Multi-photon emission from a resonantly pumped quantum dot

Resonance fluorescence of natural or artificial atoms constitutes a prime method for generating non-classical light. While most efforts have focused on producing single photons, multi-photon emission is unavoidably present in the resonant driving of an atom. Here, we study the extent to which these processes occur: we quantify the multi-photon emission statistics in a resonantly driven two-level artificial atom---a semiconductor quantum dot in a micropillar cavity---when pumped by short optical pulses. By measuring auto-correlation functions up to the fourth order, we observe up to four photons emitted after a single pumping pulse and investigate the emission dynamics with finely resolved temporal measurements. Furthermore, we propose a method based on acquisition time gating to enhance the purity of a single-photon source while maintaining high efficiencies. Our results deepen the understanding of the photon emission processes in coherently driven atomic systems and suggest a simple but effective technique to reduce the multi-photon components of a single-photon source.