Mixedness and entanglement for two-mode Gaussian states
  We analytically exploit the two-mode Gaussian states nonunitary dynamics. We show that in the zero temperature limit, entanglement sudden death (ESD) will always occur for symmetric states (where initial single mode compression is $z_0$) provided the two mode squeezing $r_0$ satisfies $0 < r_0 < 1/2 \log (\cosh (2 z_0)).$ We also give the analytical expressions for the time of ESD. Finally, we show the relation between the single modes initial impurities and the initial entanglement, where we exhibit that the later is suppressed by the former. 
L. A. M. Souza, R. C. Drumond, M. C. Nemes and K. M. Fonseca Romero
quant-ph