Giacomo Bruno, Morgane Zeoli

On Feb 1, 2020 the R&D EU Interreg project E-TEST officially started. It involves 11 institutes from Belgium, Germany and Netherlands and will carry on crucial detector developments for the Einstein Telescope (ET) - a 3rd generation antenna of gravitational waves, related mostly to cryogenic operations of large mass mirrors and their suspensions, ultra-precise metrology and sensing, as well as to advanced geological studies in the region (the ET is a deep-underground detector). The CP3 group is a partner in this project and is working on work package 1 : "Ultra-cold vibration control" and in particular on a cryogenic superconducting inertial sensor.

Gravitational wave signals below a frequency of about 10 Hz are obscured by thermal noise in current detectors. Because temperature is the vibration of atoms in some respect, making the distance measurement between the mirror surfaces more challenging, the mirrors of future detectors will need to be cooled down to temperatures around 10 K. We need to control the motion of some of the cold objects, for which we develop inertial sensors that can survive this harsh environment. The interferometric readout of the inertial sensor also serves as to monitor a ringdown or the E-TEST mirror. After it is excited by a tiny hammer strike, the interferometer follows the ringdown and can determine the quality factor. Additionally, we are investigating an alternative suspension technique, where instead of long fibres under tension, we use short flexures under compression in combination with long, fat rods so we obtain good thermal conductivity and low stiffness suspension.

CP3 members collaborate mostly with KU Leuven (we are collaborating to develop cryogenic readout electronics for the sensor) and ULiège (we align our sensor efforts), RWTH Aachen (they are preparing a cryostat where we will test the inertial sensor).

External collaborators: C. Collette (Liege), S. Hild (Maastricht), A. Bertolini (Nikhef), A. Gatto (KULeuven) and E-TEST collaboration.

Charlotte Bragard, Giacomo Bruno, Ricardo Cabrita, Pavel Demin, Swapnil Dhage, Rodney Eckler, Vito Garzone, Aaron Goodwin-Jones, Alexander Moncello, Nicolas Szilasi, Morgane Zeoli, Jue Zhang

Gravitational wave detectors based on Michelson interferometry (such as LIGO, Virgo, ET) use very high laser powers and quantum-optics to overcome fundamental noises associated with the Heisenberg Uncertainty Principle. However, these high optical powers present a number of challenges including:
- Themoelastically driven changes in the mirror radii of curvature. This leads to:
+ Reduced optical power, therefore reduced sensitivity,
+ Modifications to the phase of optical control fields, limiting the operational time and power of the detectors
+ Quantum hyperloss, leading to reduced sensitivity
- Parametric instability - an optomechanical coupling that degrades interferometer sensitivity
- Angular instability, such as siggs-sidles.

ET-OPT is a next-generation high-power interferometry platform being developed at UCLouvain to advance optical technologies for future gravitational-wave observatories such as the Einstein Telescope and Virgo. The project focuses on developing real-time optical mode control loops in a prototype scale infrastructure. This unlocks the following possibilities:
- Increase optical power in today's gravitational wave detectors (such as Virgo)
- A concrete route towards the 3 MW of optical power targeted by ET
- New possibilities, such as the use of higher order modes for coating brownian noise reduction
- Dynamic control of gouy phase, without causing hyperloss, thus facilitating PI suppression

The work is carried out in close collaboration with Virgo and ET scientific consortia.

External collaborators: Nicolas Letendre (Laboratoire d'Annecy de Physique des Particules), Alain Masserot (Laboratoire d'Annecy de Physique des Particules), Emmanuel Pacaud (Laboratoire d'Annecy de Physique des Particules), Carl Blair (University of Western Australia), Haochen Zhu (University of Western Australia), Chunnong Zhao (University of Western Australia), Liu Jian (University of Western Australia), Christophe Collette (ULiege), Matteo Tacca (Nikhef), Martin van Beuzekom (Nikhef), Sebastian Steinlechner (ETpf), André Füzfa (UNamur).