LIGO Document P2200287-v1

[Thesis] Catching the Waves in the Sea of Noise: Squeezed Light in Advanced LIGO and Beyond

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In 2015, LIGO’s first gravitational wave detection opened up a new window to the universe and marks the beginning of the gravitation-wave astronomy era. In 2017, the first binary neutron star coalescence with electromagnetic counterparts was detected and marked the beginning of multi-messenger astronomy. The advanced LIGO detector has become so sensitive that its performance is broadly limited by quantum noise – the random behavior of photons that are dictated by the Heisenberg Uncertainty Principle called shot noise. Even though the idea of injecting squeezed light to help manipulate the quantum noise in LIGO was first proposed in 1981, it took nearly 4 decades before the squeezed light source was integrated as part of a normal operation in advanced LIGO. This thesis firstly highlights work done on the squeezed light source, sensing and control implementation at LIGO Hanford. The result is a robust system that runs 100% of the detector observation time and an improvement of ∼3 dB in the detector sensitivity. With ∼50% more of the universe volume covered scientists were able to detect new events such as a black hole-neutron star coalescence and a mysterious astronomical object that falls within the mass-gap region.
The squeezed light source installed in advanced LIGO is frequency independent. Shot noise improvement at high frequency was achieved at an expense of quantum noise performance at low frequency, the radiation pressure noise. In 2020-2021, the aLIGO detector is being upgraded to an A+ detector with the goal of 6 dB squeezing. A 300-meter long filter cavity will be integrated as part of the squeezed light system to allow for frequency-dependent squeezing. This thesis secondly proposes an alternative sensing and control scheme for the squeezed light source that utilizes the filter cavity, resulting in a better phase noise performance as both phase noise and optical losses can degrade the amount of squeezing the detector can achieve.
LIGO detectors were designed to be sensitive to the coalescence of black holes and neutron stars at the 100 Hz frequency range. However, important information on matter at extreme density can be found at the 1 kHz frequency range. A smaller scale detector such as the Neutron Star Extreme Matter Observatory (NEMO) has been proposed in order to cover more of the gravitational wave spectrum. This thesis thirdly discusses the idea of using a twin-signal-recycling interferometer technique combined with internal squeezing to enhance the performance of a kHz scale detector. The final part of this thesis demonstrates a table-top twin-signal-recyling interferometer with a newly designed sensing and control scheme that is compatible with the low noise requirement of the internal squeezing. The experiment marks the first step towards an internal squeezing inside a twin-signal-recycling interferometer experiment.
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