Using quantum correlations to study black holes: squeezing techniques for quantum noise reduction in gravitational wave detectors

Since 2015, the gravitational-wave detectors LIGO and Virgo have opened a new window on the universe, detecting hundreds of signals and launching a new era of astronomy with profound impact on relativity, astrophysics, and cosmology. To listen deeper into the cosmos, detectors must become increasingly sensitive. One of the main limitations is quantum noise, ultimately arising from vacuum fluctuations entering the instrument. By “squeezing” this vacuum, i.e. manipulating its noise properties, LIGO and Virgo have already extended their reach by up to 65%, revealing events that would otherwise remain hidden. This article provides an overview of squeezing techniques for gravitational-wave detectors, from their origins to the most recent advances.