In Bell test experiments, there may be problems of experimental design or set-up that affect the validity of the experimental findings. These problems are often referred to as "loopholes". See the article on Bell's theorem for the theoretical background to these experimental efforts (see also J. S. Bell). The purpose of the experiment is to test whether nature is best described using a local hidden variable theory or by the quantum entanglement theory of quantum mechanics.
The measurement results (corresponding to quantum mechanic predictions) also follow classical Malus' law and the additional assumption, that "classical" interpretation needs polarizations to be only in perpendicular directions, is dubious.
The "detection efficiency", or "fair sampling" problem is the most prevalent loophole, and affects all experiments performed to date except one. Another loophole that has more often been addressed is that of communication, i.e. locality. To date, no test has simultaneously closed all loopholes.
In some experiments there may be additional defects that make "local realist" explanations of Bell test violations possible; these are briefly described below.
Many modern experiments are directed at detecting quantum entanglement rather than ruling out local hidden variable theories, and these tasks are different since the former accepts quantum mechanics at the outset (no entanglement without quantum mechanics). This is regularly done using Bell's theorem, but in this situation the theorem is used as an entanglement witness, a dividing line between entangled quantum states and separable quantum states, and is as such not as sensitive to the problems described here.
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