John Francis Clauser is an American experimental and theoretical physicist. He is best known for his contributions to the foundations of quantum mechanics, in particular for the Clauser-Horne-Shimony-Holt (CHSH) inequality, for the first experimental proof that non-local quantum entanglement is real (Freedman-Clauser), and for the formulation of the theory of Local Realism (Clauser-Horne). Together with the 1969 CHSH prediction, the 1972 Freedman-Clauser experiment is deemed sufficiently important that it has now been repeated by others, literally hundreds of times in a wide variety of configurations, at many laboratories around the world. It is now commonly performed by students in undergraduate laboratories as a part of a standard physics curriculum. It is also widely considered to be for quantum mechanics, what the Michelson-Morley experiment is for special relativity.