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Second-generation interferometric gravitational-wave detectors will be operating at the Standard Quantum Limit, a sensitivity limitation set by the trade off between measurement accuracy and quantum back action, which is governed by the Heisenberg Uncertainty Principle. We review several schemes that allows the quantum noise of interferometers to surpass the Standard Quantum Limit significantly over a broad frequency band. Such schemes may be an important component of the design of third-generation detectors.
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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: Quantum measurement theory in gravitational-wave detectors [Living Rev. Relativity 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.
Identifying the properties of the first generation of seeds of massive black holes is key to understanding the merger history and growth of galaxies. Mergers between ~100 solar mass seed black holes generate gravitational waves in the 0.1-10Hz band that lies between the sensitivity bands of existing ground-based detectors and the planned space-based gravitational wave detector, the Laser Interferometer Space Antenna (LISA). However, there are proposals for more advanced detectors that will bridge this gap, including the third generation ground-based Einstein Telescope and the space-based detector DECIGO. In this paper we demonstrate that such future detectors should be able to detect gravitational waves produced by the coalescence of the first generation of light seed black-hole binaries and provide information on the evolution of structure in that era. These observations will be complementary to those that LISA will make of subsequent mergers between more massive black holes. We compute the sensitivity of various future detectors to seed black-hole mergers, and use this to explore the number and properties of the events that each detector might see in three years of observation. For this calculation, we make use of galaxy merger trees and two different seed black hole mass distributions in order to construct the astrophysical population of events. We also consider the accuracy with which networks of future ground-based detectors will be able to measure the parameters of seed black hole mergers, in particular the luminosity distance to the source. We show that distance precisions of ~30% are achievable, which should be sufficient for us to say with confidence that the sources are at high redshift.
We consider enhancing the sensitivity of future gravitational-wave detectors by using double optical spring. When the power, detuning and bandwidth of the two carriers are chosen appropriately, the effect of the double optical spring can be described as a negative inertia, which cancels the positive inertia of the test masses and thus increases their response to gravitational waves. This allows us to surpass the free-mass Standard Quantum Limit (SQL) over a broad frequency band, through signal amplification, rather than noise cancelation, which has been the case for all broadband SQL-beating schemes so far considered for gravitational-wave detectors. The merit of such signal amplification schemes lies in the fact that they are less susceptible to optical losses than noise cancelation schemes. We show that it is feasible to demonstrate such an effect with the {it Gingin High Optical Power Test Facility}, and it can eventually be implemented in future advanced GW detectors.
Spring-antispring systems have been investigated as possible low-frequency seismic isolation in high-precision optical experiments. These systems provide the possibility to tune the fundamental resonance frequency to, in principle, arbitrarily low values, and at the same time maintain a compact design of the isolation system. It was argued though that thermal noise in spring-antispring systems would not be as small as one may naively expect from lowering the fundamental resonance frequency. In this paper, we present a detailed calculation of the suspension thermal noise for a specific spring-antispring system, namely the Roberts linkage. We find a concise expression of the suspension thermal noise spectrum, which assumes a form very similar to the well-known expression for a simple pendulum. It is found that while the Roberts linkage can provide strong seismic isolation due to a very low fundamental resonance frequency, its thermal noise is rather determined by the dimension of the system. We argue that this is true for all horizontal mechanical isolation systems with spring-antispring dynamics. This imposes strict requirements on mechanical spring-antispring systems for the seismic isolation in potential future low-frequency gravitational-wave detectors as we discuss for the four main concepts: atom-interferometric, superconducting, torsion-bars, and conventional laser interferometer.
The first detection of a gravitational-wave signal of a coalescence of two black holes marked the beginning of the era of gravitational-wave astronomy, which opens exciting new possibilities in the fields of astronomy, astrophysics and cosmology. The currently operating detectors of the LIGO and Virgo collaborations are sensitive at relatively high frequencies, from 10 Hz up to about a kHz, and are able to detect gravitational waves emitted in a short time frame of less than a second (binary black holes) to minutes (binary neutron stars). Future missions like LISA will be sensitive in lower frequency ranges, which will make it possible to detect gravitational waves emitted long before these binaries merge. In this article, we investigate the possibilities for parameter estimation using the Fisher-matrix formalism with combined information from present and future detectors in different frequency bands. The detectors we consider are the LIGO/Virgo detectors, the Einstein Telescope (ET), the Laser Interferometer Space Antenna (LISA), and the first stage of the Deci- Hertz Interferometer Gravitational wave Observatory (B-DECIGO). The underlying models are constructed in time domain, which allows us to accurately model long-duration signal observations with multiband (or broadband) detector networks on parameter estimation. We assess the benefit of combining information from ground-based and space-borne detectors, and how choices of the orbit of B-DECIGO influence parameter estimates.
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