  We propose an optical scheme to prepare large-scale entangled networks of W
states. The scheme works by simultaneously fusing three polarization-encoded W
states of arbitrary size via accessing only one qubit of each W state. It is
composed of a Fredkin gate (controlled-swap gate), two fusion gates [as
proposed in New J. Phys. 13, 103003 (2011)] and an H-polarized ancilla photon.
Starting with three $n$-qubit W states, the scheme prepares a new W state with
$3(n-1)$-qubits after postselection if both fusion gates operate successfully,
i.e. a four-fold coincidence at the detectors. The proposed scheme reduces the
cost of creating arbitrarily large W states considerably when compared to
previously reported schemes.
