Observation of the two- and four-photon bunching effects at a beamsplitter

Summary form only given. When the two signals are aligned into the opposite input ports of a beamsplitter and their relative arrival time is varied by using a controlled delay, we count the events where all four photons exit together. We observe a more than five fold (ideally sixfold) increase of this probability when the four photons arrive at the beamsplitter simultaneously.