  We demonstrate an optical gate that increases the size of polarization-based
W states by accessing only one of the qubits. Using this gate, we have
generated three-photon and four-photon W states with fidelities $0.836\pm 0.042
$ and $0.784\pm 0.028$, respectively. We also confirmed existence of pairwise
entanglement in every pair of the qubits including the one that was left
untouched by the gate. The gate is applicable to any size of W states and hence
is a universal tool for expanding entanglement.
