GWADW2022 - Approaching the low-frequency design sensitivity of ground-based detectors

23 May 2022, 00:00 → 28 May 2022, 01:00 Asia/Tokyo

GatherTown and ZOOM

During the last observation period, gravitational wave detectors have discovered an impressive number of events, and reduced upper limits on expected but yet undiscovered sources. Improved detectors are currently being commissioned and will soon be ready for another one-year data collection. In the mean time, preparation work for the next generation observatories is picking up pace.
The improvement of gravitational wave detectors’ sensitivity has been uneven. Good results have been obtained for quantum noise reduction and a significant progress in coating thermal noise is expected. However, the low-frequency noise has been for many years well above the design level. For this year workshop, held again virtually only, the low-frequency sensitivity of ground based interferometers will be the central subject, covering topics from the scientific motivation for low frequency, to the current status of noise sources, and plan for diagnosis, mitigation and possibly suppression by design.

There are 2 sessions per day and each session is for 2 hours. No parallel sessions. No pre-recorded talks. There are 3 poster sessions and each poster is presented at one of the sessions. We will use ZOOM for oral presentations and GatherTown for poster presentations. 

We have increased the poster sessions this year as we think more interactive discussions would be possible with GatherTown. While all the oral sessions are dedicated to the low-frequency sensitivity, any subject is welcome in the poster session. We expect many of you to give a presentation there.

The registration opens from April 1st through May 15th and the abstract submission deadline is April 30th. The registration fee is 2800 JPY (~23 USD) per person, which mainly covers the fee to use GatherTown. The ZOOM and GatherTown information will be sent to those who completed the registration and payment. (Do not forward the information to others.)

book-of-abstracts.pdf ProgramGWADW2022revised4.pdf sessionI.pdf sessionIII.pdf TimeTableGWADW2022v9.pdf

Monday, 23 May

13:30 → 15:30 Plenary

Convener: Kentaro Somiya (Tokyo Institute of Technology)

13:30 Introduction of the meeting

Speaker: Kentaro Somiya (Tokyo Institute of Technology)

13:35 Observing black holes throughout the Universe

I will overview how extending detector sensitivities at low gravitational wave frequencies will let us observe the first merging stellar black holes forming at cosmic dawn, and unveil the still elusive population of intermediate-mass black holes, which provides clues on the yet unknown origins of the massive black holes lurking at the centres of today's galaxies.

Speaker: Prof. Colpi Monica (University of Milano Bicocca)

14:15 Multi-messenger observations of gravitational-wave transients

Starting from the example of GW170817, I will discuss prospects for multi-messenger gravitational-wave astronomy, highlighting opportunities to clarify currently open questions in the field, while expanding multi-messenger studies beyond the realm of compact binary mergers.

Speaker: Alessandra Corsi (Texas Tech University)

14:35 Approaches to low frequency noise in LIGO

This talk will overview the current views of low frequency noise contributions in LIGO. This overview provides context to discuss potential upgrade options either to reduce direct noise contributions or to address underlying causes of complex bilinear noises.

Speaker: Lee McCuller (Caltech)

14:55 AdV+ low frequency noises challenges and plans

Advanced Virgo Plus is a two step plan for improving the sensitivity
of Advanced Virgo for the O4 and O5 observing runs. After discussing
the low frequency noises that have limited AdV in O3, we will present
the planned AdV+ upgrades and adjustments that aim at reducing the low
frequency fundamental and technical noises for O4 and O5.

Speaker: Michal Was (LAPP/CNRS)

22:30 → 00:30 Scatter light for LF: Scattered light for LF

Conveners: Alena Ananyeva (Caltech), Siddharth Soni

Tuesday, 24 May

00:30 → 01:00 Review of LIGO's LF workshop

00:30 Review of LIGO's LF workshop

Speaker: Peter Fritschel (MIT)

13:30 → 15:30 Poster session I

13:30 Angled beam expander telescopes for the Michelson beams in third generation Gravitational Wave Observatories

Third generation of Gravitational Wave detectors like the Einstein Telescope or the
Cosmic Explorer will be Michelson interferometers with Fabry-Perot cavities in the arms,
using mirror test masses with diameter at the limit of technical feasibility. Unlike other
detectors, the Einstein Telescope will have a 60° angle between the arms. Because of its
larger incidence angle, at any given beam size, it would require beam splitters almost double
in size and much heavier than the 90° case. It is proposed here to install beam expander
telescopes with angled mirrors located inside the Michelson interferometer between the
Fabry-Perot cavities and the beam splitter. Beyond reducing the beam sizes and the
beam splitter to manageable sizes, the proposed solution allows to bring the optimal
recombination angle to 90°. The proposed geometry offers a natural way to separate the
beam splitters of different detectors into individual, smaller and more stable caverns, thus
improving observatory observation-time efficiency, to provide needed beam diagnostic
points and convenient degrees of freedom for beam alignment into both the Fabry-Perot
cavities and the beam splitter, as well as to provide a method for maintaining optimal mode
matching of the two arms onto the beam splitter without thermal compensation plates.

