  We analyze spontaneous parametric down-conversion in various experimentally
feasible 1D quadratic nonlinear waveguide arrays, with emphasis on the
relationship between the lattice's topological invariants and the biphoton
correlations. Nontrivial topology results in a nontrivial "winding" of the
array's Bloch waves, which introduces additional selection rules for the
generation of biphotons. These selection rules are in addition to, and
independent of existing control using the pump beam's spatial profile and phase
matching conditions. In finite lattices, nontrivial topology produces single
photon edge modes, resulting in "hybrid" biphoton edge modes, with one photon
localized at the edge and the other propagating into the bulk. When the single
photon band gap is sufficiently large, these hybrid biphoton modes reside in a
band gap of the bulk biphoton Bloch wave spectrum. Numerical simulations
support our analytical results.
