Ch 23.1 A quantum state described by a single ket $|\psi\rangle$ (or equivalently a density matrix $\hat{\rho} = |\psi\rangle\langle\psi|$ with $\text{Tr}(\hat{\rho}^2) = 1$). Represents maximal knowledge of the system. *See also:* Mixed state, Density matrix, Superposition.