13:30 Current Status of Quantum Locking Experiment for Space Gravitational Wave Antenna DECIGO

The DECi-hertz Interferometer Gravitational wave Observatory (DECIGO) is the future Japanese space mission with 1,000 km arm cavities. One of the main objectives of DECIGO is the detection of primordial gravitational waves (PGWs) produced in the inflation period. We should improve DECIGO’s target sensitivity, which is limited by quantum noise, to enhance the possibility of the detection of PGWs.

The standard squeezing techniques to reduce the quantum noise are not effective because of the large diffraction loss in DECIGO due to the long arm length. Therefore, we proposed a new method, quantum locking with an optical spring, to reduce the quantum noise in a relatively broad frequency band. Quantum locking is the technique, in which each mirror of the long arm cavity (main cavity) is shared by two short-arm cavities (sub-cavities). Then the sub-cavities control the mirrors’ motion of the main cavity. Interferometer signals obtained from the main cavity and the two sub-cavities can be combined to optimize the sensitivity of DECIGO.

In parallel with the theoretical analysis of the technique, we have been performing the experiment to verify the principle of the theory. In this poster session, we explain the current status of the quantum locking experiment.

13:30 Displacement-noise-free neutron interferometer for gravitational wave detection at low frequencies

Improvement of the sensitivity of gravitational-waves (GWs) detectors at lower frequencies is still challenging on account of displacement noise sources, such as thermal noise, seismic noise, and radiation pressure noise. One of the solutions is the displacement-noise-free interferometer (DFI). At frequencies lower than 1Hz, however, the DFI has less sensitivity to GWs because the propagation time of light is much shorter than the period of the GWs. To resolve this problem, DFI with neutrons instead of laser, which is called a neutron DFI, was proposed. In a neutron DFI with neutrons propagating much more slowly than light, the neutron propagation time can be comparable to the period of GWs at lower frequencies. This enables us to cancel displacement noise without cancellation of the GW signals. Also, we proposed a simplification of the detector configuration by taking advantage of the ability to adjust the neutron speeds depending on the configuration. In our poster, we discuss the principle of the neutron DFI as well as a plan of the demonstration experiment.

13:30 Gravitational wave sources in the low frequency region and their distances

Pulsars are expected to be strong sources of low frequency gravitational waves in ground based interferometers. The knowledge of their distances is a key parameter to estimate the gravitational emission. The pulsar distances are usually estimated using dispersion measure. The Gaia data release provides information on the distance, kinematic and photometric properties of nearly two billions astronomical sources, among them some pulsars and accreting neutron star systems.
The Gaia based distances of some systems relevant for gravitational astronomy will be discussed.

13:30 Localization of gravitational waves using machine learning

An observation of gravitational waves is a trigger of the multi-messenger search of an astronomical event. A combination of the data from two or three gravitational wave detectors indicates the location of a source and low-latency data analysis is key to transferring the information to other detectors sensitive at different wavelengths. In contrast to the current method, which relies on the matched-filtering technique, we proposed the use of machine learning that is much faster and possibly more accurate than matched filtering.
Our machine-learning method is a combination of the method proposed by Chatterjee et al. and a method using the temporal convolutional network.
We demonstrate the sky localization of a gravitational-wave source using four detectors: LIGO H1, LIGO L1, Virgo, and KAGRA, and compare the result in the case without KAGRA to examine the positive influence of having the fourth detector in the global gravitational-wave network.

13:30 Optimization of design parameters for Gravitational Wave detector DECIGO including fundamental noises

The DECi-hertz Interferometer Gravitational-Wave Observatory (DECIGO) is a space gravitational wave (GW) detector. DECIGO was originally designed to be sensitive enough to observe primordial GW background (PGW). However, due to the lowered upper limit of the PGW by the Planck observation, further improvement of the target sensitivity of DECIGO is required. In the previous studies, DECIGO’s parameters were optimized to maximize the signal-to-noise ratio (SNR) of the PGW to quantum noise including the effect of diffraction loss. To simulate the SNR more realistically, we optimize DECIGO’s parameters considering the GWs from double white dwarfs (DWDs) and the thermal noise of test masses. We consider two cases of the cutoff frequency of GWs from DWDs. In addition, we consider two kinds of thermal noise: thermal noise in a residual gas and internal thermal noise. To investigate how the mirror geometry affects the sensitivity, we calculate it by changing the mirror mass and thickness. As a result, we obtained the optimums for the parameters that maximize the SNR that depends on the mirror radius. This result shows that a thick mirror with a large radius gives a good SNR and enables us to optimize the design of DECIGO.

