New method for protecting heralded single photons in hot-atom biphoton generation

Biphotons are pairs of single photons. Through physical processes such as spontaneous parametric down-conversion (SPDC) and spontaneous four-wave mixing (SFWM) process, pairs of photons exhibiting entanglement and strong temporal correlations can be generated simultaneously. One photon can serve as a heralding photon, while the other, the heralded photon, can be utilized for quantum operations. An ideal biphoton source should exhibit a high generation rate, a high pairing probability, a narrow linewidth, flexible linewidth tunability, and especially low noise. In this work, we realized such a biphoton source using SFWM process in a hot atomic vapor. We are the first to propose and systematically investigate a counterintuitive approach—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 enhances both the signal-to-background ratio (SBR) and SB. As a result, the performance of the source approaches that of an ideal biphoton source and becomes competitive with crystal- or chip-based biphoton sources. In summary, this work introduces a new tuning parameter into the SFWM process, deepens our understanding of biphoton generation, and significantly improves the overall performance of the biphoton source.