Telecom–Visible Biphotons with Subnanosecond Correlations from a Warm Atomic Ensemble

Zi-Yu Liu^1,2*, Jiun-Shiuan Shiu^1,2, Yu-Xiang Huang¹, Yong-Fan Chen^1,2

¹Department of Physics, National Cheng Kung University, Tainan, Taiwan
²Center for Quantum Frontiers of Research & Technology, Tainan, Taiwan

* Presenter:Zi-Yu Liu, email:liuziyu.ncku@gmail.com

Telecom–visible biphotons provide a direct interface between heterogeneous quantum systems and are highly desirable for fiber-compatible quantum networking [1] and integrated photonic quantum technologies [2]. We generate biphotons at 1529 and 780 nm through a diamond-type transition in a warm Rb-87 vapor. The biphoton waveform exhibits a subnanosecond correlation width, largely resulting from interference among Doppler-broadened atomic velocity classes. The associated broadband characteristic suppresses background noise and enables a high signal-to-background ratio. We also observe that increased optical depth leads to a further reduction of the correlation time through a superradiant effect within a similar temperature range. The measurements agree well with theoretical predictions. Importantly, we show for the first time that the pairing ratio [3], which characterizes the source purity, is enhanced by increasing the optical depth. This warm-atom scheme offers a practical and scalable platform for generating broadband biphotons spanning telecom and visible wavelengths, supporting integrated quantum networks across disparate frequency bands.

References
[1] H. J. Kimble, Nature 453, 1023 (2008).
[2] J. Wang et. al., Nat. Photon. 14, 273 (2020).
[3] J.-S. Shiu et. al., Phys. Rev. Res. 6, L032001 (2024).

Keywords: telecom–visible biphotons, Doppler-broadened warm atomic ensemble, diamond-type four-wave mixing