Impact of backscattered light in a squeezing-enhanced interferometric gravitational-wave detector
S. Chua, S. E. Dwyer, L. Barsotti, Daniel Sigg, R M S Schofield, V V Frolov, K Kawabe, M. Evans, G D Meadors, M Factourovich, R Gustafson, N. D. Smith, C Vorvick, M. Landry, A Khalaidovski, M S Stefszky, C M Mow-Lowry, B. C. Buchler, D A Shaddock, Ping Koy Lam, R Schnabel, N. Mavalvala, D E McClelland
Squeezed states of light have been recently used to improve the sensitivity of laser-interferometric gravitational-wave detectors beyond the quantum limit. To completely establish quantum engineering as a realistic option for the next generation of detectors, it is crucial to study and quantify the noise coupling mechanisms which injection of squeezed states could potentially introduce. We present a direct measurement of the impact of backscattered light from a squeezed-light source deployed on one of the 4 km long detectors of the laser interferometric gravitational wave observatory (LIGO). We also show how our measurements inform the design of squeezed-light sources compatible with the even more sensitive advanced detectors currently under construction, such as Advanced LIGO.
Squeezed states of light have been recently used to improve the sensitivity of laser-interferometric gravitational-wave detectors beyond the quantum limit. To completely establish quantum engineering as a realistic option for the next generation of detectors, it is crucial to study and quantify the noise coupling mechanisms which injection of squeezed states could potentially introduce. We present a direct measurement of the impact of backscattered light from a squeezed-light source deployed on one of the 4 km long detectors of the laser interferometric gravitational wave observatory (LIGO). We also show how our measurements inform the design of squeezed-light sources compatible with the even more sensitive advanced detectors currently under construction, such as Advanced LIGO.
Squeezed states of light have been recently used to improve the sensitivity of laser-interferometric gravitational-wave detectors beyond the quantum limit. To completely establish quantum engineering as a realistic option for the next generation of detectors, it is crucial to study and quantify the noise coupling mechanisms which injection of squeezed states could potentially introduce. We present a direct measurement of the impact of backscattered light from a squeezed-light source deployed on one of the 4 km long detectors of the laser interferometric gravitational wave observatory (LIGO). We also show how our measurements inform the design of squeezed-light sources compatible with the even more sensitive advanced detectors currently under construction, such as Advanced LIGO.
Nearly a century after Einstein first predicted the existence of gravitational waves, a global network of Earth-based gravitational wave observatories 1-4 is seeking to directly detect this faint radiation using precision laser interferometry. Photon shot noise, due to the quantum nature of light, imposes a fundamental limit on the attometre-level sensitivity of the kilometre-scale Michelson interferometers deployed for this task. Here, we inject squeezed states to improve the performance of one of the detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) beyond the quantum noise limit, most notably in the frequency region down to 150 Hz, critically important for several astrophysical sources, with no deterioration of performance observed at any frequency. With the injection of squeezed states, this LIGO detector demonstrated the best broadband sensitivity to gravitational waves ever achieved, with important implications for observing the gravitational-wave Universe with unprecedented sensitivity.
Recent experiments have demonstrated that squeezed vacuum states can be injected into gravitational wave detectors to improve their sensitivity at detection frequencies where they are quantum noise limited. Squeezed states could be employed in the next generation of more sensitive advanced detectors currently under construction, such as Advanced LIGO, to further push the limits of the observable gravitational wave Universe. To maximize the benefit from squeezing, environmentally induced disturbances such as back scattering and angular jitter need to be mitigated. We discuss the limitations of current squeezed vacuum sources in relation to the requirements imposed by future gravitational wave detectors, and show a design for squeezed light injection which overcomes these limitations.
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 worldwide network of laser interferometric gravitational-wave detectors is currently in an upgrade phase and will most likely detect gravitational waves within the next few years. Their strain sensitivity will be limited by photon shot noise over a large frequency band. A measurement beyond the shot noise level has already been accomplished via injecting squeezed vacuum states of light into the interferometers. In this proceeding, I review the current status in developements for the next-generation squeezed-light sources for future gravitational-wave detectors. A newly designed doubly-resonant squeezed light source is discussed, as well as squeezed light generated at the wavelengths of 1550 nm and 532 nm. Furthermore, high efficiency second harmonic generation of laser light for gravitational wave detectors will be presented. The injection of squeezed light as a key technology is well on schedule for improving the sensitivity of future gravitational-wave detectors.
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.