The Laser Interferometer Gravitational-Wave Observatory (LIGO) has been bargaining with the limits of quantum mechanics. In its observations of gravitational waves, which distort its 4 km interferometer arms by less than the width of a proton, quantum uncertainty is the dominant source of noise. Fortunately, the Heisenberg uncertainty principle is willing to make a deal: You can measure one quantity, such as a particle’s position or a wave’s amplitude, as precisely as you like, but at the expense of increasing the uncertainty elsewhere, such as in the particle’s momentum or the wave’s phase.
For LIGO, which makes ultraprecise comparisons of light waves traversing different paths, phase is the most important thing. So ever since the beginning of its third observing run in 2019, it’s been using so-called squeezed states of light, engineered to have reduced uncertainty in phase and increased uncertainty in amplitude. (See Physics Today, November 2011, page 11, and the Quick Study by Sheila Dwyer, November 2014, page 72.)
Overall, squeezed light has been a boon to LIGO’s ability to detect gravitational waves. But there’s a fly in the ointment: For detection of low-frequency signals, phase squeezing makes the noise worse. That’s because at low frequencies, the dominant source of quantum uncertainty isn’t phase noise but rather radiation pressure noise—the force of the squeezed light hitting the suspended mirrors and jiggling them around—which is compounded by increased amplitude fluctuations.
Now, in the fourth observing run, LIGO researchers are getting the best of both worlds: They’re using light that’s squeezed in phase at high frequencies and in amplitude at low frequencies. Importantly, “frequency” refers not to the light’s frequency but rather to that of the quantum fluctuations. Such an optical state—which simultaneously has higher and lower phase uncertainty, depending on how it’s measured—is a challenge to even describe, let alone create. But today’s gravitational-wave observatories stand on the shoulders of decades of theory, and the road map to frequency-dependent squeezing was laid out in a paper in 2001.
As merging pairs of black holes or neutron stars—the gravitational-wave sources that LIGO detects—orbit each other faster and faster in their final instants before colliding, they show up at LIGO as rising-frequency chirps. (See Physics Today, April 2016, page 14.) Sensitivity across a wide frequency range helps a lot in identifying those characteristic signals. Now that they’ve implemented frequency-dependent squeezing, the LIGO researchers estimate that the volume of space from which they can detect events is 65% larger than it was before. (D. Ganapathy et al. [LIGO O4 Detector Collaboration], Phys. Rev. X 13, 041021, 2023.)