Conservation laws protect dynamic spin correlations from decay: Limited   role of integrability in the central spin model
  Mazur's inequality renders statements about persistent correlations possible. We generalize it in a convenient form applicable to any set of linearly independent constants of motion. This approach is used to show rigorously that a fraction of the initial spin correlations persists indefinitely in the isotropic central spin model unless the average coupling vanishes. The central spin model describes a major mechanism of decoherence in a large class of potential realizations of quantum bits. Thus the derived results contribute significantly to the understanding of the preservation of coherence. We will show that persisting quantum correlations are not linked to the integrability of the model, but caused by a finite operator overlap with a finite set of constants of motion. 
Goetz S. Uhrig, Johannes Hackmann, Daniel Stanek, Joachim Stolze and   Frithjof B. Anders
cond-mat.mes-hall