  We show how observations of multiply-imaged quasars at high redshift can be
used as a probe of dark matter clumps (subhalos with masses ~ 10^9 solar
masses) within the virialized extent of more massive lensing halos. A large
abundance of such satellites is predicted by numerical simulations of galaxy
formation in cold dark matter (CDM) cosmogonies. Small-scale structure within
galaxy halos affects the flux ratios of the images without appreciably changing
their positions. We use numerical simulations to quantify the effect of dark
matter substructure on the distribution of magnifications, and find that the
magnification ratio of a typical image pair will deviate significantly from the
value predicted by a smooth lensing potential if, near the Einstein radius,
only a few percent of the lens surface density is contained in subhalos. The
angular size of the continuum source dictates the range of subclump masses that
can have a detectable effect: to avoid confusion with gravitational
microlensing caused by stars in the lens galaxy, the background source must be
larger than the optical continuum-emitting region of a QSO. We also find that
substructure will cause distortions to images on milli-arcsecond scales and
bias the distribution of QSO magnification ratios -- two other possible methods
of detection.
