A sub nrad beam pointing monitoring and stabilization system for   controlling input beam jitter in GW interferometers
  In this paper a simple and very effective control system to monitor and suppress the beam jitter noise at the input of an optical system, called Beam Pointing Control (BPC) system, will be described showing the theoretical principle and an experimental demonstration for the application of large scale gravitational wave interferometers, in particular for the Advanced Virgo detector. For this purpose the requirements for the control accuracy and the sensing noise will be computed by taking into account the Advanced Virgo optical configuration and the outcomes will be compared with the experimental measurement obtained in the laboratory. The system has shown unprecedented performance in terms of control accuracy and sensing noise. The BPC system has achieved a control accuracy of ~ $10^{-8}$ rad for the tilt and ~ $10^{-7}$ m for the shift and a sensing noise of less than 1 nrad/$\sqrt{Hz}$ resulting compliant with the Advance Virgo gravitational wave interferometer requirements. 
Benjamin Canuel, Eric Genin, Maddalena Mantovani, Julien Marque, Paolo   Ruggi and Matteo Tacca
physics.optics