Ch 23.1 The operator $\hat{\rho} = \sum_i p_i|\psi_i\rangle\langle\psi_i|$ that describes both pure states ($\hat{\rho} = |\psi\rangle\langle\psi|$) and mixed states (statistical mixtures). Properties: $\hat{\rho}^\dagger = \hat{\rho}$, $\text{Tr}(\hat{\rho}) = 1$, $\hat{\rho} \geq 0$. A state is pu