Short answer: Gravitational-wave detectors measure tiny length changes with laser light, and quantum noise in that light limits their sensitivity over much of the detection band. Injecting squeezed states of light reduces that noise. A 2018 review covers the method, a 2022 article explains the role of a filter cavity, and a 2024 Science paper reports that LIGO with a squeezed-light and filter-cavity upgrade improved sensitivity over a wide frequency range and, over a smaller range, went below the standard quantum limit. This is quantum-enhanced sensing in a working instrument.
Quantum noise in the detector
A 2018 review in Reports on Progress in Physics notes that LIGO made the first direct detection of gravitational waves a century after general relativity, and says the current generation of detectors are the least sensitive devices that could have done the job, so improved detectors are needed to use the new window fully. The review covers using squeezed vacuum states of light to reduce quantum noise, which it says currently limits performance in much of the detection band. The 2024 Science paper explains the underlying limit: kilometre-scale interferometers measure tiny expansions and contractions of spacetime, and precision is limited by quantum mechanics. The Heisenberg uncertainty principle prevents measuring a mirror's position and momentum with arbitrary precision at once, giving a standard quantum limit (SQL).
Why a filter cavity
A 2022 article in Galaxies gives the design logic. Once other noise sources are low enough that radiation pressure dominates at low frequencies, injecting frequency-independent squeezing degrades sensitivity at some frequencies. Reflecting the squeezed field off a filter cavity rotates and filters the squeezing in a frequency-dependent way, giving a broadband improvement. Its worked example chooses parameters giving 6 dB of shot-noise reduction and 8 dB of increase in quantum radiation pressure noise without a filter cavity, versus a 6 dB improvement in radiation pressure noise with one. The article also shows a layout with frequency-dependent squeezing injected at the output port, described as planned for the advanced LIGO and Virgo detectors in their fourth observing run.
The LIGO measurement
The 2024 Science paper reports the performance of LIGO after frequency-dependent squeezing was realised in the instrument, designed to surpass the SQL. For the LIGO Livingston detector, the authors found the upgrade reduces quantum noise below the SQL by up to three decibels between 35 and 75 hertz, while giving a broadband improvement. The summary says the upgrades improved detector sensitivity across a wide frequency range, and over a smaller range surpassed the quantum limit.
What this does not tell you
The improvement is quoted for one detector and particular frequency bands. The papers describe quantum noise reduction and do not by themselves say how many astrophysical events a detector will see.
FAQ
What limits gravitational-wave detectors?
Quantum noise in the laser light, over much of the detection band, per the 2018 review.
What is squeezed light?
A state of light with reduced uncertainty in one quantity at the cost of increased uncertainty in another.
Why add a filter cavity?
It makes the squeezing frequency dependent, so sensitivity improves across the band.
What did LIGO report?
Quantum noise reduced below the standard quantum limit by up to 3 dB between 35 and 75 Hz.
Is this only a lab demonstration?
It was measured on the LIGO Livingston detector itself.
All field notes