Commissioning of Quantum Noise Reduction for AdV+ and Study of Non-Equilibrium Thermal Noise

The pioneering detection of gravitational waves has opened a new era of astronomy. Since the first detection in 2015, interferometric gravitational wave detectors have undergone several upgrades to improve sensitivity and be able to detect more events. These efforts paid off during the last scientific observation (O3) conducted by the LIGO-Virgo collaboration, with the detection of about one gravitational wave event per week with 74 potential candidates in less than a year. For the following scientific observation (O4), a planned two-year break was set to bring about significant improvements in the sensitivity of gravitational wave interferometers across their entire frequency spectrum. One crucial aspect to address is quantum noise. During O3, quantum noise was partially mitigated in the two Advanced LIGO and in Adv Virgo through the application of Frequency Independent Squeezing (FIS), which aimed to reduce noise in the high-frequency range. However, to tackle broadband quantum noise, the implementation of Frequency-Dependent Squeezing (FDS) is necessary. FDS involves the utilization of a detuned cavity, enabling the rotation of the squeezing ellipse to suppress the quantum noise at the same time both at low and high frequencies. The first part of this thesis delves into the technical aspects of the FDS technique and its integration into the Virgo interferometer. After an initial theoretical introduction to understand how squeezed states can improve interferometer sensitivity, an overview of the Quantum Noise Reduction system (QNR) in Virgo is provided. The work done for this thesis focuses on the methods employed to create an efficient and stable stand-alone FDS and on the characterization of the system, in particular, the losses that can affect the SQZ injection. Finally, the last section of this first part documents the efforts for the first application of FIS injection as a preliminary step during the interferometer’s commissioning for the upcoming O4 scientific run. Another source of noise that limits the sensitivity of gravitational wave interferometers in the low and medium frequency region is thermal noise. It arises from the intrinsic fluctuations within the materials constituting the interferometer’s components like the suspension fibers and the mirrors. This thesis focuses on the implications of relaxing the standard assumption of thermodynamic equilibrium for the parts contributing to thermal noise. The second part of this thesis details an experimental investigation designed to measure thermal noise both in thermodynamic equilibrium and out of equilibrium. The experiment is focused on measuring the thermal noise of a mechanical piece through its longitudinal resonance mode, in and out of equilibrium due to thermal differences between the extremes. The oscillator motion is recorded with an interferometric readout. After a description of the experiment, all the work done to improve the experiment and its calibration is presented, including the reduction of various noise sources. Finally, thermal noise measurements at thermodynamic equilibrium are shown