Bright Heralded Single-Photon Superradiance in a High-Density Thin Vapor Cell

Superradiance is a hallmark of cooperative quantum emission, where radiative decay is collectively enhanced by coherence among emitters. Here, extending superradiant effects to photon pair generation from multi-level atoms, two-photon process offers a pathway to novel quantum light sources and a useful case for practical superradiance. We report bright heralded single-photon superradiance via spontaneous four-wave mixing in a 1-mm-long, high-density cesium vapor cell. By reducing the average distance between atoms in the atomic vapor to 0.29 times the idler photon wavelength, we observe a dramatic narrowing of the temporal two-photon wavefunction. This compression of temporal two-photon wavefunction evidences the superradiance of heralded photons in the collective two-photon emission dynamics. Furthermore, our heralded single-photon superradiance is accompanied by a coincidence-to-accidental ratio of 200 and the detected photon-pair counting exceeding 10^6 pairs/s. These findings establish dense thin atomic vapors as a practical, robust medium for realizing superradiant photon sources, with immediate relevance for quantum optics and the development of efficient photonic quantum technologies.