This research focuses on advanced noise suppression technologies for high-precision measurement systems, particularly addressing the limitations of classical noise reducing approaches. The noise level of laser sources is a crucial factor that directly affects the measurement sensitivity in applications such as gravitational wave detection and biomedical imaging. Classical feedback control technologies are effective but often encounter a bottleneck resulting from the classical noise suppression limits. To cope with these challenges, a novel method integrating quantum squeezed light with classical feedback control systems to reduce intensity noise is proposed in this work. By employing an amplitude-squeezed light field, a quantum-enhanced feedback control model is developed, thereby theoretically examining the influence of both the feedback loop gain and the degree of squeezing on the noise suppression performance. The results show that the injection of squeezed light significantly reduces the intensity noise, approaching the shot noise limit (SNL), thereby improving the system sensitivity beyond the classical noise reduction boundaries. Specifically, –10 dB squeezed state injection into the feedback system yields an additional noise suppression of approximately 8.97 dB, exceeding what is achievable using classical feedback alone. This demonstrates that the potential of the proposed method can enhance measurement precision close to the quantum noise limits without increasing the laser power. The analysis highlights the asymmetric noise suppression effects between the inner feedback loop and outer feedback loop. Although the outer loop benefits significantly from the squeezed light injection and achieves noise levels that are unattainable by classical feedback methods, the inner loop shows relatively minor improvements. This asymmetry is attributed to the inherent characteristics of quantum squeezing and the limitations of the feedback loop design. Further investigation into the individual noise components reveals that the primary contributors to the intensity noise include input noise, photodetector noise, and beam splitter-induced vacuum fluctuations. The injection of squeezed light effectively mitigates these vacuum fluctuations, which are typically a major noise source in high-precision laser systems. Theoretical research results show that the use of squeezed light in feedback control systems can effectively enhance noise suppression, equivalent to a nine fold increase in detected optical power, without the physical drawbacks of increasing laser power such as thermal noise. In conclusion, this study provides a theoretical validation of combining quantum squeezed states with classical feedback control to exceed classical noise suppression limits. The integration of a –10 dB squeezed state demonstrates a significant noise reduction, showing that this hybrid approach could revolutionize noise management in precision measurement applications. The results pave the way for further exploring quantum-enhanced control technologies in fields such as gravitational wave detection, quantum communication, and advanced optical sensing, providing a pathway for improving sensitivity and noise suppression without increasing additional power requirements.
This research focuses on advanced noise suppression technologies for high-precision measurement systems, particularly addressing the limitations of classical noise reducing approaches. The noise level of laser sources is a crucial factor that directly affects the measurement sensitivity in applications such as gravitational wave detection and biomedical imaging. Classical feedback control technologies are effective but often encounter a bottleneck resulting from the classical noise suppression limits. To cope with these challenges, a novel method integrating quantum squeezed light with classical feedback control systems to reduce intensity noise is proposed in this work. By employing an amplitude-squeezed light field, a quantum-enhanced feedback control model is developed, thereby theoretically examining the influence of both the feedback loop gain and the degree of squeezing on the noise suppression performance. The results show that the injection of squeezed light significantly reduces the intensity noise, approaching the shot noise limit (SNL), thereby improving the system sensitivity beyond the classical noise reduction boundaries. Specifically, –10 dB squeezed state injection into the feedback system yields an additional noise suppression of approximately 8.97 dB, exceeding what is achievable using classical feedback alone. This demonstrates that the potential of the proposed method can enhance measurement precision close to the quantum noise limits without increasing the laser power. The analysis highlights the asymmetric noise suppression effects between the inner feedback loop and outer feedback loop. Although the outer loop benefits significantly from the squeezed light injection and achieves noise levels that are unattainable by classical feedback methods, the inner loop shows relatively minor improvements. This asymmetry is attributed to the inherent characteristics of quantum squeezing and the limitations of the feedback loop design. Further investigation into the individual noise components reveals that the primary contributors to the intensity noise include input noise, photodetector noise, and beam splitter-induced vacuum fluctuations. The injection of squeezed light effectively mitigates these vacuum fluctuations, which are typically a major noise source in high-precision laser systems. Theoretical research results show that the use of squeezed light in feedback control systems can effectively enhance noise suppression, equivalent to a nine fold increase in detected optical power, without the physical drawbacks of increasing laser power such as thermal noise. In conclusion, this study provides a theoretical validation of combining quantum squeezed states with classical feedback control to exceed classical noise suppression limits. The integration of a –10 dB squeezed state demonstrates a significant noise reduction, showing that this hybrid approach could revolutionize noise management in precision measurement applications. The results pave the way for further exploring quantum-enhanced control technologies in fields such as gravitational wave detection, quantum communication, and advanced optical sensing, providing a pathway for improving sensitivity and noise suppression without increasing additional power requirements.
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.
