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1. Broadband quantum enhancement of the LIGO detectors with frequency-dependent squeezing

   Ganapathy, D. ; Jia, W. ; Nakano, M. ; Xu, V. ; Dwyer, S.E. ; Mullavey, A. ; McCuller, L. ( October 2023 , Physical review X)

   Quantum noise imposes a fundamental limitation on the sensitivity of interferometric gravitational-wave detectors like LIGO, manifesting as shot noise and quantum radiation pressure noise. Here we present the first realization of frequency-dependent squeezing in full-scale gravitational-wave detectors, resulting in the reduction of both shot noise and quantum radiation pressure noise, with broadband detector enhancement from tens of Hz to several kHz. In the LIGO Hanford detector, squeezing reduced the detector noise amplitude by a factor of 1.6 (4.0 dB) near 1 kHz, while in the Livingston detector, the noise reduction was a factor of 1.9 (5.8dB). These improvements directly impact LIGO’s scientific output for high-frequency sources (e.g., binary neutron star post-merger physics). The improved low-frequency sensitivity, which boosted the detector range by 15–18 % with respect to no squeezing, corresponds to an increase in astrophysical detection rate of up to 65%. Frequency-dependent squeezing was enabled by the addition of a 300-meter long filter cavity to each detector as part of the LIGO A+ upgrade. 

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2. Automated evaluation of environmental coupling for Advanced LIGO gravitational wave detections

   https://doi.org/10.1088/1361-6382/ad5139  

   Helmling-Cornell, A_F ; Nguyen, P. ; Schofield, R_M_S ; Frey, R. ( June 2024 , Classical and Quantum Gravity)

   The extreme sensitivity required for direct observation of gravitational waves by the Advanced LIGO detectors means that environmental noise is increasingly likely to contaminate Advanced LIGO gravitational wave signals if left unaddressed. Consequently, environmental monitoring efforts have been undertaken and novel noise mitigation techniques have been developed which have reduced environmental coupling and made it possible to analyze environmental artifacts with potential to affect the 90 gravitational wave events detected from 2015–2020 by the Advanced LIGO detectors. So far, there is no evidence for environmental contamination in gravitational wave detections. However, automated, rapid ways to monitor and assess the degree of environmental coupling between gravitational wave detectors and their surroundings are needed as the rate of detections continues to increase. We introduce a computational tool,PEMcheck, for quantifying the degree of environmental coupling present in gravitational wave signals using data from the extant collection of environmental monitoring sensors at each detector. We study its performance when applied to 79 gravitational waves detected in LIGO’s third observing run and test its performance in the case of extreme environmental contamination of gravitational wave data. We find thatPEMcheck’s automated analysis identifies only a small number of gravitational waves that merit further study by environmental noise experts due to possible contamination, a substantial improvement over the manual vetting that occurred for every gravitational wave candidate in the first two observing runs. Building on a first attempt at automating environmental coupling assessments used in the third observing run, this tool represents an improvement in accuracy and interpretability of coupling assessments, reducing the time needed to validate gravitational wave candidates. With the validation provided herein;PEMcheckwill play a critical role in event validation during LIGO’s fourth observing run as an integral part of the data quality report produced for each gravitational wave candidate.

    

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3. Incorporating information from LIGO data quality streams into the PyCBC search for gravitational waves

   https://doi.org/10.1103/PhysRevD.106.102006  

   Davis, Derek ; Trevor, Max ; Mozzon, Simone ; Nuttall, Laura K. ( November 2022 , Physical Review D)

   We present a new method which accounts for changes in the properties of gravitational-wave detector noise over time in the PyCBC search for gravitational waves from compact binary coalescences. We use information from LIGO data quality streams that monitor the status of each detector and its environment to model changes in the rate of noise in each detector. These data quality streams allow candidates identified in the data during periods of detector malfunctions to be more efficiently rejected as noise. This method allows data from machine learning predictions of the detector state to be included as part of the PyCBC search, increasing the total number of detectable gravitational-wave signals by up to 5%. When both machine learning classifications and manually generated flags are used to search data from LIGO-Virgo’s third observing run, the total number of detectable gravitational-wave signals is increased by up to 20% compared to not using any data quality streams. We also show how this method is flexible enough to include information from large numbers of additional arbitrary data streams that may be able to further increase the sensitivity of the search. 

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4. Laser frequency noise in next generation gravitational-wave detectors

   https://doi.org/10.1364/OE.439253  

   Cahillane, Craig ; Mansell, Georgia_L ; Sigg, Daniel ( December 2021 , Optics Express)

   Future ground-based gravitational-wave detectors are slated to detect black hole and neutron star collisions from the entire stellar history of the universe. To achieve the designed detector sensitivities, frequency noise from the laser source must be reduced below the level achieved in current Advanced LIGO detectors. This paper reviews the laser frequency noise suppression scheme in Advanced LIGO, and quantifies the noise coupling to the gravitational-wave readout. The laser frequency noise incident on the current Advanced LIGO detectors is$8\times <\#comment/>{10}^{-<\#comment/>5}\mathrm{H}\mathrm{z}/\sqrt{\mathrm{H}\mathrm{z}}$at 1~kHz. Future detectors will require even lower incident frequency noise levels to ensure this technical noise source does not limit sensitivity. The frequency noise requirement for a gravitational wave detector with arm lengths of 40~km is estimated to be$7\times <\#comment/>{10}^{-<\#comment/>7}\mathrm{H}\mathrm{z}/\sqrt{\mathrm{H}\mathrm{z}}$. To reach this goal a new frequency noise suppression scheme is proposed, utilizing two input mode cleaner cavities, and the limits of this scheme are explored. Using this scheme the frequency noise requirement is met, even in pessimistic noise coupling scenarios.

    

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5. Reducing scattered light in LIGO's third observing run

   S Soni, C Austin ( December 2020 , Classical and quantum gravity)

   null (Ed.)

   Noise due to scattered light has been a frequent disturbance in the advanced LIGO gravitational wave detectors, hindering the detection of gravitational waves. The non stationary scatter noise caused by low frequency motion can be recognized as arches in the time-frequency plane of the gravitational wave channel. In this paper, we characterize the scattering noise for LIGO and Virgo's third observing run O3 from April, 2019 to March, 2020. We find at least two different populations of scattering noise and we investigate the multiple origins of one of them as well as its mitigation. We find that relative motion between two specific surfaces is strongly correlated with the presence of scattered light and we implement a technique to reduce this motion. We also present an algorithm using a witness channel to identify the times this noise can be present in the detector. 

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