Additionally, in order to confirm that the cause of the low visibility was not jitter, we performed a HOM dip using only the modelocked laser as the common pump of both SPDC sources, with no synchronization. In this setup there is no synchronization jitter from the pump photons. The resulting visibility and dip widths were the same. 5 These two results combined lead to the conclusion that the loss of visibility is not caused by jitter and our media converter synchronization works as intended. Other possible reasons for the low visibility have been investigated. The probability of creating a pair per pulse has been set to approximately 10% in both sources, so the production of extra pairs should increase the visibil- ity by at most 0.9 %. Spectral undistinguishability has been measured with careful characterization of the fil- ters. The difference between their spectral width leads to a decrease in visibility of at most 1% and their cen- tral wavelength has been shown to be stable over many weeks. Pair production statistics has been measured, fol- lowing the theory presented in [31], and the results show that stimulated emission occurs as expected. A different result would have led to an increase of visibility. Walk- off in the waveguide due to group velocity dispersion has been calculated to be 7.3 ps, entailing a visibility reduc- tion of 0.2 %. Polarization is adjusted using polarization controllers and a polarizer, for an estimated visibility re- duction of 3 %. The total visibility, taking into account all these sources of distinguishability in the worst sce- nario, should have been at least 0.306, far from the ex- perimental result. We note that our results are comparable to those of other groups working with waveguides, for example Laiho et al. working with periodically poled potassium titanyl phosphate (PPKTP) waveguides and a common source also achieved a 78% maximal fidelity [32]. Such visibil- ities are sufficient to demonstrate nonlocality [33]. On the other hand, some of us achieved a 99% net visibility using separated PPLN waveguides pumped by a common modelocked laser [34]. where λ$_{0}$ is the central wavelength and Δ λ is the spectral FWHM. In order to increase the coherence length and thus effectively reduce r$_{j}$ , a small Δ λ is necessary. The count rates are recorded using a time-to-digital converter (TDC) both for true coincidences ( ∼ 1 . 4 s − $^{1}$) and for photons which did not arrive at the same time at the beamsplitter, allowing a measure of the noise due to unsynchronized dark counts. The noise is substracted and the resulting dip (Fig. 11) has a net visibility of V = 0 . 26 ± 0 . 011 and a width corresponding to 126 ± 8 ps. Note that the maximum visibility that can be found is 0.33 because the sources are probabilistic [30], so that we reach 78% of the expected value. In conclusion, we have shown that it is possible to build pulsed photon sources with picosecond pulse lengths us- ing different pumps. It is possible to synchronize two sources to such an extent that the jitter and path length fluctuations are not a cause of loss of visibility. Such sources are required to build quantum repeaters with multiple locations for the photon sources. The loss of visibility cannot be due to inaccurate syn- chronization. Considering the photon’s coherence length and the measured jitter, the width of the dip should have been 173 ps. To fully account for the reduced visibility, a jitter of 128 ps would be needed, which would correspond to a dip width of 209 ps. 7. CONCLUSIONS 6. HOM DIP FIG. 11: Measured HOM dip using two independent and sep- arated sources. FIG. 10: Average visibility as a function of the dimensionless ratio r j and as a function of the timing jitter for the specific case of a 117 ps coherent pulse length.  0.55  0.6  0.65  0.7  0.75  0.8  0.85  0.9  0.95  1  0  0.5  1  1.5  2  0  50  100  150  200 Visibility r$_{j}$ Timing jitter (ps)  0.7  0.75  0.8  0.85  0.9  0.95  1  1.05  1.1  1.15  1.2  16  17  18  19  20  21  22  23  24 arb. Position [cm] Normalized dip = 26 +/- 1 % = 38.0 +/- 2.2 pm