  Bell's theorem reveals contradictions between the predictions of quantum
mechanics and the EPR postulates for a pair of particles only in situations
involving imperfect statistical correlations. However, with three or more
particles, contradictions emerge even for perfect correlations. We describe an
experiment which can be realized in the laboratory, using four-photon entangled
states generated by parametric down-conversion, to demonstrate this
contradiction at the level of perfect correlations.