13:30 Practical quantum noise estimate of optical-spring quantum locking for space gravitational wave detector DECIGO

The DECi-hertz Interferometer for Gravitational-wave Observatory(DECIGO) aims mainly at the detection of primordial gravitational waves (PGWs) originating from inflation. Recent observations by the Planck satellite and others have lowered the upper limit of PGWs. Thus, it is necessary to improve the target sensitivity of DECIGO. A newly proposed method to reduce the quantum noise of DECIGO is quantum locking with an optical spring. In this method, a short cavity is added to the main cavity, sharing one mirror of both cavities. The error signal in this auxiliary cavity is obtained properly in a homodyne detection, and fed back to the shared mirror to cancel the radiation pressure noise of the main cavity. In our previous study, the optimal sensitivity assuming ideal homodyne detection without any additional noise was obtained by simulation. In this study, we investigate a more realistic design, taking into account the mixture of the vacuum fluctuations incident to the homodyne detection system. In this poster, we explain the latest results of this investigation

13:30 Probing dipole radiation with the low-frequency gravitational-wave observatories

Atom-interferometer gravitational-wave (GW) observatory, as a new design of ground-based GW detector for the near future, is sensitive at a relatively low frequency for GW observations. Taking the proposed atom interferometer Zhaoshan Long-baseline Atom Interferometer Gravitation Antenna (ZAIGA), and its illustrative upgrade (Z+) as examples, we investigate how the atom interferometer will complement ground-based laser interferometers in testing the gravitational dipole radiation from binary neutron star (BNS) mergers. A test of such kind is important for a better understanding of the strong equivalence principle laying at the heart of Einstein's general relativity. To obtain a statistically sound result, we sample BNS systems according to their merger rate and population, from which we study the expected bounds on the parameterized dipole radiation parameter $B$. Extracting BNS parameters and the dipole radiation from the combination of ground-based laser interferometers and the atom-interferometer ZAIGA/Z+, we are entitled to obtain tighter bounds on $B$ by a few times to a few orders of magnitude, compared to ground-based laser interferometers alone, ultimately reaching the levels of $|B| < 10^{-9}$ (with ZAIGA) and $|B| < 10^{-10}$ (with Z+).

low-frequency poster.pdf

13:30 Prospects for Detecting Exoplanets around Double White Dwarfs with LISA and Taiji

Recently, Tamanini & Danielski discussed the possibility of detecting circumbinary exoplanets (CBPs) orbiting double white dwarfs (DWDs) with the Laser Interferometer Space Antenna (LISA). Extending their methods and criteria, we discuss the prospects for detecting exoplanets around DWDs not only by LISA, but also by Taiji, a Chinese space-borne gravitational-wave (GW) mission. We first explore how different binary masses and mass ratios affect the abilities of LISA and Taiji to detect CBPs. Second, for certain known detached DWDs with high signal-to-noise ratios, we quantify the possibility of CBP detections around them. Third, based on the DWD population obtained from the Mock LISA Data Challenge, we present basic assessments of the CBP detections in our Galaxy during a 4 yr mission time for LISA and Taiji. We discuss the constraints on the detectable zone of each system. With the DWD population, we further inject two different planet distributions with an occurrence rate of 50% and constrain the total detection rates. We briefly discuss the prospects for detecting habitable CBPs around DWDs with a simplified model. These results can provide helpful inputs for upcoming exoplanetary projects and help analyze planetary systems after the common envelope phase.

Prospects for detecting exoplanets around double white dwarfs with LISA and Taiji_Kang.jpeg

13:30 Realistic Detection and Early Warning of Binary Neutron Stars with Decihertz Gravitational-wave Observatories

We investigated the detection and localization of binary neutron star (BNS) populations with decihertz gravitational-wave observatories in a realistic detecting strategy, including real-time observations and early warnings. Assuming 4 years' operation of B-DECIGO, we found that the detected BNSs can be divided into three categories: (a) sources that merge within 1 year, which could be localized with an uncertainty of $\Delta\Omega \sim 10^{0}$deg$^2$; (b) sources that merge in 1-4 years, which take up three quarters of the total events and yield the most precise angular resolution with $\Delta \Omega\sim 10^{-2}$deg$^2$ and time-of-merger accuracy with $\Delta t_c\sim 10^{-1}$s; and (c) sources that do not merge during the 4-yr mission window, which enable possible early warnings, with $\Delta \Omega\sim 10^{-1}$deg$^2$ and $\Delta t_c\sim 10^{0}$s. Furthermore, we compared the pros and cons of B-DECIGO with the Einstein Telescope, and explored the prospects of detections using 3 other decihertz observatories and 4 BNS population models. In realistic observing scenarios, we found that decihertz detectors could even provide early-warning alerts to a source decades before its merger while their localizations are still more accurate than ground-based facilities.

