Loopholes in Bell Inequality Tests of Local Realism

TL;DR

This paper reviews the various loopholes in Bell inequality experiments that challenge the validation of local realism, emphasizing the importance of loophole-free tests for quantum foundations and cryptography.

Contribution

It provides a comprehensive review of known loopholes in Bell tests and discusses strategies to perform loophole-free experiments, advancing the understanding of quantum nonlocality.

Findings

Identification of key loopholes affecting Bell tests

Analysis of how loopholes impact interpretation of experimental results

Discussion on the importance of loophole-free Bell tests for quantum security

Abstract

Bell inequalities are intended to show that local realist theories cannot describe the world. A local realist theory is one where physical properties are defined prior to and independent of measurement, and no physical influence can propagate faster than the speed of light. Quantum-mechanical predictions for certain experiments violate the Bell inequality while a local realist theory cannot, and this shows that a local realist theory cannot give those quantum-mechanical predictions. However, because of unexpected circumstances or "loopholes" in available experiment tests, local realist theories can reproduce the data from these experiments. This paper reviews such loopholes, what effect they have on Bell inequality tests, and how to avoid them in experiment. Avoiding all these simultaneously in one experiment, usually called a "loophole-free" or "definitive" Bell test, remains an open task, but is very important for technological tasks such as device-independent security of quantum cryptography, and ultimately for our understanding of the world.