more often than the second one.

We measure the photon-triplet events using two superconducting nanowire single photon detectors (SNSPDs) with $\eta_\mathrm{det,i1}=0.6$ and $\eta_\mathrm{det,i2}=0.7$ of detection efficiency (Opus One, Quantum Opus/PicoQuant Photonics North America Inc.), as well as one InGaAs avalanche photo diode with $\eta_\mathrm{det,s1}=0.25$ (ID230-SMF-FR, ID Quantique SA) in conjunction with a time-tagging module. Optical and electronic path differences between the three detectors have been compensated for, such that the expected, time-correlated detection events can be registered at around zero delay with respect to each other. The measurements have been performed for $11.5$ hours with very high stability. This is indicated by the plot in Fig. \[stability\], where we show the relative change of the idler 1 count rate with respect to the average value. We chose the idler 1 detection events for monitoring the stability, because they occur orders of magnitudes more often and with significantly better signal-to-noise ratios than secondary PDC detection events. The single event rates of signal 2 and idler 2 are orders of magnitudes lower than the respective detector noise count rates and have not been considered as stability indicators.

The continuous-wave-equivalent pump power of $P_\mathrm{pump}=\left(10.0\pm0.1\right)\,\mu\mathrm{W}$ at a repetition rate of $10\,\mathrm{MHz}$ corresponds to a 