13:30 Space gravitational wave antenna DECIGO

DECi-hertz Interferometer Gravitational-wave Observatory (DECIGO) is a future Japanese space gravitational-wave antenna with a frequency band of 0.1 Hz to 10 Hz. DECIGO aims at detection of primordial gravitational waves, which could have been produced during the inflationary period right after the birth of the universe. There are many other scientific objectives of DECIGO, including the direct measurement of the acceleration of the expansion of the universe, and reliable and accurate predictions of the timing and locations of neutron star/black hole binary coalescences. DECIGO consists of four clusters of observatories placed in the heliocentric orbit. Each cluster consists of three spacecraft, which form three differential Fabry-Perot interferometers with an arm length of 1,000 km. Three clusters of DECIGO will be placed far from each other, and the fourth cluster will be placed in the same position as one of the three clusters to obtain the correlation signals for the detection of the primordial gravitational waves. In this presentation, we will explain the aimed sciences, the mechanical and optical design, and the current status of DECIGO.

13:30 Space GW Antenna B-DECIGO

B-DECIGO is a space gravitational wave antenna mission. While it is a precursor mission of DECIGO, we can expect fruitful sciences with B-DECIGO. One of the most exciting science cases is detection of compact binary system before merger. It will enlarge the possibility of multi-messenger astronomy with electro-magnetic wave observations at the time of merger. In this presentation , we will review the science cases and mission concept of B-DECIGO.

13:30 The Current Status of TOrsion-Bar Antenna (TOBA) Experiment

Torsion-bar antenna (TOBA) is a ground-based gravity gradiometer proposed for measurement of gravity gradient fluctuations such as gravitational waves and gravity gradient noise. TOBA consists of two perpendicular torsion pendulum, and the low mechanical resonant frequency of torsion pendulums enables us to measure gravity gradient of frequencies around 0.1 Hz. TOBA aims to achieve the sensitivity 10^(-19) / √Hz at 0.1 Hz.
For the final sensitivity goal we are developing a prototype Phase-III TOBA in order to investigate technical issues and establish noise reduction scheme. One of the key topic of Phase-III TOBA is cryogenic suspension system for the reduction of the thermal noise. Another key point is the readout system with monolithic interferometer. We will show the current situation of the developments and future upgrade plans for further improvement.

13:30 The Sar-Grav Laboratory at the Sos Enattos site, one of the quietest site in the 2-10 Hz frequency range

Einstein Telescope (ET) will be the third generation of gravitational wave interferometer to be built in Europe. One of the sites candidates to host ET is located in Sardinia (Italy), near the Sos Enattos mine, where a seismometer’s net already proves the quietness of the site. The Sar-Grav laboratory, a seed of ET, aims to host underground experiments, cryogenic payloads, low frequency and cryogenic sensor development that need low seismic and anthropogenic noise. On the surface there are a hangar of about 900 square meters, an optical laboratory and a control room; a 20 tons crane and cleaned rooms are planned to be installed. Underground, an area of 250 square meters and small experimental areas are planned to be built, while different stations at different depths are hosting sensors like seismometers and magnetometers. A fundamental physics experiment, Archimedes, is under installation in the surface area and will be moved underground in the future.
The site will host the test of the preliminary seismic isolation system, currently under studies , that will be designed to improve seismic attenuation in the low frequency region (0.1-10 Hz) and reduce the frequency of mechanical resonances.

22:30 → 00:30 Suspensions for LF

Conveners: Joris van Heijningen (UCLouvain), Lucia Trozzo (Istituto Nazionale di Fisica Nucleare)

22:30 Introduction

Speaker: Joris van Heijningen (UCLouvain)

22:35 The latest development in ALFRA, the UWA low-frequency rotational accelerometer

Ground rotation sensors at low-frequency have a vital role in improving seismic isolation systems in advanced and 3G gravitational wave detectors. It was found that using seismometers as the sole source of ground motion measurement results in undistinguishable motion detection between horizontal and tilt motion, especially below 100 mHz. Therefore, pure angular motion measurement is necessary to separate tilt and translation to be applied as feedback in active isolation control. ALFRA is a low-frequency rotational accelerometer developed at the University of Western Australia (UWA) to detect ground tilt motion. It is a compact, inertial reference style rotation sensor that can be mounted in three orientations to detect ground tilt around a different axis of interest. A preliminary study of a prototype showed that ALFRA can achieve high readout sensitivity of few nrad/√Hz above 20 mHz and 0.1 nrad/√Hz above 50 mHz in measuring ground tilt. In this work, we will present our latest design for ALFRA and highlight several improvements that will be added to our previous design to enhance several aspects of the sensor and increase its sensitivity, usability, and convenience of adjustment.

Speaker: Dr Ammar Al-Jodah (The University of Western Australia)

22:55 Update on sensing seismic platform relative motion using Digital Interferometry

The relative motion of seismic platforms, via coupling to the auxiliary length controls of the suspended optics, are predicted to be the limiting noise source for future gravitational-wave detectors at frequencies below 1 Hz. By measuring, then stabilizing this relative motion, the effective control feedback to the optics will be reduced and hence the noise coupling will be less, and potentially improve detector noise performance. The measurement of the relative motion with forms of suspension platform interferometry is an ongoing area of interest and research. Digitally-enhanced Interferometry is a decade-mature technique for sensing relative motion, by providing time-tagged pseudorandom phase modulation to isolate signals based on time-of-flight delay. The application of digitally-enhanced interferometry for suspension sensing is an active area of development within the Newtonian Noise research program at the Australian National University, and offers another potential method for sensing relative platform motion. We present an update on recent developments of digitally-enhanced interferometry towards suspension sensing and measurement.

