We show that a beam splitter of reflectivity one-third can be used to realize a quantum phase gate operation if only the outputs conserving the number of photons on each side are post-selected.
@article{arxiv.quant-ph/0111092,
title = {Quantum phase gate for photonic qubits using only beam splitters and post-selection},
author = {Holger F. Hofmann and Shigeki Takeuchi},
journal= {arXiv preprint arXiv:quant-ph/0111092},
year = {2009}
}