  The magnification bias of large-scale structures, combined with galaxy
biasing, leads to a cross-correlation of distant quasars with foreground
galaxies on angular scales of the order of arc minutes and larger. The
amplitude and angular shape of the cross-correlation function w_QG contain
information on cosmological parameters and the galaxy bias factor. While the
existence of this cross-correlation has firmly been established, existing data
did not allow an accurate measurement of w_QG yet, but wide area surveys like
the Sloan Digital Sky Survey now provide an ideal database for measuring it.
However, w_QG depends on several cosmological parameters and the galaxy bias
factor. We study in detail the sensitivity of w_QG to these parameters and
develop a strategy for using the data. We show that the parameter space can be
reduced to the bias factor b, Omega_0 and sigma_8, and compute the accuracy
with which these parameters can be deduced from SDSS data. Under reasonable
assumptions, it should be possible to reach relative accuracies of the order of
5%-15% for b, Omega_0, and sigma_8. This method is complementary to other
weak-lensing analyses based on cosmic shear.
