  Low-noise quantum frequency conversion in periodically-poled nonlinear
crystals has proved challenging when the pump wavelength is shorter than the
target wavelength. This is - at least in large part - a consequence of the
parasitic spontaneous parametric down-conversion of pump photons, whose
efficiency is increased by fabrication errors in the periodic poling. Here we
characterise the poling quality of commercial periodically-poled bulk potassium
titanyl phosphate (ppKTP) by measuring the sum-frequency generation (SFG)
efficiency over a large phase mismatch range from 0 to more than 400$\pi$. Over
the probed range the SFG efficiency behaves nearly ideally and drops to a
normalised efficiency of $10^{-6}$. Our results demonstrate that any background
pedestal which would be formed by random duty cycle errors in ppKTP is
substantially reduced when compared to periodically poled lithium niobate. The
standard deviation of the random duty cycle errors can be estimated to be
smaller than 2% of the domain length. From this, we expect a noise spectral
density which is at least 20 times smaller than in current state-of-the-art
frequency converters.
