The von Neumann entropy and unitary equivalence of quantum states
  K. He, J. Hou, and M. Li have recently given a sufficient and necessary condition for unitary equivalence of quantum states. This condition is based on the von Neumann entropy. In this note we first give a short proof of their result, and then we improve it. 
Roman Drnov\v{s}ek
math.QA math.FA