Thermal biphotons

TL;DR

This paper introduces thermal biphotons, a new incoherent light source with spatial entanglement, demonstrating how their unique correlations violate classical optical relations and offer insights into quantum coherence properties.

Contribution

The paper proposes and experimentally demonstrates thermal biphotons, revealing their impact on the HBT effect and coherence properties, connecting entanglement with classical optical phenomena.

Findings

Thermal biphotons exhibit a HBT peak width determined by correlations.

Violation of the Siegert relation is observed with thermal biphotons.

Spatial entanglement influences the coherence properties of thermal light.

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 (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.