From Quantum Fluctuations to Self-Quenching: The Role of Photon Number in Single-Atom Lasers

Traditional laser theory typically involves large atomic ensembles, whereas cavity quantum electrodynamics (QED) enables lasing from a single atom in the strong-coupling regime. In this study, the dynamics of a three-level atom coupled to an optical cavity are mapped across three regimes defined by photon number: deep quantum, intermediate quantum, and semi-classical. The findings indicate that traditional laser behavior, characterized by stable phase and coherent statistics, emerges in the intermediate regime, with the threshold crossed at approximately three photons. As the field exceeds ten photons, coherence deteriorates, indicating the onset of self-quenching. Through analysis of population inversion, emission rates, and Wigner phase-space representations, the transition from quantum noise to a displaced Gaussian state is characterized. These results confirm that a single-atom system can replicate the statistical signatures of a macroscopic laser before reaching the quenching limit, thereby providing insight into the fundamental boundaries between quantum and classical light sources.