Abstract
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.
Highlights
St to the standard Hanbury Brown and Twiss (HBT) effect with thermal light, for thermal biphotons, the quantum and the classical fluctuating-speckle pictures are not equivalent
We show that thermal biphotons exhibit a 2:1 HBT peak, as in the standard HBT effect, indicating their thermal nature
We consider the case of thermal biphotons, in which a source of size D is comprised of two-photon emitters, each having a characteristic size of d [Fig. 1(b)]
Summary
St to the standard HBT effect with thermal light, for thermal biphotons, the quantum and the classical fluctuating-speckle pictures are not equivalent. The second-order coherence function g(2)(Δθ) exhibits an 2:1 HBT peak [Fig. 2(a), black dots], yet its width is determined by the size of the twophoton emitters rather than by the total width of the source.
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