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Accepted: Nov. 15, 2020

Posted: Nov. 18, 2020

Published Online: Dec. 25, 2020

The Author Email: Han Seb Moon (hsmoon@pusan.ac.kr)

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Jiho Park, Heonoh Kim, Han Seb Moon. Second-order interference of true thermal light from a warm atomic ensemble in two independent unbalanced interferometers[J]. Photonics Research, 2021, 9(1): 01000049

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## Abstract

We report the demonstration of a second-order interference experiment by use of thermal light emitted from a warm atomic ensemble in two spatially separated unbalanced Michelson interferometers (UMIs). This novel multipath correlation interference with thermal light has been theoretically proposed by Tamma [New J. Phys.18, 032002 (2016)NJOPFM1367-263010.1088/1367-2630/18/3/032002]. In our experiment, the bright thermal light used for second-order interference is superradiantly emitted via collective two-photon coherence in Doppler-broadened cascade-type $Rb87$ atoms. Owing to the long coherence time of the thermal light from the atomic ensemble, we observe its second-order interference in the two independent UMIs by means of time-resolved coincidence detection. The temporal waveforms of the interfering thermal light in the two spatially separated UMIs exhibit similarities with the temporal two-photon waveform of time–energy entangled photon pairs in Franson interferometry. Our results can contribute toward a better understanding of the relation between first- and second-order interferences that are at the heart of photonics-based quantum information science.