  Generating photon pairs via spontaneous parametric down-conversion (SPDC) in
nonlinear crystals is important for a wide range of quantum optics experiments
with spectral properties such as their bandwidths often being a crucial
concern. Here, we show the generic existence of particular phase-matching
conditions in quasi-phase matched KTP, MgO:LN and SLT crystals that lead to
ultra-broadband, widely non-degenerate photon pairs. It is based on the
existence of group-velocity matched, far apart wavelength pairs and for 2 mm
long crystals results in SPDC bandwidths between 15 and 25 THz (FWHM) for
photon pairs with the idler photon in the technologically relevant mid-IR range
3-5 {\mu}m and the signal photon in the NIR below 1100 nm. We experimentally
demonstrate this type of broadband phase-matching in ppKTP crystals for photon
pairs centered at 800 nm and 3800 nm and measure a bandwidth of 15 THz. This
novel method of generating broadband photon-pairs will be highly beneficial for
SPDC-based imaging, spectroscopy, refractometry and OCT with undetected mid-IR
photons.

Quantum Physics;Optics