Spatially addressable readout and erasure of an image in a gradient echo memory

We show that portions of an image written into a gradient echo memory can be individually retrieved or erased on demand, an important step toward 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. (paper)