  Optical quantum interferometry represents the oldest example of quantum
metrology and it is at the source of quantum technologies. The original
squeezed state scheme is now a significant element of the last version of
gravitational wave detectors and various additional uses have been proposed.
  Further quantum enhanced schemes, from SU(1,1) interferometer to twin beam
correlation interferometry, have also reached the stage of proof of principle
experiments enlarging the field of experimental quantum interferometry and
paving the way to several further applications ranging from Planck scale
signals search to small effects detection. In this review paper I introduce
these experimental achievements, describing their schemes, advantages,
applications and possible further developments.
