  We report the demonstration of a sensitive absolute gravity gradiometer based
on light-pulse atom interference techniques. The gradiometer consists of two
absolute accelerometers operated in a differential mode. We report a
differential acceleration sensitivity of 4e-9 g/Hz^(1/2) and an inferred
differential acceleration accuracy of less than 1e-9 g. This corresponds to a
gravity gradient sensitivity of 4 E/Hz^(1/2) [1 E = 1e-9 s^(-2)] and an
accuracy of better than 1 E for a 10 m separation between accelerometers. We
demonstrate that the instrument can be used to detect nearby masses in a
vibrationally noisy environment and characterize instrument sensitivity to
spurious acceleration and rotation noise.
