  A simple experimental setup consisting of a spontaneous parametric
down-conversion source and passive linear optics is proposed for conditional
preparation of a maximally entangled polarization state of two photons.
Successful preparation is unambiguously heralded by coincident detection of
four auxiliary photons. The proposed scheme utilizes the down-conversion term
corresponding to the generation of three pairs of photons. We analyze imperfect
detection of the auxiliary photons and demonstrate that its deleterious effect
on the fidelity of the prepared state can be suppressed at the cost of
decreasing the efficiency of the scheme.
