Two-photon superbunching of thermal light via multiple two-photon path interference

We report on the two-photon superbunching effect of thermal light via constructive multiple two-photon path interference, showing the possibility of controlling the bunching property of thermal light in a linear optical system. By inserting n pairs of mutually first-order incoherent optical channels as a cascade into the traditional Hanbury Brown and Twiss (HBT) interferometer, the two-photon bunching peak-to-background ratio of thermal light can be increased by up to 2×1.5ⁿ, surpassing the theoretical upper limit of 2 for a traditional HBT interferometer. Experimentally, the ratio was measured to be 2.4±0.1 when one pair of optical channels was inserted. Such a scheme can be viewed as a prototype of a two-photon grating.