Speaker: Sheon Chua (ANU Centre for Gravitational Astrophysics - OzGrav)

23:15 New Generation Superattenuators for Einstein Telescope

Seismic noise and local disturbances are dominant noise below 10 Hz (0.1-10 Hz).With the introduction of high performance seismic isolation systems based on mechanical pendula,the 2nd generation GW antennas have reached the scientific goal of the direct observation of GW signals thanks to the extension of the frequency band down to 10 Hz. Now,the 3rd generation instrument era is approaching and the Einstein Telescope giant interferometer is becoming a reality with the possibility to install the detector in an underground site where seismic noise is 100 times smaller then on surface. Moreover,new available technologies and the experience acquired in operating advanced detectors are key points to further extend the detection bandwidth down to 2-3 Hz with the possibility to suspend cryogenic payload and then mitigating Thermal Noise too.In this talk, we present the preliminary studies devoted to improve seismic attenuation performance of the Advanced VIRGO Superattenuator in the low frequency region.Following the experimental lines,we analyze the possibility to improve the vertical attenuation performance with a multistage pendulum chain equipped with magnetic anti-springs that is hung to a double Inverted Pendulum in nested configuration.The feedback control requirements and the possible strategies to be adopted for this last element, will be presented.

Speaker: Lucia Trozzo (Istituto Nazionale di Fisica Nucleare)

23:35 E-TEST: A Compact Isolation Concept for Future Einstein Telescope

This study presents a low frequency isolation system in the framework of E-TEST project which is a research facility for Einstein Telescope. The isolation system combines a passive inverted pendulum and an active inertial platform. The design of this isolator allows reducing the overall height of the isolation system. We address the isolation system design, its dynamics and the control strategy applied. The simulation results show that the seismic noise could successfully be reduced by about 3 orders of magnitude at 1 Hz in horizontal when the control is applied. To avoid spoiling the performance at high frequency, the inertial platform is designed in such a way that the first flexible internal mode appears above 300 Hz.

Speaker: AMEER SIDER (Precision Mechatronics Laboratory, Université de Liège, B-4000 Liège, Belgium)

Wednesday, 25 May

13:30 → 15:30 Technical noise for LF

Conveners: Daniel Brown (University of Adelaide), Jenne Driggers (Caltech, LIGO Hanford)

22:30 → 00:30 Poster session II

Thursday, 26 May

13:30 → 15:50 Cryogenics for LF

Conveners: Ettore Majorana, Takafumi Ushiba (ICRR)

13:30 Sapphire suspension in KAGRA –Current design and perspective-

Cooling mirrors and their suspensions are promising way to reduce thermal noise. To maximize the benefits of cooling, utilizing low mechanical loss material at cryogenic temperature is essential; therefore, KAGRA mirrors and their suspension fibers are made of sapphire. To achieve good cryogenic suspension, there are several technical difficulties because the suspension needs not only low mechanical loss for thermal noise reduction but also high thermal conductivity for keeping mirror temperature low. In this talk, we present a review of the current KAGRA sapphire suspension and its perspective.

Speaker: Dr Takafumi Ushiba (ICRR)

14:10 Silicon suspensions: thermal noise and mechanical properties

The design of cryogenic suspensions for mass tests for future generation GW detectors is based on the balancing of several factors: mechanical properties, thermal conductivity, resonance frequencies, thermo-mechanical stress and generally any parameter that aims to reduce thermal noise of test masses. The talk will present the state of the art of studying the mechanical and thermal properties of silicon as a function of the orientation of the axes and how these can influence the design of a monolithic suspension. Furthermore, the measurements of the limit load on fibers produced at the maximum of the current technique will be described and compared with the expected limit load, trying to understand the limiting factors. Finally, we will describe the tests done so far to measure the mechanical dissipations of the mono-crystalline silicon fibers and the limits found.

Speaker: Flavio Travasso (University of Camerino - INFN Perugia)

14:30 Thermal and mechanical simulation of the cryo-payload: status and preliminary results.

In the design of the cryogenic payload, thermal and mechanical FEA models are used for optimizing the system both for its structural and thermal behavior. The thermal study is important to have the temperature distribution along the suspension wires and the thermal resistances of to the various interconnections between the parts of the suspension. The mechanical study gives the estimation of the losses present in the system. We will present the status of this combined method to have an estimation of the suspension thermal noise with the Levin method.

