The hybrid quantum system integrating optical nanofibers (ONFs) with cold atoms has emerged as a paramount platform for investigating atom-light interactions and advancing quantum information processing, benefiting from the strong local field confinement of nanostructures and the long coherence times of laser-cooled atoms. In this work, we experimentally demonstrate a robust and controllable ONF-cold atom interface based on a dual-magneto-optical trap (MOT) configuration and perform in-situ characterization via photon correlation measurements. The dual-MOT setup consists of a two-dimensional MOT (2D-MOT) and a three-dimensional MOT (3D-MOT), ensuring a robust environment for efficient coupling between a cold cesium ensemble and the ONF. The ONF, fabricated with a uniform waist diameter of approximately 450 nm, facilitates enhanced evanescent field interactions for both atom excitation and high-efficiency fluorescence collection. Cold cesium atoms are trapped in the 3D-MOT , which is spatially overlapped with the ONF waist region. Fluorescence emitted by the atom ensemble is efficiently coupled into both ends of the ONF, and second-order intensity correlation measurements are performed on the collected photons.
By analyzing the second-order correlation function and fitting the data to multi-atom correlation theory, we observe a pronounced anti-bunching dip at zero time delay, a clear signature of the quantum nature of emission from a few-atom ensemble. This method allows for the simultaneous and precise determination of the effective atom number and the effective Rabi frequency of the atom–light interaction. We demonstrate that the effective atom number can be finely tuned from 0.52 to 2.42 by adjusting the power of the push beam that transports atoms from the 2D-MOT to the 3D-MOT, achieving a remarkable control precision of ±0.14. Furthermore, we systematically investigate the dependence of the atom–light interaction on the excitation laser’s power and frequency detuning. The fluorescence count rate exhibits a non-monotonic behavior with increasing excitation power; it initially increases due to higher excitation rates but subsequently decreases at higher powers due to light-induced heating and atom loss. We also verified that the square of the effective Rabi frequency scales linearly with the excitation power, while the effective Rabi frequency increases linearly with the absolute value of detuning. This work establishes a reliable and quantitative method for calibrating essential parameters in waveguide quantum electrodynamics systems. Our results provide experimental evidence for future studies of collective radiation, quantum light sources and nonlinear quantum optics in fiber-integrated atomic platforms.