-variable models fail to predict observed quantum correlations, as evidenced by the violation of Leggett-type inequalities.
• The experimental design uses polarization-entangled photon pairs in two-dimensional systems to directly compare quantum predictions with non-local realism.
• The findings imply that any future theoretical framework must reconsider the classical assumption of realism to accurately capture quantum phenomena.

An Experimental Test of Non-Local Realism

The paper "An experimental test of non-local realism" conducted by researchers from various renowned institutions tests a class of non-local hidden-variable theories. It rigorously scrutinizes the assumption that external reality is independent of the act of observation—a concept fundamental to realism—while also incorporating non-local influences. This work is premised on the incompatibility theorem introduced by Leggett, which challenges the traditional Eurocentric perspective epitomized by Bell's theorem. This paper extends Leggett's framework, both theoretically and experimentally, to elucidate the contradiction between certain non-local realistic models and observable quantum correlations.

Core Concepts and Experimental Approach

This exploration into non-local realism confronts Bell’s theorem, which asserts that all local hidden-variable theories based on realism and locality conflict with quantum predictions. In the reviewed experiment, quantum predictions for correlations are juxtaposed against a class of non-local hidden-variable models posited by Leggett. The authors derive new inequalities that hold for the models under investigation, thereby providing a litmus test for these alternative theories in actual experimental scenarios.

Central to the test are the experiments' two-dimensional quantum systems, specifically the polarization degree of photons. The researchers generate polarization-entangled photon pairs to evaluate these systems. The adopted experimental method closes previous empirical gaps related to locality and detection while addressing theories that allow for non-local dependencies by modifying measurement setups to probe correlations beyond the local realistic hypothesis.

Findings and Statistical Indications

The experimental results provide a critical examination of non-local realism. The observed data violates the derived Leggett-type inequality, highlighting the discrepancies between non-local hidden-variable models and quantum mechanical predictions. For instance, the maximum violation occurs at a specific difference angle, demonstrating quantum theory's robustness despite the surveyed class of models.

The confidence in experimental accuracy is bolstered by exceptional visibility in polarization entangled photon counts, with reported visibility rates exceeding the critical threshold necessary to challenge inequality (9). The reported values for different polarization bases and maximal violation attest to the experiments' precision and effectively exclude the class of non-local hidden-variable theories under review from explaining quantum correlations.

Implications and Future Directions

The implications of these findings are significant: they validate the continued fidelity of quantum mechanics in predicting correlations that non-local realist theories fail to accommodate. This strengthens the stance that any forthcoming extensions or modifications to quantum theory must radically reevaluate classical assumptions about realism and potentially surrender intuitively appealing features like definite polarization.

Moreover, this work broadens the investigation's spectrum, probing new theoretical territories. The results suggest that feasible models deviating from realism might involve concepts such as non-classical dimensions or communication, as hinted at by quantum entanglement explanations in Bohmian mechanics. These alternate frameworks may serve as fertile ground for developing a more comprehensive understanding of the nature of reality in terms of quantum interactions.

Conclusion

In dissecting certain non-local realist approaches, this paper underscores the complex interplay between classical physics principals and quantum phenomena. The rigorous empirical constraint of non-local hidden-variable theories reaffirms quantum mechanics' salient predictive power and suggests that any successor theories must embrace non-intuitive, perhaps paradoxical, mechanisms to faithfully reinterpret the quantum world. As the scientific endeavor persists, these findings will undoubtedly inform and inspire forthcoming research into the foundational aspects of physical realism, quantum mechanics, and hidden-variable theories.