Local expansion of photonic W state using a polarization dependent   beamsplitter
  We propose a simple probabilistic optical gate to expand polarization entangled W states. The gate uses one polarization-dependent beamsplitter and a horizontally polarized single photon as an ancilla. The gate post-selectively expands $N$-photon W states to $(N+1)$-photon W states. A feasibility analysis considering the realistic experimental conditions show that the scheme is within the reach of the current quantum optical technologies. 
Toshiyuki Tashima, Sahin Kaya Ozdemir, Takashi Yamamoto, Masato   Koashi, and Nobuyuki Imoto