Squeezed vacuum states are now employed in gravitational-wave interferometric detectors, enhancing their sensitivity and thus enabling richer astrophysical observations. In future observing runs, the detectors will incorporate a filter cavity to suppress quantum radiation pressure noise using frequency-dependent squeezing. Interferometers employing internal and external cavities decohere and degrade squeezing in complex new ways, which must be studied to achieve increasingly ambitious noise goals. This paper introduces an audio diagnostic field (ADF) to quickly and accurately characterize the frequency-dependent response and the transient perturbations of resonant optical systems to squeezed states. This analysis enables audio field injections to become a powerful tool to witness and optimize interactions such as intercavity mode matching within gravitational-wave instruments. To demonstrate, we present experimental results from using the audio field to characterize a 16-m prototype filter cavity.
Squeezed vacuum states are now employed in gravitational-wave interferometric detectors, enhancing their sensitivity and thus enabling richer astrophysical observations. In future observing runs, the detectors will incorporate a filter cavity to suppress quantum radiation pressure noise using frequency-dependent squeezing. Interferometers employing internal and external cavities decohere and degrade squeezing in complex new ways, which must be studied to achieve increasingly ambitious noise goals. This paper introduces an audio diagnostic field (ADF) to quickly and accurately characterize the frequency-dependent response and the transient perturbations of resonant optical systems to squeezed states. This analysis enables audio field injections to become a powerful tool to witness and optimize interactions such as intercavity mode matching within gravitational-wave instruments. To demonstrate, we present experimental results from using the audio field to characterize a 16-m prototype filter cavity.
Quantum vacuum fluctuations impose strict limits on precision displacement measurements, those of interferometric gravitational-wave detectors among them. Introducing squeezed states into an interferometer's readout port can improve the sensitivity of the instrument, leading to richer astrophysical observations. However, optomechanical interactions dictate that the vacuum's squeezed quadrature must rotate by 90° around 50 Hz. Here we use a 2-m-long, high-finesse optical resonator to produce frequency-dependent rotation around 1.2 kHz. This demonstration of audio-band frequency-dependent squeezing uses technology and methods that are scalable to the required rotation frequency and validates previously developed theoretical models, heralding application of the technique in future gravitational-wave detectors.
During the recent years an international network of interferometric gravitational wave (GW) detectors has been commissioned and searches for various classes of GWsignals are ongoing.Although the observatories are beginning to reach astrophysically interesting sensitivity levels, extensive upgrades are aimed for the next decade bringing us into the era of gravitational wave astronomy.One approach to enhance the detector sensitivity is the injection of squeezed vacuum states of light replacing the vacuum fluctuations entering the detector through its antisymmetric port.In this thesis the unprecedented generation of squeezed vacuum states with a noise power 6.5 dB below vacuum noise within the entire detection bandwidth of ground-based GW-detectors (10 Hz -10 kHz) was experimentally demonstrated at a wavelength of 1064 nm.For the first time a coherent control scheme was employed for comprehensive phase control of the experimental setup.This scheme is directly applicable to future GW-detector topologies.In addition, parasitic interference induced by scattered light was found to be a dominant noise source degrading squeezed states in the audio band.The compatibility of squeezed field injection into a detuned operated dual-recycled Michelson interferometer was demonstrated in a table top experiment.A detected non-classical sensitivity improvement of up to 3.0 dB within the entire detection bandwidth (5 -15 MHz) was accomplished by optimal preparation of the injected squeezing by means of a detuned locked filter cavity.In a further experiment strongly squeezed vacuum states were generated in the MHz-regime.With a maximum measured quantum noise reduction of 11.5 dB below the shot noise limit a new benchmark was set.This was achieved by suppression of phase noise and a remarkable reduction of optical loss in the experimental setup.Based on the squeezed light sources developed in this thesis and the experimental results achieved, a conceptual design of a compact squeezed light source was devised, allowing for a high duty cycle sensitivity improvement of GW-detectors.
