is negligible.

![Measured three-fold coincidences around the expected arrival times (left). The central peak contains three-fold coincidences, which overcome the average noise background by a manifold and indicate strong time-correlations. We infer raw three-fold coincidence rates of $33$ per 11.5 hours. Note that we merged the acquired data to time bins of $\tau_\mathrm{16bin}=\left(1.317\pm0.002\right)\,\mathrm{ns}$, in order to take the joint timing jitter of the detection apparatus into account. Comparison of the absolute bin occupation in two temporal dimensions for a large analysis window (right). Our pump laser runs at a peak-to-peak repetition time of $100\,\mathrm{ns}$. The expected photon triplets reside in the time bin at around zero time delay between the three detectors. Neighboring three-fold coincidences with significantly lower absolute frequency are also present at timing distances of multiple integers of the inverse laser repetition rate. These accidental triple-coincidences represent the impact of higher-order photons and nonlinear Cherenkov-type PDC on the measurement. Note that we merged our data to time bins of around $10.5\,\mathrm{ns}\times10.5\,\mathrm{ns}$ for improved visualization. The adjacent bar charts stem from the cross-cuts along the white dotted lines in the color-coded graph.[]{data-label="16-bin"}](fig04.eps "fig:"){width="0.42\linewidth"} ![Measured three-fold coincidences around the expected arrival times (left). The central peak contains three-fold coincidences, which 