SQUEEZED LIGHT FOR ADVANCED GRAVITATIONAL WAVE INTERFEROMETER

The sensitivity of gravitational wave interferometric detectors is ultimately limited by the quantum noise, which arises from the quantum nature of light and it is driven by vacuum fluctuations of the optical field entering from the dark port of the interferometer. Quantum optics methods, in particular the injection of squeezed states of light in the interferometer, are applied to reduce quantum noise. Vacuum fluctuations determine two complementary effects: the shot-noise and the radiation-pressure noise. The former depends on phase fluctuations of the optical field and dominates at high frequencies. The latter depends on amplitude fluctuation of the optical field and dominates at low frequencies. Injecting Frequency-Independent Squeezed vacuum (FIS) into the dark port, a reduction of phase fluctuations is achieved, but at the same time, due to the Heisenberg’s uncertainty principle, the amplitude fluctuations are larger, therefore we need to produce Frequency-Dependent Squeezing (FDS) in order to achieve a broadband mitigation of quantum noise. During the last observing run (O3), FIS up to 3.2 dB was injected into the interferometer, improving the sensitivity curve at high frequencies. For the next observing run (O4), it is necessary to mitigate quantum noise in the entire sensitive bandwidth. The simplest method to produce FDS, which was installed for the ongoing upgrades in Virgo and LIGO detectors, is to reflect the frequency-independent squeezed vacuum though an optical filter cavity. Another technique to achieve quantum noise reduction over the entire sensitivity band exploits the Einstein-Podolsky-Rosen (EPR) entanglement. With this technique two entangled beams, at different frequencies, would be injected into the dark port of the interferometer, which would then be used both as a filter cavity and as a GW detector. I contributed to both FDS experiment for Advanced Virgo Plus and an EPR experiment for possible application in future gravitational wave detectors.