Research Facilities for Europe’s Next Generation Gravitational-Wave Detector Einstein Telescope

The Einstein Telescope is Europe’s next generation gravitational-wave detector. To developall necessary technology, four research facilities have emerged across Europe: The Amaldi ResearchCenter (ARC) in Rome (Italy), ETpathfinder in Maastricht (The Netherlands), SarGrav in the SosEnattos mines on Sardinia (Italy) and E-TEST in Liége (Belgium) and its surroundings. The ARCpursues the investigation of a large cryostat, equipped with dedicated low-vibration cooling lines, totest full-scale cryogenic payloads. The installation will be gradual and interlaced with the payloaddevelopment. ETpathfinder aims to provide a low-noise facility that allows the testing of fullinterferometer configurations and the interplay of their subsystems in an ET-like environment.ETpathfinder will focus amongst others on cryogenic technologies, silicon mirrors, lasers and opticsat 1550 and 2090 nm and advanced quantum noise reduction schemes. The SarGrav laboratory has asurface lab and an underground operation. On the surface, the Archimedes experiment investigatesthe interaction of vacuum fluctuations with gravity and is developing (tilt) sensor technology for theEinstein Telescope. In an underground laboratory, seismic characterisation campaigns are undertakenfor the Sardinian site characterisation. Lastly, the Einstein Telecope Euregio meuse-rhine Site &Technology (E-TEST) is a single cryogenic suspension of an ET-sized silicon mirror. Additionally,E-TEST investigates the Belgian–Dutch–German border region that is the other candidate site forEinstein Telescope using boreholes and seismic arrays and hydrogeological characterisation. In thisarticle, we describe the Einstein Telescope, the low-frequency part of its science case and the fourresearch facilities.