quantum optical applications. By contrast to schemes that utilize continuous-wave pump lasers, we deploy pump pulses, which makes our source compatible to synchronized quantum network architectures.

Device design and theoretical implications {#sec:device}
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We designed and fabricated an integrated device, which is illustrated and explained in Fig. \[triplet-chip\]. A coupled-waveguide structure of constant waveguide width and based on titanium-diffusion [@Schmidt1974; @Krapick2013] has been introduced to a $76\,\mathrm{mm}$ long lithium niobate chip. In front of and behind the integrated wavelength division multiplexer (WDM) we have implemented two differently poled structures, which act as guided-wave PDC sources, by periodically inverting the nonlinear susceptibility using a pulsed electric-field-poling technique. Although our waveguides in principle support both polarizations, we restrict ourselves here to PDC processes, where only TM modes are involved, since the highest nonlinear coefficient $d_{33}$ can be deployed for both down-conversions. The choice of only one polarization also implies that the designed integrated WDM has to act only as a wavelength demultiplexer, but not as a polarization-wavelength-splitting element.

The actual photon-triplet generation process is considered as follows: picosecond pulses at $\lambda_\mathrm{p}=532\,\mathrm{nm}$ serve as the pump and drive the first type-0 PDC process in order to produce pairs comprising signal 1 photons (s1) 