Low random duty-cycle errors in periodically-poled KTP revealed by sum-frequency generation

Single photons from nitrogen-vacancy centers in diamond are a promising building block in the realisation of quantum networks [1] [2] . However quantum frequency conversion (QFC) [3] of these red single photons remains an indispensable requirement, allowing the scale of potential networks to achieve distances accessible with telecom fibers [4] . QFC can be realised in a periodically-poled nonlinear crystal with a χ (2) -nonlinearity using sum-frequency or difference-frequency generation (SFG/DFG). A major problem for scenarios where the pump wavelength is shorter than the target wavelength is the parasitic spontaneous parametric down-conversion of pump photons, whose efficiency is increased by fabrication errors of the periodic poling: It was shown by [5] that these fabrication errors, namely random duty cycle errors [6] , introduce a pedestal in the phase-matching curve at detunings far from the design wavelengths.