On preparation of the W-states from atomic ensembles
  A scheme, where three atomic ensembles can be prepared in the states of the W-class via Raman type interaction of strong classical field and a projection measurement involved three single-photon detectors and two beamsplitters, are considered. The obtained atomic entanglement consists of the Dicke or W-states of each of the ensembles. 
V.N. Gorbachev, A.A. Rodichkina, A.I. Trubilko