  In this work the degree of entanglement of two-photon states generated by
parametric-down conversion with a phase mismatch is computed in terms of the
temperature of the nonlinear medium where the interaction takes place. The
minimum interaction time, named {\it birth time of entanglement}, needed before
entanglement is attained, is computed in terms of the temperature and of the
phase mismatch. Our results show that, for each value of the phase mismatch,
there is a critical temperature above which entanglement disappears. We are
able to identify the regions of parameters, temperature and mismatch, where the
states are entangled and where they are separable. We also propose a
entanglement witness based in a dynamical invariant of the nonlinear
interaction.
