  The production of pairs of entangled photons simply by focusing a laser beam
onto a crystal with a non-linear optical response was used to test quantum
mechanics and to open new approaches in imaging. The development of the latter
was enabled by the emergence of single photon sensitive cameras able to
characterize spatial correlations and high-dimensional entanglement. Thereby
new techniques emerged such as the ghost imaging of objects - where the quantum
correlations between photons reveal the image from photons that have never
interacted with the object - or the imaging with undetected photons by using
nonlinear interferometers. Additionally, quantum approaches in imaging can also
lead to an improvement in the performance of conventional imaging systems.
These improvements can be obtained by means of image contrast, resolution
enhancement that exceed the classical limit and acquisition of sub-shot noise
phase or amplitude images. In this review we discuss the application of quantum
states of light for advanced imaging techniques.
