The ETpathfinder facility in Maastricht has reached several important milestones on its way to demonstrating key technologies for the future Einstein Telescope. Designed as a low-noise testbed for next-generation gravitational-wave detectors, ETpathfinder is now transitioning from infrastructure installation to subsystem commissioning. 

A major achievement is the successful installation of two large suspended optical benches using an innovative off-location assembly and testing approach. This method allows complex seismic isolation systems to be assembled, tested and commissioned outside of the vacuum system before being transported and re-tuned at their final location. In October 2025, the suspension system of the Beam Splitter Tower was successfully hoisted into the vacuum system followed by the installation of the Injection Tower suspension system in March 2026. Video footage of both milestones is available via the respective links: BST and INT.

With the suspended benches installed, the first laser measurements in the ETpathfinder towers became possible, using a 1550 nm pre-stabilized laser developed by the Albert Einstein Institute and marking the first laser light to travel into the vacuum system.

Progress is also being made on the cryogenic technologies that will be needed for Einstein Telescope’s unprecedented low-frequency sensitivity. High-purity silicon mirrors have been produced by IKZ Berlin and polished by Zeiss MT to a quality comparable to those used in operating gravitational-wave observatories, while the subsystems of ETpathfinder’s cryogenic system are currently being assembled under the lead of Demcon Kryoz and the University of Twente. In the upcoming month, the assembly of the inner mechanics of the South Input Tower including the mirror suspension chain and the cryogenic system will begin. The tower’s cryostat will house two silicon mirrors which allows configuring a simple Michelson interferometer with 15K end mirrors. This interferometer configuration is an intermediate step towards a cryogenic Fabry-Perot Michelson interferometer.

For readers interested in the technical details, the following presentation given by Luise Kranzhoff at GWADW in May 2026 provides a comprehensive overview of the latest developments across the ETpathfinder collaboration: ET Docs

More ETpathfinder presentations at GWADW:

• poster presentation: ‘Off-Location Installation Approach for ETpathfinder Bench-Tower Mechanics’ by Dr. Marco Vardaro
• poster presentation: ‘The 1550 nm Injection System of ETpathfinder’ by Elise Van den Bossche