We present a novel type of incoherent light, thermal biphotons, that exhibit an Hanbury Brown and Twiss peak with a different width compared to classical thermal light, leading to the breakdown of the Siegert relation.
The observation of the Hanbury Brown and Twiss (HBT) effect with thermal light marked the birth of quantum optics. All the thermal sources considered to date did not feature quantum signatures of light, as they consisted of independent emitters that emit uncorrelated photons. Here, we propose and demonstrate an incoherent light source based on phase-randomized spatially entangled photons, which we coin thermal biphotons. We show that in contrast to thermal light, the width of the HBT peak for thermal biphotons is determined by their correlations, leading to violation of the Siegert relation and breakdown of the speckle-fluctuations interpretation. We further provide an alternative interpretation of the results by drawing a connection between the HBT effect and coherent backscattering of light. Finally, we discuss the role of spatial entanglement in the observed results, deriving a relation between the Schmidt number and the degree of violation of the Siegert relation under the double-Gaussian approximation of spontaneous parametric down conversion. Our work reflects new insights on the coherence properties of thermal light in the presence of entanglement, paving the way for entanglement certification using disorder averaged measurements.
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 $2\ifmmode\times\else\texttimes\fi{}1.{5}^{n}$, surpassing the theoretical upper limit of 2 for a traditional HBT interferometer. Experimentally, the ratio was measured to be $2.4\ifmmode\pm\else\textpm\fi{}0.1$ when one pair of optical channels was inserted. Such a scheme can be viewed as a prototype of a two-photon grating.