Quality factor and pairing probability of biphoton sources

Biphotons are pairs of photons. In quantum light sources, processes such as spontaneous parametric down-conversion (SPDC) and spontaneous four-wave mixing (SFWM) process are commonly used to simultaneously generate photon pairs that exhibit entanglement and strong temporal correlations. One photon can serve as a heralding photon, while the other, the heralded photon, can be employed in quantum operations. In this work, we realized a biphoton source based on SFWM process in a hot atomic vapor, achieving a high generation rate, a narrow linewidth, and flexible linewidth tunability. We proposed a novel technique—introducing a far-detuned coupling field into the SFWM process—that successfully enhances the generation rate while simultaneously narrowing the spectral linewidth. Compared to the resonant coupling field, the spectral brightness (SB) can be improved by an order of magnitude. This approach also effectively suppresses noise generation, increases the photon-pairing probability, and simultaneously improves both the signal-to-background ratio (SBR) and SB. These striking results bring the source closer to an ideal biphoton source. In addition, we developed a new theoretical framework that incorporates the effect of impurity atoms, achieving excellent agreement with the experimental data. We further established a new physical metric to quantify how close a biphoton source approaches the ideal limit, namely the absence of noise generation. In summary, this work introduces a new tuning parameter into the SFWM process, deepens our understanding of biphoton generation, and substantially enhances the overall performance of the biphoton source.