Natural and man-made in quantum imaging

As in the case of quantum information and communication, in the field of quantum imaging entanglement represents an invaluable resource. Processes such as parametric down-conversion (PDC) and four-wave mixing provide natural sources of quantum entangled beams endowed with spatial entanglement properties that are useful in quantum imaging. The authors will review these well known features in the case of PDC, and the technique which allows for an imaging with a sensitivity beyond the standard quantum limit, which has been theoretically predicted and experimentally demonstrated.