Stabilization of the squeezed vacuum source for the Virgo interferometer

In this thesis an alternative method to stabilize the phase of a squeezed vacuum field in the framework of the Virgo gravitational wave detector is designed and realized. A brief introduction about the nature of gravitational waves and their detection is presented in chapter 1, with particular attention to the actual sensitivity limitations on the interferometers employed. Chapter 2 focuses on the quantum nature of the electromagnetic field. A nonclassical state of the light, the squeezed state, is described, as well as how it is is produced and how can be observed. This kind of radiation field is employed to increase the sensitivity of the detectors beacause with its injection into the interferometer the quantum noise that affects the measure is partially reduced. In order to avoid any technical noise contamination, a second copropagating and frequency shifted field is used to provide the error signal for the alignment and to allow the phase locking between the interferometer main laser and the squeezed light, as discussed in chapter 3, where the technique called coherent control loop is explained. Finally, in chapter 4, the optical bench and the control electronics realized at LNL to test the stability of the control loop are presented; the system will be implemented in Virgo in the next months. The difference between the method discussed here and that usually employed is that the error signal is not used to correct the path of the squeezed vacuum field, but it is instead used to close a cascade loop on a PLL. Thus all the actuators are electronic and no optical actuators are used, the stray light issue is minimized. The control loop is stronger at low frequencies in order to correct the seismic noise that dominates in this region