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International Journal of Modern Physics D

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

Dezhi Zhang, Haixing Miao

Published Jul 26, 2025• 0 citations
Publisher

Abstract

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.

DOI
10.1142/s0218271825420040

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Laser interferometers are quantum instruments. This chapter presents the quantum theory of laser interferometer gravitational wave detectors. We show the basics for analysing the quantum noise in the detector, and for deriving the associated standard quantum limit (SQL) for the sensitivity. By providing different perspectives on the origin of the SQL, we illustrate the motivations behind different approaches for surpassing the SQL. Introduction The most difficult challenge in building a laser interferometer gravitational wave (GW) detector is isolating the test masses from the rest of the world (e.g. random kicks from residual gas molecules, seismic activities, acoustic noises, thermal fluctuations, etc.) whilst keeping the device locked around the correct point of operation (e.g. pitch and yaw angles of the mirrors, locations of the beam spots, resonance condition of the cavities, and darkport condition for the Michelson). Once all these issues have been solved, we arrive at the issue that we are going to analyse in this chapter: the fundamental noise that arises from quantum fluctuations in the system. A simple estimate (following the steps of Braginsky, 1968) will already lead us into the quantum world – as it will turn out, the superb sensitivity of gravitational wave detectors will be constrained by the standard quantum limit (SQL), which relates to the fundamental heisenberg uncertainty principle . Further improvements of detector sensitivity beyond this require us to manipulate the quantum coherence of light to our advantage. In this chapter, we will introduce how to analyse GW detectors quantum mechanically, and will describe several advanced configurations to surpass the SQL.
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