Observation of four-photon interference with a beam splitter by pulsed parametric down-conversion

When four photons arrive at a beam splitter, two from each side, a four-pho ton, six-path interference effect occurs to yield a sixfold enhancement of the probability for all four photons to exit together from the beam splitte r. We produce the four-photon state by using the stimulated emission proces s in a pulsed parametric down-conversion and measure the probability for al l four photons to exit from one side of the beam splitter. The observed enh ancement factor is in good agreement with a multimode treatment of pulsed d own-conversion.