Broadband quantum enhancement of the LIGO detectors with frequency-dependent squeezing

Quantum noise imposes a fundamental limitation on the sensitivity of interferometric gravitational-wave detectors like LIGO, manifesting as shot noise and quantum radiation pressure noise. We present the realization of frequency-dependent squeezing for the fourth astrophysical observing run of the LIGO detectors, resulting in the reduction of both shot noise and quantum radiation pressure noise, alongside broadband detector sensitivity enhancement from tens of hertz to several kilohertz. Frequency-dependent squeezing was enabled by the addition of a 300-meter-long in-vacuum filter cavity at each detector, as part of the LIGO A+ upgrade. For the LIGO Hanford detector, squeezing reduced the detector noise power by up to 5 dB near 1 kHz; for the Livingston detector, detector noise power was reduced by up to 6 dB. These improvements directly impact LIGO's scientific output for high-frequency sources (e.g., binary neutron star postmerger physics). The improved low-frequency sensitivity, which boosted the detector range by almost 20% with respect to no squeezing, corresponds to increased astrophysical detection rates of up to 70%.