Quantum enhancement of gravitational-wave detectors: A fundamental quantum limit framework

Advanced gravitational-wave detectors, which have enabled groundbreaking discoveries, are large-scale laser interferometers. They rank among the most sensitive instruments ever conceived, so sensitive that even the minuscule quantum fluctuations of light become a primary limiting factor by introducing quantum noise. In this paper, we provide an overview of various quantum techniques developed to surpass these quantum limits. We demonstrate how these techniques can be interpreted and categorized within a unified framework defined by the fundamental quantum limit, which is determined by the quantum fluctuations of the optical energy stored within the interferometer. This framework not only facilitates comparisons between the ultimate sensitivities achievable by different interferometer configurations but also supports the systematic generation of new configurations that coherently integrate multiple quantum-enhancement techniques. Through further refinement and dedicated experimental efforts, these quantum technologies will pave the way for constructing gravitational-wave detectors with unprecedented sensitivity.