Ch 23.3 The quantum analogue of Shannon entropy: $S(\hat{\rho}) = -\text{Tr}(\hat{\rho}\ln\hat{\rho})$. For a pure state, $S = 0$; for a maximally mixed state of dimension $d$, $S = \ln d$. Measures the degree of mixedness or ignorance about the state. *See also:* Density matrix, Entanglement entrop