Squeezed states of light have been successfully employed in interferometric gravitational-wave detectors to reduce quantum noise, thus becoming one of the most promising options for extending the astrophysical reach of the generation of detectors currently under construction worldwide. In these advanced instruments, quantum noise will limit sensitivity over the entire detection band. Therefore, to obtain the greatest benefit from squeezing, the injected squeezed state must be filtered using a long-storage-time optical resonator, or ``filter cavity,'' so as to realize a frequency-dependent rotation of the squeezed quadrature. While the ultimate performance of a filter cavity is determined by its storage time, several practical decoherence and degradation mechanisms limit the experimentally achievable quantum noise reduction. In this paper we develop an analytical model to explore these mechanisms in detail. As an example, we apply our results to the 16 m filter cavity design currently under consideration for the Advanced LIGO interferometers.
The ability to directly detect gravitational waves will open a completely new branch of astronomy to view the Universe, one that is inaccessible to electromagnetic-based astronomy. First generation ground-based interferometric gravitational-wave detectors have achieved strain sensitivities of order one part in ten to the twenty-one, at 100 Hz detection frequency. A new generation of detectors are under construction, designed to improve on the sensitivity of the first-generation detectors by a factor of 10. The quantum nature of light will broadly limit the sensitivity of these new instruments. This quantum noise will originate from the quantum vacuum fluctuations that enter the unused port of the interferometer. One of the most promising options for reducing the quantum noise impact and further increasing the sensitivity is applying quantum squeezed vacuum states. These squeezed states have lower noise in one quadrature than the vacuum state. By replacing the quantum vacuum fluctuations entering the interferometer with squeezed vacuum states, the quantum noise impact is reduced. This thesis firstly details the development of a squeezed light source that produces squeezed states applicable for enhancing interferometric gravitational-wave detectors. A doubly-resonant, travelling-wave bow-tie cavity squeezed light source is presented. For the first time, greater than 10 dB of quantum noise suppression across the gravitational-wave detection band is directly observed, and 11.6 dB of quantum noise suppression is observed above 200 Hz. This squeezing cavity design also has benefits with intrinsic isolation to backscattered light. Experiments that quantify this isolation are reported. The properties affecting squeezing magnitude and low-frequency squeezing measurement are discussed. In addition, a modified squeezing-ellipse-phase control technique for squeezed vacuum states is presented. This thesis secondly presents results from the LIGO Squeezed Light Injection Experiment, undertaken to test squeezed light injection into a 4 km interferometric gravitational-wave detector. The results of the experiment show the first measurement of squeezing-enhancement in a 4 km gravitational-wave detector, with 2.15 dB measured above 250 Hz. This represents the best sensitivity to gravitational waves yet achieved at these frequencies by any single gravitational-wave detector to date. An unknown area was whether the addition of a squeezer would introduce noise couplings that degrade the crucial low frequency sensitivity. The results demonstrate that injected squeezed states are compatible with low frequency gravitational-wave measurement. The characterisation of squeezing-injection optical losses and fluctuations of the squeezing angle are also reported. The knowledge and processes gained, from both the squeezed light source development work and the LIGO Squeezed Light Injection Experiment, will inform the design, planning and implementation of squeezed states in future gravitational-wave detectors.
The era of gravitational-wave astronomy has begun, with the detection of 5 confirmed binary black holes and a binary neutron star coalescence by the Advanced Laser Interferometer Gravitational- wave Observatory (aLIGO), and later with the advanced Virgo detector. These detections have already revealed a wealth of discoveries across the fields of nuclear physics, general relativity and astrophysics. The work presented in this thesis is part of the ongoing effort to improve the sensitivity of ground-based interferometric gravitational-wave detectors. The sensitivity of aLIGO, and other interferometric detectors, is broadly limited by quantum noise. Improving on the quantum noise will increase the astrophysical range of the detectors, and improve the source parameter estimation. One way to reduce quantum noise is to inject audio- band squeezed vacuum states into the detection port. This technique has been demonstrated on the initial LIGO and GEO600 detectors. A squeezed light source for aLIGO must meet stringent requirements in terms of optical loss, phase noise, and scattered light. The squeezer must produce high levels of audio-band squeezing and operate under vacuum, to take advantage of the excellent existing isolation systems and to minimise optical loss. At design sensitivity, squeezed states whose quantum noise depends on frequency will be required. We have demonstrated an ultra-stable glass-based squeezed light source, the first experiment of this kind to operate under vacuum. The squeezer