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

• Published in: Optics letters Vol. 45; no. 24; p. 6748
• Main Authors: Lee, Gyu-Hyeok, Im, Dong-Gil, Kim, Yosep, Kim, U-Shin, Kim, Yoon-Ho
• Format: Journal Article
• Language: English
• Published: United States 15.12.2020
• ISSN: 1539-4794
• DOI: 10.1364/OL.413287

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.