Proof that nonlocal quantum entanglement is real

Experiments and theory now prove that nonlocal quantum entanglement is real. Einstein, Podolsky, and Rosen (EPR, 1935) proposed Local Hidden Variables as a completion of quantum mechanics for entangled-state systems. John Bell (1964) ironically showed that EPR’s hidden variables are incompatible with the predictions of quantum mechanics. Inspired by his mathematical result, Clauser, Horne, Shimony, and Holt (CHSH, 1969) designed an experiment employing entangled photon pairs. Assuming it is governed by a general Local Hidden Variable Theory, they derived the CHSH Bell inequality as an experimental prediction for it. Assuming it is governed by quantum mechanics gives a different prediction. The two theories are thus experimentally distinguishable. The first experimental test was by Freedman-Clauser (1972). It measured the polarization correlation of entangled photon pairs produced by an atomic cascade in Calcium. The second experimental test [Clauser (1976)] used photon pairs produced by Mercury. These two experiments were the first observations of a violation of the CHSH inequality, and disproved general Local Hidden Variable Theories.

Clauser and Horne (1974) formulated Local Realism as a significant generalization of Local Hidden Variable Theories, and derived its experimental prediction, the Clauser-Horne (CH) Bell inequality. Unlike CHSH, it requires no auxiliary assumptions. It is, however, much more difficult to test than is the CHSH inequality. Observations of its violation have occurred in 2014 and 2017. In addition to closing loopholes left by CHSH inequality tests, Local Realism and the CH inequality have fostered experimental tests that undermine and refute the minimal elements of Einstein’s theoretical platform for providing a mathematical description of nature in a space-time framework.