In quantum mechanics, wave function collapse (also called collapse of the state vector or reduction of the wave packet) is the phenomenon in which a wave function—initially in a superposition of several different possible eigenstates—appears to reduce to a single one of those states after interaction with an observer. In simplified terms, it is the reduction of the physical possibilities into a single possibility as seen by an observer. It is one of two processes by which quantum systems evolve in time, according to the laws of quantum mechanics as presented by John von Neumann. The reality of wave function collapse has always been debated, i.e., whether it is a fundamental physical phenomenon in its own right or just an epiphenomenon of another process, such as quantum decoherence. In recent decades the quantum decoherence view has gained popularity and is commonly taught at the graduate level (e.g. Cohen-Tannoudji's standard textbook). Collapse may be understood as an update in a probabilistic model, given the observed result. The quantum filtering approach and the introduction of quantum causality non-demolition principle allowed for a derivation of quantum collapse from the stochastic Schrödinger equation.
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