Speaker: Dr Paola Puppo (INFN Roma)

14:50 A new approach for suspending cryogenic mirrors

The Einstein Telescope will increase the sensitivity to gravitational wave detections with respect to the current detectors, especially in the low-frequency band (down to 2 Hz). Reaching such sensitivities at low frequencies implies great technology challenges: in order to reduce the seismic and the thermal noise very soft suspensions and cryogenic temperatures are needed. A big challenge arises here: to bring and to keep a test mass at temperatures around 10 K we need to extract heat from it. The only way to do this is via thermal conductivity of the test mass suspension elements. However, soft suspensions are non-compatible with the need of extracting heat which would require short suspensions with a large cross section. We propose here a new suspension morphology in order to allow a good heat extraction without spoiling the softness of the suspensions. This should provide a good solution to one of the biggest technological problems of building the Einstein Telescope.

Speaker: Francesca Badaracco (UCLouvain)

15:30 Sapphire technology for cryogenic detectors: new results from Lyon

The institutes iLM and iP2i in Lyon are involved on a significant effort to develop the sapphire technology for the future cryogenic detectors. iLM is taking care of the crystalline growth of ultra-low level of optical absorption and mechanical losses of mirror substrates and last stage suspensions. iP2i through the platform LMA develops the ultra-low level of total optical losses of mirrors, working on aberration, scattering and absorption. The collaboration is finalising the development of a large oven able to grow 500kg sapphire ingots for the production of 450mm diameter mirrors. In parallel the investigation on the growth of ultra-low absorption sapphire is ongoing and the details on the most recent result of 10ppm/cm sapphire will be presented. The impact that this new advancement will have on the ET suspensions will be given too.

Speaker: Teo Aventin (iLM)

22:30 → 00:30 Missing issues for LF

Conveners: Gabriele Vajente (Caltech), Matthew Evans (MIT)

Friday, 27 May

13:30 → 15:30 Poster session III

13:30 Angular Signal Amplification with a Coupled Cavity for Torsion-Bar Antenna

Torsion-Bar Antenna (TOBA) is a ground-based gravitational wave detector using a torsion pendulum. The resonant frequency of torsional motion is ~1 mHz, therefore TOBA has good design sensitivity of $10^{-19} \, / \sqrt{\mathrm{Hz}}$ at 0.1 Hz in low frequencies (0.1 Hz – 10 Hz). TOBA can detect intermediate mass black hole binary mergers, Newtonian noise, and so on. A prototype detector Phase-III TOBA with a 35 cm-scale pendulum is under development to demonstrate noise reduction. The target sensitivity is set to $10^{-15} \, / \sqrt{\mathrm{Hz}}$ at 0.1 Hz. To achieve our target sensitivity, we need to measure the pendulum rotation precisely. We propose a coupled wavefront sensor (Coupled WFS) as an angular sensor for Phase-III TOBA. In our method, an auxiliary cavity is used to compensate Gouy phase of a main cavity and enhance the first-order TEM modes in the main cavity. The experimental demonstration was successfully performed in 2021. In this workshop, we will show the principle and demonstration results of a Coupled WFS.

13:30 CO2 mode cleaner for Thermal Compensation System of Advanced Virgo+

As planned for its fifth observation run O5, Advanced Virgo+ will have 80 Watts in main laser. The absorption of laser power in the interferometer's core optics leads to thermal effects causing optical aberrations, ultimately preventing interferometer's operation. To recover detector's ideal operation, Thermal Compensation System (TCS) is needed to correct wavefront distortions. In particular, to correct the axisymmetric part of the spurious thermal lens in the power recycling cavity, a heating pattern is projected on a compensation plate using Double Axicon System (DAS) where a 50 Watts CO2 laser beam is reshaped using axicons. Due to O5 stringent requirements on the residual of DAS correction, heating pattern distortions caused by the known amount of higher-order modes (HOM) in the CO2 beam cannot be tolerated. To remove these HOMs, we are constructing an optical mode cleaner which will allow us to retain 95% of the CO2 laser power for compensation with a strong reduction of HOM related residual correction. To our knowledge, this is the first time a mode cleaner is designed for a high power CO2 laser. We present here the requirements, motivation and current status of the work, discussing the issues related to the CO2 wavelength and power.

13:30 Compensating decoherence of squeezed light in cavity-enhanced quantum metrology

Quantum states of light are being more commonly used to increase the sensitivity of various sensors. They allow to reach high sensitivity without using significant light power, and thus find application in various fields, from biological sensing to gravitational-wave detection. At the same time, these states are very fragile, and even a small amount of decoherence can significantly reduce their benefit. We propose a new approach that allows to compensate part of quantum decoherence, thus increasing the sensitivity beyond the previously established decoherence-induced quantum limit. To achieve this, we use an optimally tuned quantum squeezer placed directly inside the detector cavity. This squeezer operates to restore the externally injected squeezing or to amplify the signal, depending on the level of loss. It can be flexibly tuned to the optimal operation. We present the first experimental combination of intra-cavity and externally injected squeezing used to enhance detector’s sensitivity. We demonstrate for the first time how optimal tuning allows to compensate quantum decoherence. Finally, we derive the new decoherence-induced quantum limit. Based on this approach, we develop the quantum expander for the detection bandwidth of GW detectors, which allows to significantly increase the sensitivity at high frequencies.

