Observation of second-order interference beyond the coherence time with true thermal photons

Abstract

It has recently been shown that counter-intuitive Franson-like second-order interference can be observed with a pair of classically correlated pseudo thermal light beams and two separate unbalanced interferometers (UIs): the second-order interference visibility remains fixed at 1/3 even though the path length difference in each UI is increased significantly beyond the coherence length of the pseudo thermal light [Phys. Rev. Lett. 119, 223603 (2017)PRLTAO0031-900710.1103/PhysRevLett.119.223603]. However, as the pseudo thermal beam itself originated from a long-coherence laser (and by using a rotating ground disk), there exists the possibility of a classical theoretical model to account for second-order interference beyond the coherence time on the long coherence time of the original laser beam. In this work, we experimentally explore this counter-intuitive phenomenon with a true thermal photon source generated via quantum thermalization, i.e., obtaining a mixed state from a pure two-photon entangled state. This experiment not only demonstrates the unique second-order coherence properties of thermal light clearly but may also open up remote sensing applications based on such effects.

Indistinguishability of temporally separated pairwise two-photon state of thermal photons in Franson-type interferometry

“…Interestingly, the second-order interference (SOI) of thermal light in two independent unbalanced interferometers beyond the photon coherence length has been theoretically and experimentally reported 14 – 18 . Although thermal photons are not energy–time entangled, this SOI beyond the coherence length in the Franson-type interferometry is a highly counterintuitive phenomenon 14 , 16 .…”

Indistinguishability of Temporally Separated Pairwise Two- Photon State of Thermal Photons in Franson-Type Interferometry

“…Interestingly, the second-order interference (SOI) of thermal light in two independent unbalanced interferometers beyond the photon coherence length has been theoretically and experimentally reported [14][15][16][17][18]. Although thermal photons are not energy-time entangled, this SOI beyond the coherence length in the Franson-type interferometry is a highly counterintuitive phenomenon [14,16].…”