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arXiv:gr-qc/0306055 · Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Quantum noise in gravitational wave interferometers: an overview and recent developments

T. R. Corbitt, N. Mavalvala

Published May 16, 2003• 6 citations
arXivHTMLPDF

Abstract

We present an overview of quantum noise in gravitational wave interferometers. Gravitational wave detectors are extensively modified variants of a Michelson interferometer and the quantum noise couplings are strongly influenced by the interferometer configuration. We describe recent developments in the treatment of quantum noise in the complex interferometer configurations of present-day and future gravitational-wave detectors. In addition, we explore prospects for the use of squeezed light in future interferometers, including consideration of the effects of losses, and the choice of optimal readout schemes.

DOI
10.1117/12.507487

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•Jan 1, 2013•1 citation
We present an overview of quantum noise in gravitational wave interferometers. Gravitational wave detectors are extensively modified variants of a Michelson interferometer and the quantum noise couplings are strongly influenced by the interferometer configuration. We describe recent developments in the treatment of quantum noise in the complex interferometer configurations of present-day and future gravitational-wave detectors. In addition, we explore prospects for the use of squeezed light in future interferometers, including consideration of the effects of losses, and the choice of optimal readout schemes.
We present an overview of quantum noise in gravitational-wave interferometers. Current gravitational-wave detectors are modified variants of a Michelson interferometer and the quantum noise limits are strongly influenced by the optical configuration of the interferometer. We describe recent developments in the treatment of quantum noise in the complex interferometers of present-day and future gravitational-wave detectors and explore prospects for beating the standard quantum limit by use of both injected and ponderomotive squeezing in future interferometers.
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
Squeezed vacuum states of light show a reduction in quantum noise along one quadrature compared to the vacuum state. This characteristic is the key to improve interferometric measurements beyond their quantum noise limitation. In particular for ground-based gravitational wave detectors [1] the application of squeezed light led to a significant sensitivity improvement of up to 6 dB. Future gravitational wave detectors, like the Einstein Telescope [2] or Cosmic Explorer [3], aim for an even higher enhancement in the range of 10 dB with the injection of squeezed states of light into the dark port of their interferometers.
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.
This paper explores the theoretical potential of utilizing quantum entanglement, specifically through the generation and application of squeezed states of light, to enhance the sensitivity of gravitational wave detectors. We delve into the fundamental limitations imposed by quantum noise in conventional interferometric detectors and investigate how the injection of squeezed states can circumvent these limitations. We present a detailed theoretical framework for implementing squeezed light in advanced gravitational wave observatories and analyze the expected improvements in signal-to-noise ratio. The analysis includes a rigorous treatment of the quantum mechanical interactions within the interferometer, considering realistic experimental imperfections. Our findings suggest that the strategic deployment of quantum entanglement offers a promising pathway towards significantly improving the detection capabilities of future gravitational wave detectors, opening new avenues for exploring the universe.
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.