Dear Editor,

please find enclosed a manuscript on a systematic evaluation of on parameters
for a scheme to generate correlated photon pairs through four wave mixing in a
cold atomic ensemble. This is a systematic study of relevant experimental
parameters that will hopefully be helpful for anyone operating a similar
narrowband photon pair source. 

Somewhat unexpectedly, we were also able to model some of the pair generation
behavior, in particular the contributions of coherent and incoherent
scattering by a remarkably simple model of three-level atoms back from
1976. This complements recent theoretical models on collective atomic behavior
for photon pair generation.

The manuscript is a detailed follow-up on our earlier work with that
system over the last years; relevant publications on these experiments were
PRL 111, 123602 (2013); PRA 90, 033819 (2014); PRL 113, 163601 (2014); PRA 91,
063829 (2015);  NJP 17, 093034 (2015); Nat. Comm. 7, 13716 (2016);
Opt. Expr. 25, 6294 (2017).

We feel that Physical Review A is an appropriate platform for this systematic
experimental work, and would ask you to consider it as a regular article.

With Best Regards on behalf of all authors,

Christian Kurtsiefer