by Gloria Nobile
Currently, the only instruments capable of detecting the faint signals produced by gravitational waves – tiny ripples in spacetime – are laser interferometers. Technologies being developed for Einstein Telescope (ET), the future observatory that will join the already active LIGO, Virgo and KAGRA, aim to increase sensitivity, enabling the observation of a volume of the universe a thousand times larger.
To achieve this, ET must address several challenges, including eliminating or mitigating various noise sources that could compromise measurements. Terrestrial interferometers operate within a frequency band ranging from a few hertz to several kilohertz. However, each frequency interval is dominated by different noise sources, each with varying impacts on the performance of experiments like Advanced Virgo or, in the future, the Einstein Telescope. Optimizing the detection of gravitational wave signals – known as the “instrument’s sensitivity” – therefore requires identifying and minimizing the most limiting noise in each frequency range.
Noise often overlaps with the signal of a gravitational wave, making it essential for ET to distinguish the desired signal from these disturbances. If not adequately reduced, such noise could make measurements impossible. «At higher frequencies, above 300 hertz, the main disturbance is quantum noise, an effect tied to the very nature of the laser light used in interferometers», explains Barbara Garaventa, head of the ET Genova research unit and part of the ETIC project. «Reducing this noise is crucial because the effects of gravitational waves are so small they can be confused with the vacuum fluctuations of light».
This noise arises both from fluctuations in the number of photons reaching the detector and from tiny variations in the mirrors’ positions due to the pressure exerted by the light itself. To mitigate these effects, a quantum optics technique called “squeezing” is used, which reduces quantum uncertainty in one of the light’s two parameters, position or momentum. «Another way to reduce quantum noise at high frequencies is to increase the input power in the interferometer», adds Lorenzo Aiello from the University of Rome Tor Vergata, also a researcher in the ETIC project. «This is one of the main reasons ET will consist of two detectors: one optimized for low frequencies, using cryogenics, and another optimized for high frequencies, with up to three megawatts of circulating power».
Thermal noise, linked to energy dissipation in the mirrors and suspension materials, dominates the frequency band between 10 hertz and a few hundred hertz. A promising solution for reducing it is cryogenics, already implemented in the Japanese KAGRA detector. This technique involves cooling the mirrors and suspensions to drastically reduce thermal fluctuations that could interfere with measurements. However, cryogenic cooling must be balanced with laser power, as the latter tends to heat the mirrors. Given that the mirrors are subjected to extreme mechanical and thermal stress, an active line of research focuses on identifying new coating materials to replace current fused silica substrates. These materials must resist laser-induced thermal stresses while also having low optical losses. Silicon, for example, is an ideal candidate for cryogenic mirrors due to its thermal conductivity and low dissipation. Advanced optical coatings are also being developed to reduce thermal noise without compromising reflectivity.
«The materials used in optics are characterized by various properties. Regarding the substrate, a key parameter is absorption, that is how much power is absorbed when a laser beam passes through», notes Aiello. «Einstein Telescope will require materials with far superior performance to current ones across several aspects. Currently, different laboratories, like ours in Tor Vergata, are working to characterize these materials and identify the most promising ones for the next generation of detectors».
However, even with improved absorption, the use of such high laser power will inevitably cause significant heat absorption in the mirrors, generating optical aberrations much larger than current ones. «If not properly compensated, optical aberrations – undesired deformations of light wavefronts – can seriously compromise the instrument’s performance», continues Aiello. «This is why, at Tor Vergata, leveraging years of experience in correcting aberrations for Advanced Virgo, we are developing new methods for detecting and correcting them in preparation for ET, with the creation of a dedicated facility».
At lower frequencies, below 10 hertz, the predominant noise is seismic, caused by natural and artificial ground vibrations. The Earth’s surface is constantly shaken by phenomena such as tectonic plate movements, wind, ocean waves, and human activities. These movements transmit through suspensions to the interferometer mirrors, altering the gravitational wave signal.
Fluctuations caused by people, vehicles, trains, and nearby activities also generate variations in the Earth’s gravitational field, known as Newtonian noise. Unlike seismic noise, this directly affects the mirrors and cannot be physically shielded. The solution is to place the telescope underground, where ground movements are significantly reduced. For this reason, the Einstein Telescope will be located approximately 200 meters underground.
Another challenge for third-generation interferometers is sensitivity to magnetic fields and their fluctuations, referred to as magnetic noise, which is particularly critical at low frequencies. Efforts to reduce this noise focus on controlling environmental noise from the detector, the so-called self-inflicted noise, and reducing the coupling coefficient, that is the efficiency with which magnetic fields interact with sensitive components of the detector, interfering with ET’s measurements. Strategies to mitigate magnetic noise near the interferometer’s sensitive parts need to be developed.
«In Genoa, we are developing the MANET facility to characterize the magnetic emission of various devices and compensate for external fields», concludes Garaventa. «One mitigation strategy involves shielding magnetic devices with layers of high magnetic permeability material. MANET will allow us to quantify shielding effectiveness by comparing results with and without shielding, as well as verify prototype quality against simulation data».
To learn more, watch the interviews with Barbara Garaventa and Lorenzo Aiello on the ET Italy YouTube channel.
Optical squeezing benches