Current interferometric gravitational wave detectors were designed to be limited, in much of their sensitive
band, by quantum noise in the form of shot noise (at high frequency) or quantum radiation pressure noise
(at low frequency). In the current observation run (O3), a squeezed light source has been used to
manipulate quantum noise and surpass the limits imposed by the circulating laser power by injecting nonclassical states of the electromagnetic field, called squeezed vacuum states, in the interferometer's dark
port. Recent O3 results show that this technique was able to reduce the shot noise contribution to the
noise spectral density by about one half. To surpass the so called standard quantum limit across the entire
sensitivity band, an even more complex variation of this technique will be implemented, which requires the
use of a so called “filter cavity” to reflect the squeezed quantum field and rotate the squeezing angle as a
function of frequency.
The performance of this techniques is however severely limited by light losses. One big contributor to
optical losses is the mismatch between the squeezed vacuum beam, the beam circulating into the
interferometer and, in the near future, the filter cavity fundamental transverse spatial mode.
To be able to monitor and actively correct mode mismatch, an accurate, online sensing technique is
needed. A possible solution consists in the introduction of an Electro-Optic Lens (EOL) that can induce
radio-frequency sidebands in the Laguerre-Gauss 10 (LG10) transverse mode of the beam. If the
modulation frequency is tuned in a way that one sideband is at the cavity LG10 frequency, then the
resonance breaks the symmetry of the two sidebands and converts phase modulation into amplitude
modulation, making it possible to retrieve an error signal directly from the cavity reflected electric field
with the same single-element photodetector used for the Pound Drever Hall (PDH) locking technique. This
error signal carries information about the magnitude and nature of the mismatch, providing all is needed to
inform a dedicated feedback loop with a suitable actuator to maintain the mismatch at or below the 1%
level.
The goal of this thesis is to demonstrate the technique on a dedicated bench-top experiment employing an
EOL prototype device. The EOL alignment and working point, as well as the cavity lock parameters were
optimized in an already existing setup. Simulations and theoretical calculations of the cavity reflected field
were performed to evaluate the system non-idealities: both Electro-Optic Modulator (for PDH technique)
and EOL modulations effects and interactions, as well as some system imperfections were considered. The
simulations confirm that this technique requires a stable cavity lock condition, and for this reason an
accurate study and optimization of the feedback loop parameters was done. In conclusion, a
characterization of the EOL error signal response to different mismatch conditions are presented.
Current interferometric gravitational wave detectors were designed to be limited, in much of their sensitive
band, by quantum noise in the form of shot noise (at high frequency) or quantum radiation pressure noise
(at low frequency). In the current observation run (O3), a squeezed light source has been used to
manipulate quantum noise and surpass the limits imposed by the circulating laser power by injecting nonclassical states of the electromagnetic field, called squeezed vacuum states, in the interferometer's dark
port. Recent O3 results show that this technique was able to reduce the shot noise contribution to the
noise spectral density by about one half. To surpass the so called standard quantum limit across the entire
sensitivity band, an even more complex variation of this technique will be implemented, which requires the
use of a so called “filter cavity” to reflect the squeezed quantum field and rotate the squeezing angle as a
function of frequency.
The performance of this techniques is however severely limited by light losses. One big contributor to
optical losses is the mismatch between the squeezed vacuum beam, the beam circulating into the
interferometer and, in the near future, the filter cavity fundamental transverse spatial mode.
To be able to monitor and actively correct mode mismatch, an accurate, online sensing technique is
needed. A possible solution consists in the introduction of an Electro-Optic Lens (EOL) that can induce
radio-frequency sidebands in the Laguerre-Gauss 10 (LG10) transverse mode of the beam. If the
modulation frequency is tuned in a way that one sideband is at the cavity LG10 frequency, then the
resonance breaks the symmetry of the two sidebands and converts phase modulation into amplitude
modulation, making it possible to retrieve an error signal directly from the cavity reflected electric field
with the same single-element photodetector used for the Pound Drever Hall (PDH) locking technique. This
error signal carries information about the magnitude and nature of the mismatch, providing all is needed to
inform a dedicated feedback loop with a suitable actuator to maintain the mismatch at or below the 1%
level.
