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.