  Since the first quantum ghost imaging (QGI) experiment in 1995, many QGI
schemes have been put forward. However, the position-position or
momentum-momentum correlation required in these QGI schemes cannot be
distributed over optical fibers, which limits their large geographical
applications. In this paper, we propose and demonstrate a scheme for long
distance QGI utilizing frequency correlated photon pairs. In this scheme, the
frequency correlation is transformed to the correlation between the
illuminating position of one photon and the arrival time of the other photon,
by which QGI can be realized in the time domain. Since frequency correlation
can be preserved when the photon pairs are distributed over optical fibers,
this scheme provides a way to realize long-distance QGI over large geographical
scale. In the experiment, long distance QGI over 50 km optical fibers has been
demonstrated.
