On parametric type interaction between light and atomic ensembles
  One-photon and Raman type interactions between two-level atoms and narrow-band light are considered. We give some exactly solvable models of these processes when only one-photon Fock states are involved in the evolution. Possible application of these models for generation and transformation of entangled states of the W-class, some of which demonstrate hierarchy structure, are discussed. Finally, we consider preparation of entangled chains of atomic ensembles. 
V.N. Gorbachev, A.I. Zhiliba, A.A. Rodichkina, A.I.Trubilko