Unlocking multiphoton emission from a single-photon source through mean-field engineering

In the single-photon emission from a two-level system, multiphotons are generally regarded as accidental, undesired, and unrelated to the mechanism. In coherently driven systems, however, they form the cornerstone of single-photon emission, which arises from quantum interferences between virtual multiphoton fluctuations of the emitter and the Poissonian superposition of all number states induced by the driving. Here, we demonstrate how one can control the multiphoton dynamics by disrupting these quantum interferences through an external homodyne control of the emitter's mean field. Experimentally, we observed a transition from single-photon to multiphoton emission, up to three-photon correlations. We show that, counterintuitively, quantum fluctuations always play a major qualitative role, even and, in fact, especially when their quantitative contribution is vanishing. Our findings provide distinct insights into the paradoxical character of quantum mechanics and open pathways for mean-field engineering as a tool for precision multiphoton control.