In one of the research corridors at Nikhef in Amsterdam, work is underway on a compact new lab for research into ultra-cold mirrors for gravitational-wave detectors. The coldest and quietest in the world.
As post-doc researcher Viola Spagnuolo opens the door to the research corridor, the visitor is greeted by the roar of pumps and cooling units. Ribbed metal hoses disappear through passages in the back wall, behind which lies the new lab. The coldest and quietest mirror lab in the world, although that is hard to imagine here at the moment.
Two doors further down the whisper-quiet clean room, the actual setup stands on a sturdy yellow metal frame. A horizontal steel vessel with a large access hatch, with hoses and cables entering through flanges.
On the top of the structure is the vessel housing a vibration-isolation system. This system was brought to life through the detailed engineering and design work of Rogier Elsinga, from Nikhef’s Mechanical Technology Department. The pioneering cooling system was developed in collaboration with the University of Twente, featuring their innovative approach to the system’s thermal storage unit.
A small porthole is currently still closed, but will eventually become the point through which lasers can shine in for measurements.
All these facilities, explains Spagnuolo, are designed to enable efficient testing of coatings for the silicon mirrors that will be used in the next generation of gravitational-wave detectors. Specifically: the proposed Einstein Telescope.
The Einstein Telescope will be an underground observatory for gravitational waves from the Universe. In 2015, such minute vibrations, which alter the fabric of spacetime, were observed for the first time using interferometers. By resolving the change in the distance between the system’s suspended mirrors, the interferometers can detect events from the deepest reaches of the Universe.
Gravitational-wave research has now become a mature scientific discipline, also at Nikhef, which is one of the partners of a large underground gravitational-wave observatory in Europe.
The exact configuration and location of the Einstein Telescope are still under active study. However, it will be a laser interferometer designed to operate at ultra-low temperatures, in order to minimise the dominant noise source, the thermal noise.
Gravitational waves are extremely weak signals by the time they reach Earth, so the detectors must be as still as possible. Mechanical noise and vibrations can now be effectively suppressed using advanced suspension systems. But even then, thermal movements of mirror surfaces still cause too much interference.
Extreme cooling to 10-20 K is the solution to this thermal noise, but glass mirrors, as the current fused silica mirrors, are unsuitable at those temperatures. The Einstein Telescope will therefore use crystalline silicon mirrors, with specific suitable coatings, to effectively reflect laser light in order to detect gravitational waves.
However, silicon also has a drawback: it absorbs the standard 1064 nm laser light used by existing detectors, which in turn causes thermal noise. The Einstein Telescope will therefore use 1550 nm lasers, to which silicon is transparent. But also this is relatively uncharted territory, requiring a great deal of research.
The cooling system and the silicon mirrors are new and require a great deal of further research and development, just like the use of 1550 nm lasers. And that is precisely why, Spagnuolo points out, this lab at Nikhef is the best place a researcher could wish for. “It is ultra-quiet and ultra-cold in an ultra-high vacuum, and we have an unprecedentedly more flexible workspace compared to previous experiments as the one in the US.”
The new Coating Thermal Noise lab was originally conceived back in 2018 by Nikhef researchers Alessandro Bertolini and Matteo Tacca and is now nearing completion. In addition to Nikhef, the construction has been funded by the Dutch Black Hole Consortium, a national interdisciplinary research collaboration in the field of black holes.
The setup in Amsterdam is still in the testing phase, but is already yielding good results, says Spagnuolo. It is now possible to cool down the system to 4 K under high vacuum. Work is currently underway on a procedure to first cool as deep as possible and then switch off the cooling and pumping for a few minutes, so as not to introduce any vibrations or noise during the coating thermal noise measurements. The aim, says the researcher, is half an hour of measurement time. She believes this is achievable.
This summer, the laser at 1550nm wavelength will also be installed on an optical bench next to the measurement setup. The start of the actual research into optimal coatings for the mirrors of the future detectors is scheduled for early 2027.