13:30 Control of Dual-Pass Fabry-Perot Cavity for space gravitational wave antennas : DECIGO and B-DECIGO

A dual-pass Fabry-Perot cavity will be used for DECIGO (DECi-hertz Interferometer Gravitational-wave observatory) and B-DECIGO. To detect gravitational waves, it is necessary to establish the method to control the dual-pass Fabry-Perot cavity. We can divide this issue in two parts, “Length control” and “Alignment control”. For Length control, it is demonstrated that we can control the length of dual-pass Fabry Perot cavity with Pound-Drever-Hall technique. On the other hand, for Alignment control, though the method was already proposed (WaveFront Sensor and Beam Pointing Control), it is not demonstrated yet. Therefore, an experiment is needed for the demonstration.In this poster, we show the principle to control the dual-pass Fabry-Perot cavity in the direction of the angle and explain the experiment to demonstrate it.

13:30 Estimating the Newtonian noise of groundwater at the KAGRA

Changes due to gravitational waves are very small, so noise is generated due to various factors. KAGRA was built 300 meters underground to reduce ground vibrations. The groundwater generated underground is discharged through pipes. The gravity gradient generated by the universal gravitation force due to the oscillation of the water surface through the pipe may cause the mirror of KAGRA to shake and become a noise to the target sensitivity of KAGRA.
Our experiment was conducted using the simulation software Flow-3D in order to know the magnitude of Newtonian noise. The Newtonian noise was evaluated by calculating the waveform of the flowing water.

13:30 Juggled interferometer for gravitational wave detection

Juggled interferometer (JIFO) is a novel type of earthbound gravitational wave detector targeting a frequency band of 0.1–10 Hz. By using repeatedly free-falling test masses, JIFO can in principle decouple test masses from the seismically noisy environment and avoid suspension thermal noise in a straightforward manner. Since the test masses are in a weightless state, as is the case with space gravitational wave detectors, JIFO would be a good testbed for technologies of space projects.

Here, the concept of the Michelson-type JIFO is introduced. Then the experiment setup and the data readout method of a JIFO are discussed. Considering the displacement noise budget of the Einstein Telescope (ET), we show that the juggled test masses could significantly improve the sensitivity at 0.1-2.5 Hz even with discontinuous data. The science cases brought with the improved sensitivity would include detecting quasi-normal modes of black holes with 104-105 Msun, testing Brans-Dicke theory with black-hole and neutron-star inspirals, and detecting primordial-black-hole-related gravitational waves.

13:30 Measuring thermal noise in gram-scale Si flexures at 123 K

Future terrestrial gravitational wave detectors are limited by fundamental noise sources, one of which is the thermal noise arising in the test masses and suspensions in the frequency band where ground-based detectors are sensitive. To mitigate this noise, future detectors are envisioned to operate at cryogenic temperatures using silicon optics as test masses and silicon ribbons to suspend the test masses. Silicon ribbons resemble cantilever topology, and therefore studying the thermal noise in the flexing of a gram-scale silicon cantilever is analogous to the suspension thermal noise encountered in these ribbon suspensions. At the Australian National University, I have built an operational cryogenic infrastructure to measure the broadband thermal noise of silicon flexures at 123 K. In this talk, I will present initial results of our cool-down tests, thermal noise measurements from the experiment and future plans.

13:30 Optical absorption of TiO$_{2}$ doped SiO$_{2}$ as a replacement high index coating material

The sensitivity of 3rd generation gravitational wave detectors is currently projected to be
limited by the level of Brownian coating thermal noise produced from highly reflecting
mirror coatings. In the current detectors layers of Ta$_2$O$_5$, a material with higher
refractive index (n = 2.14), has higher levels of optical absorption compared the lower index
layers of SiO$_ {2}$ (n=1.44). To improve detector sensitivity and duty cycle, the optical and
mechanical properties of new doped coating materials are being investigated. This work
presents measurements of the optical absorption of SiO$_2$ and SiO$_2$ doped with
TiO$_2$ layers as part of a highly reflecting coating stack. Utilising the photothermal
common-path interferometry technique, we discuss the effects of heat treatment and
crystallisation on the optical performance of the material compared to current aLIGO
coatings.

13:30 SpicyPy: a common python tool for signal processing and control systems

Within gravitational wave research community (including but not limited to LIGO-Virgo-KAGRA, Einstein Telescope and LISA collaborations) a lot of work in understanding and improving the detectors involves signal processing and modelling of control systems. Historically, different software tools were used for these purposes. We believe that it is possible to create a single software tool that can be useful for many different applications in these domains. This would help to facilitate exchange of knowledge between collaborations, and could be used in teaching.

