  Parametric nonlinear optical processes are instrumental in optical quantum
technology for generating entangled light. However, the range of materials
conventionally used for producing entangled photons is limited. Metal-organic
frameworks (MOFs) have emerged as a novel class of optical materials with
customizable nonlinear properties and proven chemical and optical stability.
The large number of combinations of metal atoms and organic ligand from which
bulk MOF crystals are known to form, facilitates the search of promising
candidates for nonlinear optics. To accelerate the discovery of next-generation
quantum light sources, we employ a multi-scale modeling approach to study
phase-matching conditions for collinear degenerate type-II spontaneous
parametric down conversion (SPDC) with MOF-based one dimensional waveguides.
Using periodic-DFT calculations to compute the nonlinear optical properties of
selected zinc-based MOF crystals, we predict polarization-entangled pair
generation rates of $\sim 10^3-10^6$ s$^{-1}$mW$^{-1}$mm$^{-1}$ at 1064 nm,
which are comparable with industry materials used in quantum optics. We find
that the biaxial MOF crystal Zn(4-pyridylacrylate)$_2$ improves two-fold the
conversion efficiency over a periodically-poled KTP waveguide of identical
dimensions. This work underscores the great potential of MOF single crystals as
entangled light sources for applications in quantum communication and sensing.
