Wigner's friend

A thought experiment placing a friend's physical measurement record inside a larger isolated quantum laboratory. Conditioned on a record the friend has a definite outcome, while an external observer using universal unitary evolution can assign an entangled laboratory state. Its contrast with a collapsed mixture is an instance of the quantum measurement problem; it does not require giving consciousness a special dynamical law. Eugene Wigner used the observer-within-the-system construction to examine such tensions.