Asymmetric Biphoton Generation under Ground‐State Decoherence and Phase Mismatch in a Cold Atomic Ensemble

Abstract An experimental investigation of how ground‐state decoherence and phase mismatch influence biphoton generation in double‐Λ spontaneous four‐wave mixing (SFWM) within a cold atomic ensemble is presented. The results reveal significant asymmetry in the Stokes and anti‐Stokes photon generation rates, arising from the distinct effects of phase mismatch and ground‐state decoherence. While phase mismatch primarily drives this asymmetry under minimal decoherence, larger decoherence further amplifies it, underscoring the complex interplay between these factors. Using the coincidence count rate representation, insights are provided into pairing ratios, demonstrating that the stimulated four‐wave mixing process inherent in SFWM explains the observed phenomena. Interestingly, although ground‐state decoherence reduces the generation of temporally correlated photons, it paradoxically enhances biphoton purity, as confirmed through conditional autocorrelation measurements. This counterintuitive phenomenon is reported here for the first time. Furthermore, unconditional autocorrelation measurements show that the generated photons follow a thermal‐state distribution, consistent with theoretical predictions. This study advances the understanding of biphoton generation dynamics and temporal photon correlations in SFWM, offering valuable insights for optimizing SFWM‐based biphoton sources and their applications in quantum technologies.