  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 (SPDC).
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.
