Improving future gravitational-wave detectors using nondegenerate internal squeezing

Abstract

Gravitational waves are "ripples" in spacetime emitted by massive astrophysical events. Over the past decade, interferometric detectors have been used to measure gravitational waves from the binary mergers of black holes and neutron stars to learn more about such systems; these gravitational waves had frequencies around 100 Hz. Other frequencies of gravitational waves are thought to exist and contain valuable information but are yet to be detected. For example, detecting kilohertz (1–4 kHz) gravitational waves from binary neutron-star mergers could be used to further constrain the neutron-star equation-of-state and better understand exotic states of matter. However, to do so, the sensitivity of current detectors will need to be extended from 100 Hz to the kilohertz regime. The kilohertz sensitivity of current gravitational-wave detectors is limited by quantum noise from the fundamental quantum uncertainties in the state of light inside the detector. This noise can be mitigated by replacing the vacuum fluctuations entering the readout port of the detector with squeezed states. In this thesis, I investigate a new technique to improve kilohertz sensitivity by placing a nondegenerate squeezer inside the detector. This technique, called nondegenerate internal squeezing, improves sensitivity by amplifying the detector's response to the gravitational-wave signal more than it increases the quantum noise. To assess its feasibility, I derive an analytic Hamiltonian model of nondegenerate internal squeezing and calculate its sensitivity and stability as well as analyse its tolerance to the realistic optical losses expected in a future gravitational-wave detector. My model indicates that nondegenerate internal squeezing is stable, robust to detection loss in the readout, and provides a viable alternative to other proposals to improve kilohertz sensitivity. I demonstrate a technique to determine its squeezing threshold and, therefore, the limits of its operation. I find that nondegenerate internal squeezing could feasibly improve the sensitivity of a future detector to 1–4 kHz gravitational waves. I also explore an alternative readout scheme that is promising for broadband 0.1–4 kHz sensitivity. Show more