Strict concavity of Von Neumann entropy

For $0<b<1$ and distinct density operators, the Von Neumann entropy of $\rho =b{\rho }_1+(1-b){\rho }_2$ is strictly larger than the corresponding average. The difference is $bD({\rho }_1\Vert \rho )+(1-b)D({\rho }_2\Vert \rho )$, positive by nonnegativity of quantum relative entropy unless both states coincide. On the closed interval $0\le b\le 1$, equality holds exactly when $b=0$, $b=1$, or ${\rho }_1={\rho }_2$.