Temporally ultralong biphotons with a linewidth of 50 kHz

Yu Sheng Wang, Kai Bo Li, Chao Feng Chang, Tan Wen Lin, Jian Qing Li, Shih Si Hsiao, Jia Mou Chen, Yi Hua Lai, Ying Cheng Chen, Yong Fan Chen, Chih Sung Chuu, Ite A. Yu

Research output: Contribution to journalArticlepeer-review


We report the generation of biphotons, with a temporal full width at the half maximum (FWHM) of 13.4 ± 0.3 μs and a spectral FWHM of 50 ± 1 kHz, via the process of spontaneous four-wave mixing with laser-cooled atoms. The temporal width is the longest, and the spectral linewidth is the narrowest to date. This is also the first biphoton result that obtains a linewidth below 100 kHz, reaching a new milestone. The very long biphoton wave packet has a signal-to-background ratio of 3.4, which violates the Cauchy-Schwarz inequality for classical light by 4.8 folds. Furthermore, we demonstrated a highly tunable-linewidth biphoton source and showed that while the biphoton source's temporal and spectral width were controllably varied by about 24 folds, its generation rate only changed by less than 15%. A spectral brightness or generation rate per pump power per linewidth of 1.2× 106 pairs/(s mW MHz) was achieved at the temporal width of 13.4 μs. The above results were made possible by the low decoherence rate and high optical depth of the experimental system, as well as a novel scheme of classical fields' and biphotons' propagation directions in the experiment. This work has demonstrated a high-efficiency ultranarrow-linewidth biphoton source and has made substantial advancements in quantum technology utilizing heralded single photons.

Original languageEnglish
Article number126102
JournalAPL Photonics
Issue number12
Publication statusPublished - 2022 Dec 1

All Science Journal Classification (ASJC) codes

  • Atomic and Molecular Physics, and Optics
  • Computer Networks and Communications


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