Expanding the Reach of Quantum Enhanced Gravitational-Wave Detectors

Abstract

TheAdvancedLIGOdetectorsarethemostprecisedisplacementsensorsevermade,operating at the cutting edge of quantum noise limited sensitivity. The introduction of non-classical squeezed states to reduce quantum shot noise during the third gravitational wave observing run O3 ushered in the era of quantum-enhanced gravitational wave interferometry. This was, however, accompanied by an increase in measurement back-action, in the form of quantum radiation pressure noise which degraded detector sensitivity at low frequencies below 100Hz. In the early 2000s, Kimble et. al. [1] proposed the use of optical filter cavities to prepare frequency dependent squeezed states which circumvent measurement back-action by suppressing radiation pressure noise at low frequencies while continuing to reduce shot noise across the rest of the gravitational wave signal band.

In this thesis, we explore frequency dependent squeezing for gravitational wave detectors, with an emphasis on optimal filter cavity design, and characterization of squeezing in optical systems. We then describe the commissioning of a 300m filter cavity for the first realization of frequency dependent squeezing in gravitational wave interferometer for the fourth gravitational wave observing run O4. Along with significantly enhancing the astrophysical sensitivity of the LIGO detectors, this is also the latest milestone in several decades of research in quantum noise reduction. We conclude the thesis by extending frequency dependent squeezing to alternate interferometer configurations by studying the feasibility of detuning the signal cavity of the interferometer to enhance sensitivity to kilohertz signals from neutron star post-mergers.