  Baez, Fritz, and Leinster derived a method for characterizing Shannon entropy
in classical systems. In this method, they considered a functor from a certain
category to the monoid of non-negative real numbers with addition as a map from
measure-preserving functions to non-negative real numbers, and derived Shannon
entropy by imposing several simple conditions. We propose a method for
characterizing von Neumann entropy by extending their results to quantum
systems.
