  Non-classical states of light find applications in enhancing the performance
of optical interferometric experiments, with notable example of gravitational
wave-detectors. Still, the presence of decoherence hinders significantly the
performance of quantum-enhanced protocols. In this review, we summarize the
developments of quantum metrology with particular focus on optical
interferometry and derive fundamental bounds on achievable quantum-enhanced
precision in optical interferometry taking into account the most relevant
decoherence processes including: phase diffusion, losses and imperfect
interferometric visibility. We introduce all the necessary tools of quantum
optics as well as quantum estimation theory required to derive the bounds. We
also discuss the practical attainability of the bounds derived and stress in
particular that the techniques of quantum-enhanced interferometry which are
being implemented in modern gravitational wave detectors are close to the
optimal ones.
