Evolution from Entanglement to Decoherence of Bipartite Mixed "X" States
  We examine a class of bipartite mixed states which we call X states and report several analytic results related to the occurrence of so-called entanglement sudden death (ESD) under time evolution in the presence of common types of environmental noise. Avoidance of sudden death by application of purely local operations is shown to be feasible in some cases. 
Ting Yu and J. H. Eberly