  We design linear optics multiqubit quantum logic gates. We assume the
traditional encoding of a qubit onto state of a single photon in two modes
(e.g. spatial or polarization). We suggest schemes allowing direct
probabilistic realization of the fundamental Toffoli and Fredkin gates without
resorting to a sequence of single- and two-qubit gates. This yields more
compact schemes and potentially reduces the number of ancilla photons. The
proposed setups involve passive linear optics, sources of auxiliary single
photons or maximally entangled pairs of photons, and single-photon detectors.
In particular, we propose an interferometric implementation of the Toffoli gate
in the coincidence basis, which does not require any ancilla photons and is
experimentally feasible with current technology.
