  Three-photon superbunching is experimentally observed with the recently
proposed superbunching pseudothermal light. To the best of our knowledge, it is
the first time that three-photon superbunching is observed in linear optical
system. From the quantum optics point of view, three-photon superbunching is
interpreted as the result of constructive-destructive three-photon
interference. The key to observe three-photon superbunching in superbunching
pseudothermal light is that all the different ways to trigger a three-photon
coincidence count are in principle indistinguishable, which is experimentally
guaranteed by putting a pinhole before each rotating groundglass to ensure that
all the passed photons are within one coherence area. The observed three-photon
superbunching is helpful to increase the visibility of ghost imaging and
understand the physics of third-order interference of light.
