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Technical and Theoretical Bridges Between Gravitational Wave Observations and Quantum Information Processing Systems

Mehmet Keçeci

Published Jul 1, 2025• 5 citations
Publisher

Abstract

This study comprehensively examines the profound technological and methodological synergies existing between observations made via interferometric detectors such as LIGO and Virgo, which have revolutionized the field of gravitational wave (GW) astrophysics, and the rapidly advancing quantum computing (QC) technologies. As both disciplines aim to perform measurements pushing the limits of precision, the effective control and mitigation of environmental and quantum-originated noise pose a critical challenge. In this context, the quantum squeezing technique stands out as a fundamental tool in both domains, employed to reduce measurement uncertainty below the quantum limit in GW detectors and to enhance the sensitivity of quantum bits (qubits) in QCs. Carlton Caves' pioneering 1980 paper [1] first theoretically established the inevitable presence of quantum mechanical radiation pressure fluctuations in laser interferometers and their impact on measurement sensitivity, thereby laying the groundwork for integrating quantum optics principles into high-precision metrology. This theoretical framework has also provided the intellectual basis for quantum noise reduction strategies developed to enhance the sensitivity of GW detectors. Similarly, Peter Shor's development of quantum error correction (QEC) codes in 1996 [3] represented a landmark, offering a solution to decoherence and operational errors-one of the biggest obstacles for QCs-and paving the way for scalable and fault-tolerant quantum computation. The present work meticulously compares the parallel technological advancements and conceptual intersections in these two pioneering fields, highlighting a rich interdisciplinary potential that can yield mutual benefits and inspire innovative solutions. In this vein, the study analyses the historical evolution and current technological challenges of both GW observations and QCs, while also envisioning potential future areas of interaction and collaboration-such as advanced sensors, novel signal processing algorithms, and the application of quantum information theory to physical systems-thereby aiming to establish a solid foundation for a deeper and more fruitful integration of quantum technologies in these two distinct yet complementary domains.

