2. Postulates: Evolution

Postulate 2: Evolution

How does the state change over time?

• The Rule: The evolution of a closed quantum system is described by a Unitary Transformation.
• The Equation: $|\psi'\rangle = U|\psi\rangle$

Key Property: Unitary matrices preserve the length of the vector (probability must sum to 1) and are reversible ($U^{-1} = U^{\dagger}$). This means quantum information is never lost.