A new axiomatic characterization of the von Neumann entropy

We lay down a series of postulates which we consider indespensable for any measure of the intrinsic dispersion of quantum states. Then we show that the v. Neumann entropy S(W)=–kTr(WlnW) is uniquely determined by these postulates up to a scaling factor. Thus the v. Neumann entropy can be characterized as the only reasonable measure of the intrinsic dispersion of quantum states. Our postulates have no direct bearing on phenomenological thermodynamics or information theory.