The quantum measurement problem asks how a unitary superposition of system and apparatus outcomes gives rise to one definite observed outcome. Quantum decoherence explains the suppression of interference between outcome branches, but by itself does not select one branch.
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.
A thought experiment correlating a microscopic quantum alternative with macroscopically distinct records, introduced by Erwin Schrödinger to sharpen the problem of applying unitary evolution to a measuring apparatus. The resulting entangled superposition and the mixture produced by selecting an outcome are different joint density operators. Quantum decoherence can make the records' reduced density operator approximately diagonal without selecting a single global component.
The problem of explaining which basis represents stable measurement records. A maximally entangled quantum premeasurement state can have several equally weighted Schmidt decompositions. Choosing a different decomposition does not alter that pure state, but declaring a projection in a different basis generally changes the resulting mixture. Environment-induced pointer-basis selection supplies a dynamical account of robust records, while leaving the further question of a single realized outcome distinct.
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