Ch 7.1 The unitary operator $\hat{U}(t) = e^{-i\hat{H}t/\hbar}$ (for time-independent $\hat{H}$) that evolves quantum states: $|\psi(t)\rangle = \hat{U}(t)|\psi(0)\rangle$. Unitarity ensures conservation of probability. *See also:* Schrodinger equation, Unitary operator, Hamiltonian.