We aim to develop a python package intended as a general tool with a simple but powerful interface to facilitate control systems modelling, signal processing, and provide an interface between the two. It may rely on other well-known and tested packages already used for these applications, but it will abstract interactions with them with a unified interface. Potential applications include time series analysis, suspensions modelling, feeding sensor signals through a control system, and more. The project is a collaborative open-source effort across the groups from the start, and new contributors are always welcome. We are currently focused on compiling software requirements specification and are in discussions with researches from different collaborations to understand the most common potential applications.

13:30 Substrate transferred aluminum gallium arsenide (AlGaAs) crystalline coatings

Substrate-transferred crystalline coatings made from aluminum gallium arsenide (AlGaAs) have very low thermal noise compared to the ion beam deposited amorphous oxides used until now in gravitational wave detectors. AlGaAs coatings also show excellent optical properties and both thermal noise and optical performance has been demonstrated in other precision optics applications. The primary challenge to using AlGaAs coatings in future detectors is the coating diameter necessary and the large mass and thickness of the test mases. We present results on 10 cm diameter AlGaAs coatings and propose multiple pathways to implement AlGaAs coatings on upgrades to current detectors with up to 40 kg masses and on future detectors with larger masses. We also show schedule and budget plans that allow AlGaAs to be used in future gravitational wave detectors.

13:30 TiO$_{2}$:SiO$_{2}$ coating thermal noise and optical studies

Current gravitational wave detectors are limited in their most sensitive frequency range by the mirror coating thermal noise which arises from the Brownian motion of the coating materials on the interferometer test mass optics. For the next generation detector upgrades and beyond, it is imperative to find coating materials/topologies that reduce this mechanical effect, whilst still meeting the desired optical requirements. Titania-doped silica had been identified as a coating material candidate which could potentially improve detector sensitivity.

We present here our investigations into the mechanical and optical properties of highly-reflective coating stacks made of pure SiO2 and TiO2 doped SiO2, deposited via ion beam sputtering (IBS). Two different concentrations of TiO2 doping in the high-refractive index layers of our coating stacks were investigated, with mechanical loss and optical absorption being measured through different steps of heat treatment for each, with the level of coating thermal noise being calculated from the former.

13:30 Vibration analysis of ETpathfinder cryogenic heat-links

The Einstein Telescope pathfinder (ETpathfinder) is a cryogenic testbed for the next generation of gravitational-waves antennas. To reach the target temperature of 18 K, ETpathfinder cryogenic payloads are designed to extract heat from the test masses by integrating low stiffness and highly conductive heat-links that connect to the cryocoolers. Since the interferometer test masses are very sensitive to mechanical vibrations, the noise from the cryocoolers through the heat conductor should be carefully monitored and controlled. This work presents the modeling and experimental measurements of the mechanical vibrations transferred by the heat-links in a dedicated payload setup. To investigate the stability criterion of the cold-head, seismic noise propagated via the heat-links is then projected to the displacement sensitivity of ETpathfinder.

13:30 Vibration Analysis of KAGRA Cryostat at Cryogenic Temperature

Large-scale Cryogenic Gravitational-Wave Telescope, KAGRA is a second-generation gravitational-wave detector (GWD) located in Japan. The features that distinguish KAGRA from other GWDs are its underground location and cryogenic operation of the four main mirrors. The underground location provides a quiet site with low seismic noise, while the cryogenic operation cools the mirrors down to 20 K, reducing the thermal noises. However, cryocooler vibration and structural resonances of the cryostat can contaminate detector sensitivity as they couple to test mass through the heat-links. Monitoring and characterization of the vibration inside the cryostat is critical for the optimum noise performance of KAGRA.
In April, 2020 KAGRA conducted an international observation run, "O3GK" along with GEO600. During this run several noise sources were identified and a noise budget was prepared. However, as the mirrors were not cooled the noise transfer via heat-links was estimated based on room temperature, in vacuum vibration measurement performed 2.5 years before O3GK. During the upcoming observation run the mirrors will be cooled down, so we performed vibration analysis of the cooling system at cryogenic temperature to study its impact on detector sensitivity. In this poster, we describe the KAGRA cooling system and discuss the results of vibration analysis.

22:30 → 00:30 Summary

Convener: Francesco Fidecaro (University of Pisa & INFN)

22:30 ET at 3 Hz - can we break the 'controls noise' wall?

The Einstein Telescope aims to have astrophysically interesting
sensitivity at 3Hz. The performance goal is a factor of ~10^6 quieter
than LIGO and Virgo at that frequency. This talk will present a recipe
for addressing controls noise in a systematic manner at the design stage
with supporting evidence from existing instruments.

Speaker: Conor Mow-Lowry (Nikhef)

22:50 Cosmic Explorer at low frequency

This talk will discuss the problems that must be solved to achieve Cosmic Explorer's low frequency sensitivity goals.

Speaker: Evan Hall

23:10 Panel discussions

23:40 GWADW2023