Demonstration of local expansion toward large-scale entangled webs.

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 $0.836\ifmmode\pm\else\textpm\fi{}0.042$ and $0.784\ifmmode\pm\else\textpm\fi{}0.028$, respectively. We also confirmed the existence of pairwise entanglement in every pair of 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.