An elementary optical gate for expanding entanglement web
  We introduce an elementary optical gate for expanding polarization entangled W states, in which every pair of photons are entangled alike. The gate is composed of a pair of 50:50 beamsplitters and ancillary photons in the two-photon Fock state. By seeding one of the photons in an $n$-photon W state into this gate, we obtain an $(n+2)$-photon W state after post-selection. This gate gives a better efficiency and a simpler implementation than previous proposals for $\rm W$-state preparation. 
Toshiyuki Tashima, Sahin Kaya Ozdemir, Takashi Yamamoto, Masato   Koashi, and Nobuyuki Imoto