Non-linear parametric processes in quantum information

In recent years, a large number of experiments aimed at the verification of fundamental aspects of quantum mechanics, such as the Bell's inequality test of quantum nonlocality, have been realized by adopting photon particles mutually interacting according the celebrated methods of non-linear optics (NLO). In addition, the sophisticated NLO methods have been extended to relevant investigations and realizations in the domain of the emerging new sciences of Quantum Information (QI) and Quantum Communication. Indeed NLO has proved to be an excellent theoretical and experimental test bench for the novel concepts introduced by QI. The present work is intended to survey the most relevant quantum processes investigated by NLO by focusing on the generation and on the manipulation of entangled photon states by the three-wave NL parametric processes.