Simple scheme for expanding a polarization-entangled W state by adding   one photon
  We propose a simple scheme for expanding a polarization-entangled W state. By mixing a single photon and one of the photons in an n-photon W state at a polarization-dependent beam splitter (PDBS), we can obtain an (n+1)-photon W state after post-selection. Our scheme also opens the door for generating n-photon W states using single photons and linear optics. 
Yan-Xiao Gong, Xu-Bo Zou, Yun-Feng Huang, and Guang-Can Guo