We introduce an elementary optical gate for expanding polarization entangled W states, in which every pair of photons are entangled alike. The gate is composed of a pair of 50:50 beamsplitters and ancillary photons in the two-photon Fock state. By seeding one of the photons in an n-photon W state into this gate, we obtain an (n+2)-photon W state after post-selection. This gate gives a better efficiency and a simpler implementation than previous proposals for W-state preparation.
@article{arxiv.0803.1897,
title = {An elementary optical gate for expanding entanglement web},
author = {Toshiyuki Tashima and Sahin Kaya Ozdemir and Takashi Yamamoto and Masato Koashi and Nobuyuki Imoto},
journal= {arXiv preprint arXiv:0803.1897},
year = {2015}
}