Experimental quantum enhanced optical interferometry

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 the SU(1,1) interferometer to twin beam correlation interferometry, have also reached the stage of proof of principle experiments, thus enlarging the field of experimental quantum interferometry and paving the way to several additional applications, from Planck scale signals search to small effect detection. In this review, I will describe these experimental achievements, and I will focus on their schemes, advantages, applications, and possible further developments.