cavity is constructed quasi-monolithically, with optics and nonlinear crystal oven optically contacted to a breadboard base. The cavity is designed to have extremely low length noise, and to produce high levels of audio-band squeezing. We have measured 8.6 ± 0.9 dB of squeezing and infer the generation of 14.2 ± 1.0 dB after accounting for all known losses. The squeezer has demonstrated record phase noise performance of 1.3 mradRMS, dominated by sources other than cavity length noise. This exceeds the phase-noise requirement for a squeezer for aLIGO. A copy of this squeezer is currently being installed in a squeezing- ellipse rotation experiment to demonstrate frequency-dependent squeezing for aLIGO. Lessons learnt during the construction and operation of the in-vacuum squeezer have helped inform the design of a frequency-independent squeezed light source currently being installed at the LIGO sites. Future gravitational-wave detectors will continue to use interferometric techniques, and will be limited by quantum noise for the foreseeable future. To improve on thermal noise limits and interferometer power handling, future detectors look to cryogenic silicon as a test mass material. To take advantage of the desirable properties of silicon, including low scatter and absorption, a longer operating wavelength is required. The proposed LIGO Voyager upgrade has an operating wavelength in the 2 μ m region, with the specific wavelength to be determined. LIGO Voyager will require a squeezed light source in the 2 μ m region to reach its design sensitivity. We present the design, characterisation, and results of the first squeezed light source in the 2 μ m region. Laser and detector technologies at 2 μ m are less developed than their 1064 nm counterparts, causing significant technical challenges. We have measured 4.0 ± 0.2 dB of squeez- ing at 1984 nm, limited by loss due to detector quantum efficiency. Accounting for known losses in the system, we infer the generation of 10 dB of squeezing. This is an important demonstra- tion of quantum noise reduction for future detectors, and a pathfinder technology for the design choices of LIGO Voyager. So far we have found no reason why a 2 μ m interferometer should not be feasible.
Gravitational-wave astronomy promises a radically new method of investigating the universe, one that does not detect elementary particles, but rather ripples in space-time itself. One might expect that with this radically new method, our understanding of the universe will make an equally giant leap. One of the most promising methods for the eventual detection of gravitational-waves is that of interferometric gravitational-wave detection. The sensitivity of these devices is nothing short of amazing, and their sensitivity continues to improve. The next generation of detectors are expected reach a point where increasing the laser power, previously used to increase the sensitivity, is no longer expected to provide any benefit. One of the most promising options for further increasing the sensitivity, through reduction of the quantum noise, is via the application of "squeezed states of light". Squeezed states of light are light fields for which the noise of some observable has been reduced below the quantum noise limit. These states have been shown for many years to offer increased sensitivity in interferometers and recently in interferometric gravitational-wave detectors. One of the most challenging tasks to make suitable squeezed light sources for these detectors is to produce low-frequency squeezing, corresponding to the detection band of these interferometers. This thesis details the doubly resonant travelling wave bow-tie squeezer as a source of squeezing for interferometric gravitational-wave detection. This squeezer achieves record results of squeezing in the gravitational-wave detection band. For the first time, 10 dB of shot noise suppression at 10Hz is directly observed and above 200Hz, 11.6 dB is observed. The work presented in this thesis provides evidence that further substantiates previous progress indicating that squeezed states of light are now ready for full-time integration into interferometric gravitational-wave detectors. The new cavity design, described in detail throughout this thesis, is shown to outperform previous designs and provide benefits when integrating squeezing into these detectors, in particular, isolation to backscattered light from the interferometer. The noise sources that typically lead to degradation in squeezing measurements are investigated and discussed. Additionally, a modification to the standard locking technique used to control these vacuum squeezed states, coherent locking, is presented and discussed. The modified technique reduces the required number of locking loops and provides a larger beat note from which to derive an error signal for one of the remaining loops. A squeezer using these design philosophies is then constructed and used to inject squeezing into the LIGO gravitational-wave detector. Injection of the squeezed state provides enhancement of the sensitivity of the detector at frequencies around 200Hz and above. The sensitivity seen was equivalent to approximately 2 dB of shot noise reduction. The enhancement is limited by the loss within the interferometer. Injection of the squeezed state did not show any degradation in the sensitivity of the device below 200Hz, indicating for the first time that noise couplings between the interferometer and the squeezer could be sufficiently suppressed at these frequencies even for the most sensitive detectors.