Quantum entanglement

Quantum entanglement, also called the quantum non-local connection (and also spooky-action-at-a-distance), is a property of certain states of a quantum system containing two or more distinct objects, in which the information describing the objects is inextricably linked such that performing a measurement on one immediately alters properties of the other, even when separated at arbitrary distances. Specifically, such a system is said to be in an entangled state if it cannot be written as the tensor product of its constituent subsystems. This discovery posed a serious conceptual challenge to physicists of the day, because faster-than-light influences were assumed to be prohibited by special relativity. In that framework, it was thought superluminal effects would lead to causal contradictions because a change of reference frame can reverse the order of the events. It is now understood that while these nonlocal correlations do occur, they cannot be used to transmit information and thus do not violate causality. Entanglement, which was a recognized consequence of the mathematical formulation of quantum mechanics during its inception, eventually stimulated the discovery of the EPR paradox by Albert Einstein, Boris Podolsky and Nathan Rosen in 1935. This thought experiment, while drawing an incorrect conclusion, helped elucidate the relationship between relativity and quantum theory, and set the stage for John S. Bell to finally resolve the contradiction in 1964 through Bell's theorem. Entanglement has since been verified by a series of experiments beginning in 1972 by Stuart Freedman and John Clauser.