Abstract

A century after the prediction of the existence of gravitational waves by A. Einstein and after over fifty years of experimental efforts, gravitational waves have been detected at Earth directly. This result is a major achievement and opens new prospectives for the exploration of our universe. Gravitational waves carry different and complementary information about the source with respect to electromagnetic signals. In particular the first detection demonstrated the existence of stellar-mass black holes, binary systems of black holes and their coalescence. The detection was made by the LIGO instruments which are twin kilometer-scale Michelson interferometers in the US. These detectors represent the second generation of gravitational wave interferometers and, for the first time, they achieved the outstanding strain sensitivity of 10^(-23) Hz^(-1/2) between 90Hz and 400Hz. In the next months the LIGO network will be joined by another second generation detector: Advanced Virgo located near Pisa, Italy. The sensitivity of these advanced detectors is set by different noise sources. In particular, in the low frequency range (below 100Hz) major contributions come from thermal noises, gravity gradient noise and radiation pressure noise; instead, the high frequency band (above 100-200Hz) is dominated by shot noise. Quantum noise (radiation pressure and shot noise) is expected to dominate the detector sensitivity in the whole frequency band at the final target laser input power. To decrease the shot noise while increasing the radiation-pressure noise, or vice-versa, Caves \cite{Caves1981} proposed in 1981 the idea of the squeezed-state technique. The LIGO collaboration demonstrated for the first time in 2011 that the injection of a squeezed vacuum state into the dark port of the interferometer can reduce the shot noise due to the quantum nature of light. This result was achieved with the German-British interferometer GEO600 and was replicated in 2013 with the LIGO interferometer at Livingston. After these results, the LIGO collaboration have pursued further the research in the squeezed-state technique which is considered mandatory for third generation of ground based interferometric detectors. In 2013, the Virgo collaboration started developing the squeezed-state technique. The subject of my thesis is the realization of a prototype of frequency independent squeezed vacuum state source to be injected in Advanced Virgo. This prototype is developed in collaboration with other Virgo groups.