Resolution of quantum and classical ghost imaging

The quantum ghost imaging phenomena, experimentally demonstrated a decade ago, exploited the apparent spooky action at a distance of entangled photon pairs and offered a novel approach toward imaging. Can ghost imaging effects be produced by "classical" light sources, such as separable systems of photon pairs or thermal light? If so, can these sources achieve the same accuracy achieved by entangled states? In order to answer these questions, we formulate the different physics behind entangled and separable systems in terms of a set of inequalities derived from the historical argument of Einstein, Podolsky, and Rosen. We show that the ghost images produced by separable sources are subject to the standard statistical limitations. However, entangled states offer the possibility of overcoming such limitations. Imaging can, therefore, achieve its fundamental limit through the high spatial resolution and nonlocal behavior of entangled systems.