Gravitational wave detectors are limited by fundamental quantum noise – an incessant “hum” that they cannot ever remove. But now physicists have recently improved a technique, called “squeezing”, that can allow the next generation of detectors to double their sensitivity.
All the gravitational waves sloshing around the universe are incredibly weak. When they wash over the Earth, even the strongest waves wiggle no more than the width of an atomic nucleus. Our detectors, like LIGO and VIRGO that bounce laser beams back and forth, need to measure these tiny differences. But when they do, they run into the fundamental uncertainty of the universe dictated by quantum mechanics.
That fundamental uncertainty is called the Heisenberg Uncertainty Principle, and it tells us that certain sets of measurements (like, say, position and momentum of a particle, or the phase and brightness of a light beam) can never be as precise as we want. That uncertainty limits the size of gravitational waves that we can detect – they have to be bigger than that background quantum “noise” due to the uncertainty principle.
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But physicists are clever folks, and they’ve come up with a way to trick the uncertainty principle by “squeezing” light. In essence, the trick works by carefully preparing the light source, making the phase measurement more precise (which is what we use to detect the gravitational waves) with an associated cost in the measurement of the brightness (which we don’t care about so much). This way, the overall uncertainty principle is obeyed while getting more precision out of the observations.
The team at one laser experiment, GEO600, was able to reduce the fundamental quantum noise by a factor of two. If their system was at something like LIGO, then LIGO would be able to detect gravitational waves twice as faint, increasing the observing capabilities of the instrument.
“We have focused on optimizing and characterizing the squeezed-light source at GEO600 and its interface to the detector. Compared to a detector without squeezing the observable volume of the Universe has now increased by a factor of 8 at high frequencies. This could help improve our understanding of neutron stars,” says Dr. James Lough, lead scientist for GEO600 and first author of the publication that appeared in Physical Review Letters.
This technology will be needed to enable the next generation of detectors. “The international community is currently planning the third generation of gravitational-wave detectors: the European Einstein Telescope and Cosmic Explorer in the US. Both will need even higher levels of squeezing than the impressive results we obtained. GEO600 is in the ideal position to further optimize this technology,” says Prof. Karsten Danzmann, director at the AEI and director of the Institute for Gravitational Physics at Leibniz Universität Hannover.