  Light with high mutual correlations at different frequencies can be used to
create heralded single-photon sources, which may serve as the basic elements of
existing quantum cryptography and quantum teleportation schemes. One of the
important examples in natural systems of light with high mutual correlations is
the light produced by spontaneous Raman scattering on an ensemble of molecules.
In this paper, we investigate the possibility of using Raman light to create a
heralded single-photon source. We show that when using Stokes scattered light
for postselection of an anti-Stokes scattered light, the latter may posses the
single-photon properties. We analyze the influence of the various negative
factors on the characteristics of such a heralded single-photon source, which
include a time delay between Stokes and anti-Stokes photons, the finiteness of
the correlation radius of an external source, and background radiation. We show
that the high purity of the single-photon source is preserved even when the
flow of uncorrelated photons exceeds the flow of correlated photons in the
scattered Raman light by an order of magnitude.