DOI
10.22541/au.175138854.46819184/v1

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This study comprehensively examines the profound technological and methodological synergies existing between observations made via interferometric detectors such as LIGO and Virgo, which have revolutionized the field of gravitational wave (GW) astrophysics, and the rapidly advancing quantum computing (QC) technologies. As both disciplines aim to perform measurements pushing the limits of precision, the effective control and mitigation of environmental and quantum-originated noise pose a critical challenge. In this context, the quantum squeezing technique stands out as a fundamental tool in both domains, employed to reduce measurement uncertainty below the quantum limit in GW detectors and to enhance the sensitivity of quantum bits (qubits) in QCs. Carlton Caves' pioneering 1980 paper [1] first theoretically established the inevitable presence of quantum mechanical radiation pressure fluctuations in laser interferometers and their impact on measurement sensitivity, thereby laying the groundwork for integrating quantum optics principles into high-precision metrology. This theoretical framework has also provided the intellectual basis for quantum noise reduction strategies developed to enhance the sensitivity of GW detectors. Similarly, Peter Shor's development of quantum error correction (QEC) codes in 1996 [3] represented a landmark, offering a solution to decoherence and operational errors-one of the biggest obstacles for QCs-and paving the way for scalable and fault-tolerant quantum computation. The present work meticulously compares the parallel technological advancements and conceptual intersections in these two pioneering fields, highlighting a rich interdisciplinary potential that can yield mutual benefits and inspire innovative solutions. In this vein, the study analyses the historical evolution and current technological challenges of both GW observations and QCs, while also envisioning potential future areas of interaction and collaboration-such as advanced sensors, novel signal processing algorithms, and the application of quantum information theory to physical systems-thereby aiming to establish a solid foundation for a deeper and more fruitful integration of quantum technologies in these two distinct yet complementary domains.
The 2015 seminal discovery of gravitational waves (GW) from the collision of the binary black holes has boosted interest to development of a more sensitive next generation gravitational-wave interferometers. As the current Advanced LIGO and Virgo detectors are limited by fundamental quantum fluctuations of light in the most of their detection band, the next generation of interferometers must use advanced quantum noise-mitigation methods and quantum non-demolition (QND) techniques to achieve the planned design sensitivity of 10 times better than the current detectors have. In this paper, we attempt to give an overview of some advanced quantum metrology techniques being considered as potential sensitivity boosters for the next generation GW detectors.
Gravitational waves are a predication of Einstein's theory of general relativity. After nearly 50 years of effort by the scientific community to construct a detector capable of directly measuring gravitational waves, we expect a breakthrough, the first ever direct measurement of a gravitational wave within the next 5 years. The Advanced LIGO detectors currently under construction are predicted to achieve not only the first detection but to open the field of gravitational-wave astronomy: as an observational window on astrophysics and with an impart on other areas such as cosmology, strong-field gravity, general relativity, and nuclear physics. \n \nThe Advanced LIGO detectors are large-scale laser interferometers that have been designed for very low technical noise so that quantum noise will be limiting their sensitivity over a wide range of their spectrum. Planning for future gravitational wave observatories is already underway and reducing the fundamental quantum noise is considered to be one of the main experimental challenges. My work over the last 4 years has focused on possible new techniques to reduce or circumvent the quantum noise in laser interferometers. \n
Interferometric gravitational wave detectors (such as advanced LIGO) employ high-power solid-state lasers to maximize their detection sensitivity and hence their reach into the universe. These sophisticated light sources are ultra-stabilized with regard to output power, emission frequency and beam geometry; this is crucial to obtain low detector noise. However, even when all laser noise is reduced as far as technically possible, unavoidable quantum noise of the laser still remains. This is a consequence of the Heisenberg Uncertainty Principle, the basis of quantum mechanics: in this case, it is fundamentally impossible to simultaneously reduce both the phase noise and the amplitude noise of a laser to arbitrarily low levels. This fact manifests in the detector noise budget as two distinct noise sources—photon shot noise and quantum radiation pressure noise—which together form a lower boundary for current-day gravitational wave detector sensitivities, the standard quantum limit of interferometry. To overcome this limit, various techniques are being proposed, among them different uses of non-classical light and alternative interferometer topologies. This article explains how quantum noise enters and manifests in an interferometric gravitational wave detector, and gives an overview of some of the schemes proposed to overcome this seemingly fundamental limitation, all aimed at the goal of higher gravitational wave event detection rates. This article is part of a discussion meeting issue ‘The promises of gravitational-wave astronomy’.
