title: Two-mode Squeezed States for Quantum-Enhanced Interferometric Gravitational Wave Detectors
Creators: name: Gould, Dan
Publisher: name: The Australian National University
Year: 2024

description: The scientific potential of gravitational-wave detection was exemplified by GW170817, the first detected gravitational waves from the inspiral and merger of two neutron stars. Thanks to the early warning provided by gravitational-wave detectors, this event was observed across a broad spectrum of electromagnetic radiation, setting a gold standard for multi-messenger observation. However, the scientific progression of gravitational-wave astrophysics and astronomy is constrained by the sensitivity of the existing network of ground-based detectors. Quantum fluctuations of the electromagnetic field set a fundamental noise limit to the sensitivity of current interferometric gravitational-wave detectors. This noise can be mitigated by forming correlations in the vacuum state entering the dark port of the interferometer, specifically by replacing the uncorrelated vacuum state with a squeezed vacuum state. The application of such a quantum state has been successfully demonstrated and the technique is now employed across the interferometric gravitational-wave detector network. Future gravitational-wave detectors will be designed such that they can best take advantage of modern quantum state and squeezed state technologies. Two-mode squeezed states, identified as a promising resource for manipulating quantum noise, exploit an entanglement engineered between two distinct modes. This approach requires different methodologies from those used with current squeezing techniques. Compared to their widespread analysis and application in areas such as quantum information processing and quantum communication, two-mode squeezed states and entanglement are underutilised in sensing and metrology. This thesis investigates possible applications of two-mode squeezed states to gravitational-wave detection. Findings are presented from two independent tabletop experiments, demonstrating experimental methodologies and discussing the potential for quantum noise reduction via quantum entanglement. A key component of broadband quantum noise reduction in current interferometric gravitational-wave detectors is the production of frequency-dependent squeezing. Although typically produced with the additional infrastructure of a filter cavity, frequency-dependent squeezing can alternatively be produced using a two-mode squeezed state and the interferometer itself. This process requires the calculation and implementation of an optimal filter to maximize the reduction of quantum noise. The first experiment in this thesis demonstrates, through optimal digital filtering of a two-mode entangled state, a 2 dB reduction in quantum noise. Furthermore, it presents the analogous configuration that would provide optimal quantum noise reduction in a gravitational-wave detector. Several studies have highlighted promising prospects for the application of two-mode squeezing in twin-interferometer topologies. These topologies have been proposed for both gravitational-wave detection and other tests of fundamental physics. The second experiment of this thesis demonstrates the use of a two-carrier heterodyne readout applied to a twin-Michelson interferometer. This readout method circumvents the inherent 3 dB signal-to-noise penalty of conventional heterodyne detection and, with the application of a two-mode squeezed state, reports a further 3.5 dB signal-to-noise improvement. This thesis discusses potential applications of two-mode squeezed states to ground-based gravitational-wave detectors. Such states offer a competitive means to reduce the quantum noise in future gravitational-wave detectors. They furthermore provide unique methods with which signals can be extracted and measured against reduced quantum noise floors. With further studies, the experimental application of two-mode squeezed states will undoubtedly offer unique ways to combine techniques from quantum information and communication with quantum-limited metrology.