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Optical and Quantum Sensing and Precision Metrology II

Advanced quantum-enhanced metrology for gravitational-wave detection

S. L. Danilishin

Published Feb 25, 2020• 1 citation
Publisher

Abstract

The 2015 seminal discovery of gravitational waves (GW) from the collision of the binary black holes has boosted interest to development of a more sensitive next generation gravitational-wave interferometers. As the current Advanced LIGO and Virgo detectors are limited by fundamental quantum fluctuations of light in the most of their detection band, the next generation of interferometers must use advanced quantum noise-mitigation methods and quantum non-demolition (QND) techniques to achieve the planned design sensitivity of 10 times better than the current detectors have. In this paper, we attempt to give an overview of some advanced quantum metrology techniques being considered as potential sensitivity boosters for the next generation GW detectors.

DOI
10.1117/12.2552683

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The 2015 seminal discovery of gravitational waves (GW) from the collision of the binary black holes has boosted interest to development of a more sensitive next generation gravitational-wave interferometers. As the current Advanced LIGO and Virgo detectors are limited by fundamental quantum fluctuations of light in the most of their detection band, the next generation of interferometers must use advanced quantum noise-mitigation methods and quantum non-demolition (QND) techniques to achieve the planned design sensitivity of 10 times better than the current detectors have. In this paper, we attempt to give an overview of some advanced quantum metrology techniques being considered as potential sensitivity boosters for the next generation GW detectors.
Quantum fluctuation of light limits the sensitivity of advanced laser interferometric gravitational-wave detectors. It is one of the principal obstacles on the way towards the next-generation gravitational-wave observatories. The envisioned significant improvement of the detector sensitivity requires using quantum non-demolition measurement and back-action evasion techniques, which allow us to circumvent the sensitivity limit imposed by the Heisenberg uncertainty principle. In our previous review article (Danilishin and Khalili in Living Rev Relativ 15:5, 2012 ), we laid down the basic principles of quantum measurement theory and provided the framework for analysing the quantum noise of interferometers. The scope of this paper is to review novel techniques for quantum noise suppression proposed in the recent years and put them in the same framework. Our delineation of interferometry schemes and topologies is intended as an aid in the process of selecting the design for the next-generation gravitational-wave observatories.
Since 2015, the gravitational-wave detectors LIGO and Virgo have opened a new window on the universe, detecting hundreds of signals and launching a new era of astronomy with profound impact on relativity, astrophysics, and cosmology. To listen deeper into the cosmos, detectors must become increasingly sensitive. One of the main limitations is quantum noise, ultimately arising from vacuum fluctuations entering the instrument. By “squeezing” this vacuum, i.e. manipulating its noise properties, LIGO and Virgo have already extended their reach by up to 65%, revealing events that would otherwise remain hidden. This article provides an overview of squeezing techniques for gravitational-wave detectors, from their origins to the most recent advances.
Advanced gravitational-wave detectors, which have enabled groundbreaking discoveries, are large-scale laser interferometers. They rank among the most sensitive instruments ever conceived, so sensitive that even the minuscule quantum fluctuations of light become a primary limiting factor by introducing quantum noise. In this paper, we provide an overview of various quantum techniques developed to surpass these quantum limits. We demonstrate how these techniques can be interpreted and categorized within a unified framework defined by the fundamental quantum limit, which is determined by the quantum fluctuations of the optical energy stored within the interferometer. This framework not only facilitates comparisons between the ultimate sensitivities achievable by different interferometer configurations but also supports the systematic generation of new configurations that coherently integrate multiple quantum-enhancement techniques. Through further refinement and dedicated experimental efforts, these quantum technologies will pave the way for constructing gravitational-wave detectors with unprecedented sensitivity.