The detection of the binary neutron star inspiral GW170817 by the Advanced Laser Interferometer Gravitational Wave Observatory (LIGO) and Advanced Virgo, followed by counterpart observations across a broad electromagnetic spectrum, marks the dawn of multi-messenger astronomy. The discovery rate and potential of gravitational-wave astronomy is limited by the detector's sensitivity. A key challenge in improving the sensitivity of gravitational-wave detectors is to reduce quantum noise which arises due to the quantum nature of light. A well established technique to reduce the quantum noise in the detector is by replacing the coherent vacuum fluctuations entering the readout port with a squeezed vacuum state. This technique has been successfully demonstrated at the GEO-600 and LIGO Hanford detector, and is now in routine operation in current gravitational wave detectors. This thesis presents research from three experiments to develop quantum noise reduction techniques for next-generation gravitational wave detector using non-classical quantum states such as squeezed states. Current proposals for future generation gravitational wave detectors, such as LIGO Voyager, plan to utilise cryogenic silicon instead of fused silica as the test mass material. This is to improve the power handling of the detector and reduce thermal noise. The switch to silicon test masses requires a change of laser wavelength to the 2 micron region. In the first experiment, we built and operated the first squeezed vacuum source at a wavelength of 1984 nm. With a stable squeezing ellipse phase control employed, 3.9 dB of squeezing was measured, predominately limited by losses due to the low quantum efficiency of the photodiode. The squeezed vacuum system presented is compatible with future detectors and is a key pathfinder experiment to inform the design of cryogenic silicon interferometers. The quantum nature of light results in quantum radiation pressure noise that limits the low frequency sensitivity of gravitational wave detectors. In collaboration with Louisiana State University, the second experiment demonstrates the reduction of quantum radiation pressure noise by injecting bright squeezed light into a centimetre-scale optomechanical cavity at room temperature. The experiment involves the development and integration of a low-frequency bright squeezed source to reduce the quantum radiation pressure noise by 1.2 dB at 20 kHz. The result also represents the lowest frequency bright squeezed state measured from a crystal-based squeezer. Quadrature rotation of the injected squeezed state is required to achieve broadband quantum noise reduction in gravitational wave detectors due to the quantum back-action effect. This requires the squeezed state to interact with a low loss optical filter with a low frequency bandwidth prior to injection and represents a significant technical challenge. The final experiment presented in this thesis demonstrates an alternate method where frequency-dependent squeezing with Einstein-Podolsky-Rosen (EPR) entangled states is generated. When integrated into a gravitational wave detector, the alternate method allows the detector to simultaneously serve as the optical filter, eliminating the need for the additional apparatus.
A century after the prediction of the existence of gravitational waves by A. Einstein and after over fifty years of experimental efforts, gravitational waves have been detected at Earth directly. This result is a major achievement and opens new prospectives for the exploration of our universe. Gravitational waves carry different and complementary information about the source with respect to electromagnetic signals. In particular the first detection demonstrated the existence of stellar-mass black holes, binary systems of black holes and their coalescence. The detection was made by the LIGO instruments which are twin kilometer-scale Michelson interferometers in the US. These detectors represent the second generation of gravitational wave interferometers and, for the first time, they achieved the outstanding strain sensitivity of 10^(-23) Hz^(-1/2) between 90Hz and 400Hz. In the next months the LIGO network will be joined by another second generation detector: Advanced Virgo located near Pisa, Italy. The sensitivity of these advanced detectors is set by different noise sources. In particular, in the low frequency range (below 100Hz) major contributions come from thermal noises, gravity gradient noise and radiation pressure noise; instead, the high frequency band (above 100-200Hz) is dominated by shot noise. Quantum noise (radiation pressure and shot noise) is expected to dominate the detector sensitivity in the whole frequency band at the final target laser input power. To decrease the shot noise while increasing the radiation-pressure noise, or vice-versa, Caves \cite{Caves1981} proposed in 1981 the idea of the squeezed-state technique. The LIGO collaboration demonstrated for the first time in 2011 that the injection of a squeezed vacuum state into the dark port of the interferometer can reduce the shot noise due to the quantum nature of light. This result was achieved with the German-British interferometer GEO600 and was replicated in 2013 with the LIGO interferometer at Livingston. After these results, the LIGO collaboration have pursued further the research in the squeezed-state technique which is considered mandatory for third generation of ground based interferometric detectors. In 2013, the Virgo collaboration started developing the squeezed-state technique. The subject of my thesis is the realization of a prototype of frequency independent squeezed vacuum state source to be injected in Advanced Virgo. This prototype is developed in collaboration with other Virgo groups.
The first detections of gravitational waves have opened an exciting new field of astronomy. One of the most fundamental limitations for the sensitivity of current and future interferometric gravitational-wave detectors is imposed by the quantum nature of light: Quantum vacuum fluctuations entering the interferometer through the readout port will contribute to the detection noise, at high frequencies in the form of shot noise and at low frequencies by radiation pressure noise. A promising way to reduce this quantum noise is the injection of squeezed states of light that have a lower uncertainty in one quadrature than the vacuum state. The GEO 600 gravitational-wave detector demonstrated the use of squeezed light in 2010 and it is now the first detector to routinely apply squeezing to improve its sensitivity beyond the limits set by classical quantum shot noise. This thesis details the practical aspects of long-term stable and efficient squeezed-light integration in a large-scale gravitational-wave detector. Imperfections that can limit the amount of observable non-classical noise improvement, such as optical losses and phase fluctuations, were studied in detail and methods for their mitigation were developed. Novel control schemes for the active stabilisation of the squeezed light field's phase and alignment were one main focus of the investigations. At the same time, important experience was gathered in the operation of the squeezed light source over long timescales. Over the course of the thesis work, improvements were implemented that significantly increased the performance of the squeezed-light application. Squeezing was injected with an overall duty cycle of 88%, reaching a noise reduction of up to 4.4 dB, corresponding to a 40% lowered shot-noise level. This work has firmly established the practical application of squeezing as a mature technology. The gained knowledge will directly inform the implementation of squeezed light for all future gravitational-wave detectors.