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In the coming years, the gravitational-wave community will be optimizing detector performance to target a variety of astrophysical sources which make competing demands on detector sensitivity in different frequency bands. In this paper we describe a number of technologies that are being developed as anticipated upgrades to the Advanced LIGO detectors and quantify the potential sensitivity improvement they offer. Specifically, we consider squeezed light injection for the reduction of quantum noise, detector design and materials changes which mitigate thermal noise and mirrors with significantly increased mass. We explore how each of these technologies impacts the detection of the most promising gravitational-wave sources and suggest an effective progression of upgrades which culminates in a twofold improvement in broadband sensitivity. [ABSTRACT FROM AUTHOR]

The LIGO gravitational wave (GW) detectors will begin collecting data in 2015, with Virgo following shortly after. These detectors are expected to reach design sensitivity before the end of the decade, and yield the first direct detection of GWs before then. The use of squeezing has been proposed as a way to reduce the quantum noise without increasing the laser power, and has been successfully tested at one of the LIGO sites and at GEO in Germany. When used in Advanced LIGO without a filter cavity, the squeezer improves the performances of detectors above ~100 Hz, at the cost of a higher noise floor in the low-frequency regime. Frequency-dependent squeezing, on the other hand, will lower the noise floor throughout the entire band. Squeezing technology will have a twofold impact: it will change the number of expected detections and it will impact the quality of parameter estimation for the detected signals. In this work we consider three different GW detector networks, each utilizing a different type of squeezer-all corresponding to plausible implementations. Using LALInference, a powerful Monte Carlo parameter estimation algorithm, we study how each of these networks estimates the parameters of GW signals emitted by compact binary systems, and compare the results with a baseline advanced LIGO-Virgo network. We find that, even in its simplest implementation, squeezing has a large positive impact: the sky error area of detected signals will shrink by ~30% on average, increasing the chances of finding an electromagnetic counterpart to the GW detection. Similarly, we find that the measurability of tidal deformability parameters for neutron stars in binaries increases by ~30%, which could aid in determining the equation of state of neutron stars. The degradation in the measurement of the chirp mass, as a result of the higher low-frequency noise, is shown to be negligible when compared to systematic errors. Implementations of a quantum squeezer coupled with a filter cavity will yield a better overall network sensitivity. They will give less drastic improvements over the baseline network for events of fixed signal-to-noise ratio but greater improvements for identical events. [ABSTRACT FROM AUTHOR]

Cosmic Explorer is a next-generation ground-based gravitational-wave observatory concept, envisioned to begin operation in the 2030s and expected to be capable of observing binary neutron star and black hole mergers back to the time of the first stars. Cosmic Explorer's sensitive band will extend below 10 Hz, where the design is predominantly limited by geophysical, thermal, and quantum noises. In this work, thermal, seismic, gravity-gradient, quantum, residual gas, scattered-light, and servo-control noises are analyzed in order to motivate facility and vacuum system design requirements, potential test mass suspensions, Newtonian noise reduction strategies, improved inertial sensors, and cryogenic control requirements. Our analysis shows that, with improved technologies, Cosmic Explorer can deliver a strain sensitivity better than 10-23 Hz-1/2 down to 5 Hz. Our work refines and extends previous analysis of the Cosmic Explorer concept and outlines the key research areas needed to make this observatory a reality. [ABSTRACT FROM AUTHOR]

Gravitational waves produced at kilohertz frequencies in the aftermath of a neutron star collision can shed light on the behavior of matter at extreme temperatures and densities that are inaccessible to laboratory experiments. Gravitational-wave interferometers are limited by quantum noise at these frequencies but can be tuned via their optical configuration to maximize the probability of postmerger signal detection. We compare two such tuning strategies to turn Advanced LIGO into a postmerger-focused instrument: first, a wideband tuning that enhances the instrument's signal-to-noise ratio 40-80% broadly above 1 kHz relative to the baseline, with a modest sensitivity penalty at lower frequencies; second, a "detuned" configuration that provides even more enhancement than the wideband tuning, but over only a narrow frequency band and at the expense of substantially worse quantum noise performance elsewhere. With an optimistic accounting for instrument loss and uncertainty in postmerger parameters, the detuned instrument has a 40 % sensitivity improvement compared to the wideband instrument. [ABSTRACT FROM AUTHOR]

Quantum vacuum fluctuations fundamentally limit the precision of optical measurements, such as those in gravitational-wave detectors. The injection of a conventional squeezed vacuum can be used to reduce quantum noise in the readout quadrature, but this reduction is at the cost of increasing noise in the orthogonal quadrature. For detectors near the limits imposed by quantum radiation pressure noise (QRPN), both quadratures impact the measurement, and the benefits of conventional squeezing are limited. In this paper, we demonstrate the use of a critically coupled 16 m optical cavity to diminish antisqueezing at frequencies below 90 Hz, where it exacerbates QRPN, while preserving beneficial squeezing at higher frequencies. This is called an amplitude filter cavity, and it is useful for avoiding degradation of detector sensitivity at low frequencies. The attenuation from the cavity also provides technical advantages such as mitigating backscatter. [ABSTRACT FROM AUTHOR]

Vacuum quantum fluctuations impose a fundamental limit on the sensitivity of gravitational-wave interferometers, which rank among the most sensitive precision measurement devices ever built. The injection of conventional squeezed vacuum reduces quantum noise in one quadrature at the expense of increasing noise in the other. While this approach improved the sensitivity of the Advanced LIGO and Advanced Virgo interferometers during their third observing run (O3), future improvements in arm power and squeezing levels will bring radiation pressure noise to the forefront. Installation of a filter cavity for frequency-dependent squeezing provides broadband reduction of quantum noise through the mitigation of this radiation pressure noise, and it is the baseline approach planned for all of the future gravitational-wave detectors currently conceived. The design and operation of a filter cavity requires careful consideration of interferometer optomechanics as well as squeezing degradation processes. In this paper, we perform an in-depth analysis to determine the optimal operating point of a filter cavity. We use our model alongside numerical tools to study the implications for filter cavities to be installed in the upcoming "A+" upgrade of the Advanced LIGO detectors. [ABSTRACT FROM AUTHOR]