Non-Markovian dynamics of a single electron spin coupled to a nuclear   spin bath
  We apply the time-convolutionless (TCL) projection operator technique to the model of a central spin which is coupled to a spin bath via nonuniform Heisenberg interaction. The second-order results of the TCL method for the coherences and populations of the central spin are determined analytically and compared with numerical simulations of the full von Neumann equation of the total system. The TCL approach is found to yield an excellent approximation in the strong field regime for the description of both the short-time dynamics and the long time behavior. 
E. Ferraro, H.-P. Breuer, A. Napoli, M. A. Jivulescu and A. Messina