an anti-bunching at zero time delay, proving the single-photon nature of the resonance fluorescence. The anti-bunching at zero delay is surrounded by a bunching at a non-zero time delay. This effect is caused by the onset of Rabi-oscillations under strong resonant driving. The cross-correlation (green curve) differs from the auto-correlation in two main respects: The $g^{(2)}(\tau)$ is a slightly asymmetric function of $\tau$ and has a time-offset towards positive $\tau$. We explain these features following Fig. \[fig:crosscorr\](d): After the emission of a radiative Auger photon, the second electron is located in a higher shell. Before re-excitation of the trion can take place, this electron has to relax down to the $s$-shell – in contrast to the resonance fluorescence where re-excitation is immediately possible. By comparing auto- and cross-correlation, we determine the relaxation time for an isolated electron to be $\tau_{\text{p}}\simeq85\ \text{ps}$. The time-scale of the electron relaxation is comparable to numbers reported for weak nonresonant excitation [@Ohnesorge1996; @Kurtze2009]. The relaxation might be caused by a multi-phonon emission process [@Li1999]. We stress the advantage of the present method: the radiative Auger process leaves a single electron in a higher shell; the hole has disappeared; the chance of creating electrons or holes 