Entropic and information inequality for nonlinearly transformed   two-qubit X-states
  The entropic and information inequalities for two qubit X-states transformed by the nonlinear channels are given in explicit form. The subadditivity condition and nonnegativity of von Neumann quantum information are studied for both initial X-state and the state after action of the nonlinear channel. Partial case of Werner state is considered in detail. We shown that the von Neumann information increases due to action of the nonlinear channel. We generalize the results obtained for Werner state in our earlier article [J. Russ. Laser Res., \textbf{35}, iss. 3 (2014)] to the case of X-state of two qubit system. We study the influence of nonlinear channel acting on X-state of two qubit system onto von Neumann mutual information. 
V.I. Man'ko and R.S. Puzko
quant-ph