Gravitational waves manifest as a time varying straining of space: they arise from the accelerating motions of large bodies of mass and propagate across the universe at the speed of light as ripples in the fabric of space-time, a fleetingly weak effect so far eluding direct detection. The detection of gravitation waves is expected to yield a rich vein to observational astronomy, complementing existing electromagnetic surveys and revealing a hitherto unexplored range of phenomena. First generation interferometric gravitational wave detectors, notably Enhanced LIGO, achieved strain sensitivities of one part in ten to power twenty-one per square-root-Hertz at 100 Hz with an expected detection rate of 2-3 events per year. Commissioning of a new generation of Advanced LIGO interferometric detectors has concluded recently with a resultant ten-fold sensitivity improvement. Overall their potential event detection space has increased by a factor of 1000. The quantum nature of light within these detectors now limits their sensitivity over most of their frequency range. This quantum noise limit is driven by the vacuum quadrature fluctuations propagated through their open detection ports and represents a fundamental noise floor to their strain sensitivity. This thesis addresses two distinct approaches to quantum noise improvement for future upgrades to advanced detectors. The first addresses the issue of quantum noise by adopting a quantum non-demolition approach to detector readout variables, the so-called `speed-meter’ design. Such a modified instrument samples test mass momentum, a quantity for which time separated measurements commute and are therefore not bound by Heisenberg-like limits. A novel polarisation-folded sloshing cavity type speed-meter is proposed where readout fields are stored and delayed in the orthogonal polarisation of the interferometer’s arms cavities. Here frequency dependence is selected to cancel position like measurements so that only test mass momentum information remains. A quantum noise propagation model is developed and a sensitivity performance is demonstrated that beats the standard quantum limit below 100 Hz over a broad range of frequencies. A second approach to achieve quantum noise enhancement in advanced detectors involves injection of quadrature-squeezed states in the place of vacuum. This dissertation details the development of a prototype squeezed vacuum source suitable to the demanding enhancement requirements for an Advanced LIGO squeezing installation. The construction of a doubly resonant, bow-tie cavity source is presented. This employs a novel monolithic all-glass cavity construction and is vacuum compatible. This design demonstrates the viability of building a cavity using optical contacting as a construction technique for attaching mounting prisms to highly polished fused-silica breadboards. Such a design can be expected to have excellent length noise stability, provide low intrinsic phase noise and would be suitable to mount on seismic isolation stages within the LIGO vacuum envelope. Further, the travelling wave cavity design should provide excellent 50 dB intrinsic backscatter isolation. We demonstrate the first operation of such a complex non-linear device under vacuum, producing 8.6 dB of measured vacuum squeezing down to 10 Hz across the advanced LIGO ‘audio-band’ detection range.
Quantum fluctuations in the phase and amplitude quadratures of light set limitations on the sensitivity of modern optical instruments. The sensitivity of the interferometric gravitational-wave detectors, such as the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO), is limited by quantum shot noise, quantum radiation pressure noise, and a set of classical noises. We show how the quantum properties of light can be used to distinguish these noises using correlation techniques. Particularly, in the first part of the paper we show estimations of the coating thermal noise and gas phase noise, hidden below the quantum shot noise in the Advanced LIGO sensitivity curve. We also make projections on the observatory sensitivity during the next science runs. In the second part of the paper we discuss the correlation technique that reveals the quantum radiation pressure noise from the background of classical noises and shot noise. We apply this technique to the Advanced LIGO data, collected during the first science run, and experimentally estimate the quantum correlations and quantum radiation pressure noise in the interferometer.
Abstract The recent discovery of gravitational waves (GW) by Advanced LIGO (Laser Interferometric Gravitational-wave Observatory) has impressively launched the novel field of gravitational astronomy and allowed us to glimpse exciting objects about which we could previously only speculate. Further sensitivity improvements at the low-frequency end of the detection band of future GW observatories must rely on quantum non-demolition (QND) methods to suppress fundamental quantum fluctuations of the light fields used to readout the GW signal. Here we present a novel concept of how to turn a conventional Michelson interferometer into a QND speed-meter interferometer with coherently suppressed quantum back-action noise. We use two orthogonal polarizations of light and an optical circulator to couple them. We carry out a detailed analysis of how imperfections and optical loss influence the achievable sensitivity. We find that the proposed configuration significantly enhances the low-frequency sensitivity and increases the observable event rate of binary black-hole coalescences in the range of 102−103 M⊙10^2 - 10^3\,M_ \odot102−103M⊙​ 1 0 2 - 1 0 3 M ⊙ by a factor of up to ~300.