  Ghost imaging enables the imaging of an object using intensity correlations
between a single-pixel detector placed behind the object and a camera that
records light that did not interact with the object. The object and the camera
are often placed at conjugate planes to ensure correlated illumination
patterns. Here, we show how the combined effect of optical reciprocity and the
memory effect in a random medium gives rise to correlations between two beams
that traverse the random medium in opposite directions. Using these
correlations, we demonstrate a ghost imaging scheme in which the object and
camera are placed at opposite ends of the random medium and illuminated by
counter-propagating beams that can potentially be emitted by two different
sources.
