  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.