High levels of laser power stability are necessary for high precision metrology applications. The classical limit for the achievable power stability is determined by the shot noise of the light used to generate a power control signal. Increasing the power of the detected light reduces the relative shot noise level and allows higher stabilities. However, sufficiently high power is not always available and the detection of high laser powers is challenging. Here, we demonstrate a nonclassical way to improve the achievable power stability without increasing the detected power. By the injection of a squeezed vacuum field of light we improve the classical laser power stability beyond its shot noise limit by 9.4_{-0.6}^{+0.6} dB at Fourier frequencies between 5 and 80 kHz. For only 90.6 μA of detected photocurrent we achieve a relative laser power noise of 2.0_{-0.1}^{+0.1}×10^{-8}/sqrt[Hz]. This is the first demonstration of a squeezed light-enhanced laser power stabilization and its performance is equivalent to an almost tenfold increase of detected laser power in a classical scheme. The analysis reveals that the technique presented here has the potential to achieve stability levels of 4.2×10^{-10}/sqrt[Hz] with 58 mA photocurrent measured on a single photodetector.
Pump power fluctuations are a critical factor in limiting the long-term stability of quantum sensing light sources. However, existing laser power stabilization techniques fail to efficiently address power fluctuations caused by downstream beam-pointing noise coupling. Here, we propose an optimized suppression scheme of pump power fluctuations that integrates a passive second harmonic process and active feedback control, achieving multiple noise suppression of both pointing and intensity noise in the pump field. Within the measurement time of 8.5 hours, we achieve a quantum sensing light source with a squeezing factor of 10.0 dB and its fluctuations below 0.4 dB. The proposed approach effectively boosts the long-term stability of the quantum squeezing source, thus paving a solid way for expanding the utilization of squeezing sources within the realm of quantum precision measurement.
Squeezed states of light are used for precision metrology and quantum-enhanced measurements, with applications spanning communication and sensing. State-of-the-art squeezed-light sources typically rely on optical cavities to achieve high, usable levels of squeezing. Recently, waveguide-based squeezed-light sources have demonstrated significant improvements in achievable squeezing, with performance currently limited by fabrication-induced losses. In this work, we present a detailed analysis of waveguide-based squeezers, examining the effects of phase noise, multiple loss mechanisms, and fundamental light leakage seeding the squeezer. We further investigate a cascaded squeezer architecture, in which a second waveguide operates as a phase-sensitive amplifier to mitigate out-coupling and detection losses. Owing to their ease of integration, robustness to high pump powers, and low intrinsic phase noise, we propose waveguide-based squeezed-light sources as a promising alternative for quantum noise reduction in future gravitational wave detectors, such as the Einstein Telescope.
Current gravitational wave detectors, such as LIGO, Virgo and KAGRA, are laser interferometers. This technique consists of injecting a laser beam through an input port, which is then separated into two arms several kilometers long by a semi-reflecting mirror. At the end of each arm, a mirror reflects the beam back to the semi-reflecting mirror. The two beams then interfere and the power of the resulting beam, measured at the detection port, contains information about the relative length of the two arms. When a gravitational wave passes through the detector, this relative length varies in time. Several noise sources degrade the quality of the recorded signal, which affects the number of gravitational waves that can be observed. Currently, this sensitivity is limited at high-frequency by the quantum noise.An electromagnetic wave described by quantum mechanics has a classical amplitude and quantum fluctuations in amplitude and phase. A state with a null classical amplitude is called a vacuum state, but still has these quantum fluctuations. Thus, in the detector, although no light beam is introduced through the detection port, the vacuum state is present. It is then the quantum fluctuations of this vacuum state that are responsible for the quantum noise of the interferometer. In particular, the phase and amplitude fluctuations are respectively responsible for the quantum noise at high and low frequencies; the separation between these two regions occurs at about 100 Hz.The work presented in this thesis focuses on the injection of squeezed vacuum states into the Virgo detector via the detection port in order to lower the quantum noise. These states are characterized by the fact that their phase (amplitude) fluctuations are reduced, which allows reducing the quantum noise at high (low) frequency. As the Virgo detector is currently limited only by high-frequency quantum noise, only phase squeezed vacuum states were used during the last observation period O3. It was first necessary to ensure that these squeezed vacuum states had the proper geometric and phase parameters with respect to the interferometer beam such that they were not degraded. Control loops allowed these parameters to be maintained at their nominal values during O3, a year long period. The decrease in high-frequency quantum noise, but also the increase in low-frequency quantum noise, have been measured for the first time with the Virgo detector. These observations were in agreement with the measurements of the losses that affect the squeezed vacuum states.From O4 , quantum noise will also be limiting at low frequency. It will then be possible to use squeezed vacuum states having their amplitude fluctuations reduced at low frequency and their phase fluctuations reduced at high frequency. Thus, the total quantum noise can be reduced. Such states can be generated using an optical cavity called a filter cavity. The second part of the work presented in this thesis consisted in defining the optical parameters of the filter cavity that will be installed on Virgo for O4. To do this, it was first necessary to study the sources of degradation of the squeezed states of the vacuum. This then allowed defining the length of the filtering cavity and the set of parameters of the mirrors that compose it. Finally, the implementation of this filter cavity within the Virgo infrastructure was studied. This has shown that the use of compressed states will effectively reduce quantum noise over the entire sensitivity spectrum of Virgo.
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.