  We experimentally demonstrate a spectral compression scheme for heralded
single photons with narrow spectral bandwidth around 795 nm, generated through
four-wave mixing in a cloud of cold Rb-87 atoms. The scheme is based on an
asymmetric cavity as a dispersion medium and a simple binary phase modulator,
and can be, in principle, without any optical losses. We observe a compression
from 20.6 MHz to less than 8 MHz, almost matching the corresponding atomic
transition.
