Erwin Schrödinger coined the word entanglement in 1935 in a three-part paper1 on the “present situation in quantum mechanics.” His article was prompted by Albert Einstein, Boris Podolsky, and Nathan Rosen’s now celebrated EPR paper that had raised fundamental questions about quantum mechanics earlier that year.
Einstein and his coauthors had recognized that quantum theory allows very particular correlations to exist between two physically distant parts of a quantum system; those correlations make it possible to predict the result of a measurement on one part of a system by looking at the distant part. On that basis, the EPR paper argued that the distant predicted quantity should have a definite value even before being measured if the theory were to claim completeness and respect locality. However, because quantum mechanics disallows such definite values prior to measuring, the EPR authors concluded that, from a classical perspective, quantum theory must be...
