  We show that portions of an image written into a gradient echo memory can be
individually retrieved or erased on demand, an important step towards
processing a spatially multiplexed quantum signal. Targeted retrieval is
achieved by locally addressing the transverse plane of the storage medium, a
warm 85Rb vapor, with a far-detuned control beam. Spatially addressable erasure
is similarly implemented by imaging a bright beam tuned near the 85Rb D1 line
in order to scatter photons and induce decoherence. Under our experimental
conditions atomic diffusion is shown to impose an upper bound on the effective
spatial capacity of the memory. The decoherence induced by the optical eraser
is characterized and modeled as the response of a two level atom in the
presence of a strong driving field.
