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International Journal of Modern Physics D

QUANTUM SQUEEZING IN ADVANCED GRAVITATIONAL WAVE DETECTORS

S. Chua, Michael Stefszky, CONOR MMOW-LOWRY, B. C. Buchler, Kirk McKenzie, D. A. Shaddock, Ping Koy Lam, David E. McClelland

Published Sep 1, 2011• 2 citations
Publisher

Abstract

Second generation ground-based gravitational wave detectors, scheduled to be operating by the middle of this decade, will be limited in sensitivity over much of their detection range by optical quantum noise. As they will be operating at power levels close to the tolerance of the optical components, significant further improvement in sensitivity will require the use of quantum optical techniques such as the injection of squeezed states. In this paper we briefly review squeezing and plans for its implementation into advanced gravitational wave detectors.

DOI
10.1142/s0218271811020159

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Second generation ground-based gravitational wave detectors, scheduled to be operating by the middle of this decade, will be limited in sensitivity over much of their detection range by optical quantum noise. As they will be operating at power levels close to the tolerance of the optical components, significant further improvement in sensitivity will require the use of quantum optical techniques such as the injection of squeezed states. In this paper we briefly review squeezing and plans for its implementation into advanced 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.
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.
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 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.
Direct detection of gravitational waves will require earth based detectors to measure strains of the order 10-21, at frequencies of 100 Hz, a sensitivity that has been accomplished with the initial generation of LIGO interferometric gravitational wave detectors. A new generation of detectors currently under construction is designed improve on the sensitivity of the initial detectors by about a factor of 10. The quantum nature of light will limit the sensitivity of these Advanced LIGO interferometers at most frequencies; new approaches to reducing the quantum noise will be needed to improve the sensitivity further. This quantum noise originates from the vacuum fluctuations that enter the unused port of the interferometer and interfere with the laser light. Vacuum fluctuations have the minimum noise allowed by Heisenberg's uncertainty principle, [Delta]X1 [Delta]X2 >/=1, where the two quadratures X1 and X2 are non-commuting observables responsible for the two forms of quantum noise, shot noise and radiation pressure noise. By replacing the vacuum fluctuations entering the interferometer with squeezed states, which have lower noise in one quadrature than the vacuum state, we have reduced the shot noise of a LIGO interferometer. The sensitivity to gravitational waves measured during this experiment represents the best sensitivity achieved to date at frequencies above 200 Hz, and possibly the first time that squeezing has been measured in an interferometer at frequencies below 700 Hz. The possibility that injection of squeezed states could introduce environmental noise couplings that would degrade the crucial but fragile low frequency sensitivity of a LIGO interferometer has been a major concern in planning to implement squeezing as part of baseline interferometer operations. These results demonstrate that squeezing is compatible with the low frequency sensitivity of a full scale gravitational wave interferometer. We also investigated the limits to the level of squeezing observed, including optical losses and fluctuations of the squeezing angle. The lessons learned should allow for responsible planning to implement squeezing in Advanced LIGO, either as an alternative to high power operation or an early upgrade to improve the sensitivity. This thesis is available at DSpace@MIT and has LIGO document number P1300006.
A fundamental noise source limiting the measurement sensitivity of interferometric gravitational wave (GW) observatories is the light's quantum noise. While the sensitivity of the first observatory generation was limited by the shot noise at Fourier frequencies above several hundred hertz, the future observatory generations are expected to be limited by quantum noise also in their low-frequency detection band. Squeezed states of the light field allow for improving the sensitivity at all quantum-noise-limited frequencies. Recently, a squeezing-reduced shot noise of the GW observatory GEO 600 was demonstrated. Based on this success, the Einstein Telescope design study proposes squeezed-light injection to reduce shot noise as well as quantum radiation pressure noise by a factor of ten in power, i.e. by 10 dB. This contribution summarizes the current state of the art in the field of squeezed-light generation for gravitational wave astronomy and the prospects for the intermediate future.