The goal of this thesis is to demonstrate the technique on a dedicated bench-top experiment employing an
EOL prototype device. The EOL alignment and working point, as well as the cavity lock parameters were
optimized in an already existing setup. Simulations and theoretical calculations of the cavity reflected field
were performed to evaluate the system non-idealities: both Electro-Optic Modulator (for PDH technique)
and EOL modulations effects and interactions, as well as some system imperfections were considered. The
simulations confirm that this technique requires a stable cavity lock condition, and for this reason an
accurate study and optimization of the feedback loop parameters was done. In conclusion, a
characterization of the EOL error signal response to different mismatch conditions are presented.
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.
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 thebeam 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 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.
In this thesis an alternative method to stabilize the phase of a squeezed vacuum field in the framework of the Virgo gravitational wave detector is designed and realized. A brief introduction about the nature of gravitational waves and their detection is presented in chapter 1, with particular attention to the actual sensitivity limitations on the interferometers employed. Chapter 2 focuses on the quantum nature of the electromagnetic field. A nonclassical state of the light, the squeezed state, is described, as well as how it is is produced and how can be observed. This kind of radiation field is employed to increase the sensitivity of the detectors beacause with its injection into the interferometer the quantum noise that affects the measure is partially reduced. In order to avoid any technical noise contamination, a second copropagating and frequency shifted field is used to provide the error signal for the alignment and to allow the phase locking between the interferometer main laser and the squeezed light, as discussed in chapter 3, where the technique called coherent control loop is explained. Finally, in chapter 4, the optical bench and the control electronics realized at LNL to test the stability of the control loop are presented; the system will be implemented in Virgo in the next months. The difference between the method discussed here and that usually employed is that the error signal is not used to correct the path of the squeezed vacuum field, but it is instead used to close a cascade loop on a PLL. Thus all the actuators are electronic and no optical actuators are used, the stray light issue is minimized. The control loop is stronger at low frequencies in order to correct the seismic noise that dominates in this region.
Quantum noise is one of the limiting factors in laser-interferometric gravitational-wave (GW) detectors. The application of squeezed states in these interferometers allows the reduction of quantum noise in one quadrature. Due to opto-mechanical coupling in a GW detector the squeezed quadrature needs to be rotated within the spectrum to achieve a broadband noise reduction. So far, the implementation of additional filter cavities is considered that allow for the optimal, frequency-dependent rotation of the squeezed quadrature. However, these cavities need to have low loss, a length in the order of \unit[100]{m} and must be situated in the vacuum system, making them cost-intensive. In 2017, Ma and coworkers proposed a scheme for the broadband quantum-noise reduction without the need of additional filter cavities. It was shown by Brown et al. that a similar scheme can be used to broadband-enhance interferometers with a detuned signal-recycling cavity. Here, we performed a proof-of-principle experiment of the proposal on a table-top-scale. Squeezed states were produced detuned to the carrier field of a \unit[2.5]{m}-linear cavity and read out in a bichromatic homodyne detection. The frequencies of the lower and upper local oscillator were at entangled sidebands of the squeezed field. Depending on the relations between the involved frequencies, we can address both variants of the proposal. We show, that the frequency-dependences of the resulting noise spectra fit to a theoretical model we derived from the theory used by Ma et al. With this work we set the path towards an implementation of these schemes in a GW-detector prototype, where the compatibility of the approach with a low-frequency suspended interferometer can be tested. Moreover, we used the same setup to show nonclassical interferometer enhancement at low frequencies by high-frequency squeezed states. Here, a heterodyne readout scheme was implemented to avoid limiting noises at low frequencies. The application of squeezed states centered around the local oscillator frequency yielded an improvement in signal-to-noise ratio of $\unit[3.4]{dB}\pm\unit[0.3]{dB}$. Additionally, I designed, built and characterized a compact source of squeezed vacuum-states at \unit[1064]{nm} with a footprint of just $\unit[0.8]{m^2}$. I show measurements of squeezed states from this source with a reduction of quantum noise of $\unit[10.7]{dB}\pm\unit[0.2]{dB}$ below the vacuum noise and present a noise reduction in the frequency range from \unit[70]{kHz} to \unit[65]{MHz}.
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.