Evolution and entanglement of Gaussian states in the parametric   amplifier
  The linear time-dependent constants of motion of the parametric amplifier are obtained and used to determine in the tomographic-probability representation the evolution of a general two-mode Gaussian state. By means of the discretization of the continuous variable density matrix, the von Neumann and linear entropies are calculated to measure the entanglement properties between the modes of the amplifier. The obtained results for the nonlocal correlations are compared with those associated to a linear map of discretized symplectic Gaussian-state tomogram onto a qubit tomogram. This qubit portrait procedure is used to establish Bell-type's inequalities, which provide a necessary condition to determine the separability of quantum states, which can be evaluated through homodyne detection. Other no-signaling nonlocal correlations are defined through the portrait procedure for noncomposite systems. 
Julio A. L\'opez-Sald\'ivar, Armando Figueroa, Octavio Casta\~nos,   Ram\'on L\'opez-Pe\~na, Margarita A. Man'ko, Vladimir I...
quant-ph