Quantum entanglement

Summary

Quantum entanglement is the property of a state of a composite quantum system that cannot be described by assigning a state to each part separately. Formally, a state on a tensor product of Hilbert spaces is entangled when it is not a separable state. Entangled states produce correlations that no local hidden-variable model reproduces, as Bell's theorem shows. Entanglement is the main resource of quantum information, used in teleportation, dense coding and quantum key distribution.