Polarization Entangled W State using Parametric Down-Conversion
  An experimental scheme for preparing a polarization entangled W states from four photons emitted by parametric down-conversion is proposed. We consider two different configurations and a method of improving the yield by using single photon sources. In the proposed scheme, one uses only linear optical elements and photon detectors, so that this scheme is feasible by current technologies. 
Takashi Yamamoto, Kiyoshi Tamaki, Masato Koashi and Nobuyuki Imoto