Abstract
We proposed and experimentally demonstrated a new idea of using a far-detuned coupling field to enhance the generation rate of the hot-atom biphoton source based on the double-Lambda spontaneous four-wave mixing (SFWM) process. Previous theories indicated that the biphoton generation rate decreases with the coupling detuning. Thus, the new idea seems counterintuitive, and no literature has explored it, to the best of our knowledge. Here, we showed that the far-detuned coupling field not only increased the biphoton generation rate but also narrowed down the biphoton linewidth, and consequently, it enhanced the spectral brightness (SB) or generation rate per linewidth by 10 folds, compared to the resonant coupling field. This surprising result led us to develop a new theoretical framework, considering that the far-detuned frequency protects the heralded single photons from a previously unexplored physical mechanism. The predictions from the new theory agree with the experimental data and indicate that the optical depth (OD) of atomic media can further amplify the enhancements of the generation rate and SB induced by the coupling detuning. Moreover, in the past, a higher SB always resulted in a lower signal-to-background ratio (SBR) by varying the OD, pump power, pump detuning, and/or coupling power in the hot-atom biphoton source. The new tuning parameter of coupling detuning significantly enhanced the heralding probability by efficiently suppressing noise-generating mechanisms and increased the SB and SBR together. Hence, the SB (or SBR) under the same SBR (or SB) of the atom-based biphoton sources with the detuned coupling field is now on a par with that of crystal-or chip-based biphoton sources. This work introduces a new parameter for tuning the double-Lambda SFWM biphoton source, advances our understanding of biphoton generation, and opens new avenues for enhancing the performance of these sources. (c) 2025 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement.