  One of the most surprising consequences of quantum mechanics is the
entanglement of two or more distant particles. Although questions regarding
fundamental issues of quantum theory still exist, quantum entanglement has
started to play important roles in practical engineering applications. Quantum
imaging is one of these exciting areas. Quantum imaging has demonstrated two
peculiar features: (1) reproducing "ghost" images in a "nonlocal" manner, and
(2) enhancing the spatial resolution of imaging beyond the diffraction limit.
In this article, we start with the review of classical imaging to establish the
basic concepts and formalisms of imaging. We then analyze two-photon imaging
with particular emphasis on the physics of spatial resolution enhancement and
the "ghost" imaging phenomenon.
