On a quantum version of Shannon's conditional entropy
  In this article we propose a quantum version of Shannon's conditional entropy. Given two density matrices $\rho$ and $\sigma$ on a finite dimensional Hilbert space and with $S(\rho)=-\tr\rho\ln\rho$ being the usual von Neumann entropy, this quantity $S(\rho|\sigma)$ is concave in $\rho$ and satisfies $0\le S(\rho|\sigma)\le S(\rho)$, a quantum analogue of Shannon's famous inequality. Thus we view $S(\rho|\sigma)$ as the entropy of $\rho$ conditioned by $\sigma$. 
Robert Schrader