  We describe a cavity-enhanced spontaneous parametric down-conversion
(CE-SPDC) source for narrowband photon pairs with filters such that over 97% of
the correlated photons are in a single mode of 4.3(4) MHz bandwidth. Type-II
phase matching, a tuneable-birefringence resonator, MHz-resolution pump tuning,
and tuneable Fabry-Perot filters are used to achieve independent signal and
idler tuning. We map the CE-SPDC spectrum using difference frequency generation
to precisely locate the emission clusters, and demonstrate CE-SPDC driven
atomic spectroscopy. The generated photon pairs efficiently interact with
neutral rubidium, a well-developed system for quantum networking and quantum
simulation. The techniques are readily extensible to other material systems.
