The Origin of Antibunching in Resonance Fluorescence

The generation of single photons is one key component for many quantum applications. It was shown, that resonance fluorescence from a quantum two-level system, e.g. a semiconductor quantum dot, enables single photon generation with high optical quality. Furthermore, excitation in the weak driving or the so called Rayleigh regime leads to an emission that inherits the excellent linewidth and coherence properties of the laser. In addition, autocorrelation measurements of the coherently scattered light have suggested that it also gains the single photon statistics from the two-level system. However, we present measurements that prove that the simultaneous observation of sub-natural linewidth and antibunching is not possible. High-resolution spectroscopy reveals the sharp spectral feature of the Rayleigh regime with a vanishing component of incoherently scattered light that is limited in its linewidth by the lifetime of the excited state of the two-level system. Filtering the emission in the order of the Fourier limited linewidth leads to the loss of antibunching in the correlation measurement. Our theoretical model identifies two-photon interference between the coherent and incoherently scattered light as the origin of antibunching of the unfiltered emission. This prefigures schemes to achieve a source of single photons with sub-natural linewidth.