Four-photon interference experiment and its implications for the Greenberger-Horne-Zeilinger theorem

Summary form only given. We report the latest experimental results on four-photon interference and discuss what they mean in terms of the GHZ theorem. The four-photon interference is the nonlocal effect proving the existence of the entangled (i.e., nonfactorizable) states of four photons. The entanglement of two photons emerging from a spontaneous parametric downconversion (SPDC) crystal is well studied. Particularly, for the type-II SPDC, when the signal and the idler photons are orthogonally polarized, it is possible to entangle them in both space and polarization. We take advantage of this to entangle two pairs of SPDC photons generated by two different, but coherently pumped, sources.