# Unitarity (physics)

## Requirement that quantum states' time evolution operators are unitary transformations / From Wikipedia, the free encyclopedia

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In quantum physics, **unitarity** is (or a **unitary process** has) the condition that the time evolution of a quantum state according to the Schrödinger equation is mathematically represented by a unitary operator. This is typically taken as an axiom or basic postulate of quantum mechanics, while generalizations of or departures from unitarity are part of speculations about theories that may go beyond quantum mechanics.^{[1]} A **unitarity bound** is any inequality that follows from the unitarity of the evolution operator, i.e. from the statement that time evolution preserves inner products in Hilbert space.

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