To study this question, we discuss a class of spatially incoherent sources, coined thermal biphotons, which are made of independent emitters, that emit pairs of photons. 10–15 An intriguing question is whether the equivalence between the Gaussian field and the photon bunching interpretations holds for other types of incoherent sources. Similarly, it is generally believed that the Gaussian fields description of thermal ghost imaging yields equivalent results to the quantum theory, up to a contrast difference. 9 For such sources, the classical interpretation of fluctuating Gaussian fields and the quantum interpretation of photon bunching predict exactly the same features of the HBT effect the second order coherence exhibits a 2:1 peak-to-background ratio with a width that is inversely proportional to the source size. To date, in all demonstrators of the spatial HBT effect, the sources were spatially incoherent thermal sources, which are modeled by a large number of independent emitters that emit uncorrelated photons. 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. 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. We further provide an alternative interpretation of the results by drawing a connection between the HBT effect and coherent backscattering of light. 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. Here, we propose and demonstrate an incoherent light source based on phase-randomized spatially entangled photons, which we coin thermal biphotons.

All the thermal sources considered to date did not feature quantum signatures of light, as they consisted of independent emitters that emit uncorrelated photons. The observation of the Hanbury Brown and Twiss (HBT) effect with thermal light marked the birth of quantum optics.

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