Loopholes in Bell inequality tests of local realism

Abstract

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