  We use a novel form of quantum conditional probability to define new measures
of quantum information in a dynamical context. We explore relationships between
our new quantities and standard measures of quantum information, such as von
Neumann entropy. These quantities allow us to find new proofs of some standard
results in quantum information theory, such as the concavity of von Neumann
entropy and Holevo's theorem. The existence of an underlying probability
distribution helps shed light on the conceptual underpinnings of these results.
