Gravitational-wave detectors like Advanced LIGO probe the most massive objects and cataclysmic events in the universe, yet they remain at the whim of minute quantum fluctuations. In recent years, we have entered the era of quantum-enhanced gravitational-wave detection, wherein squeezed states are injected into the interferometers to mitigate high-frequency quantum fluctuations. As gravitational-wave detectors continue to improve, radiation pressure noise is increasingly becoming a limiting factor at low frequencies. A frequency-dependent squeezed source can suppress both of these noises to achieve a broadband improvement in sensitivity. In this work, we use a long, high-finesse optical cavity to demonstrate this technique at gravitational-wave frequencies.

Meanwhile, gravitational-wave detectors are also now reaching the sensitivities required to observe quantum effects on the kilogram-scale of the test masses. We show how the superb displacement precision of Advanced LIGO can be used to feedback-cool the test mass differential motion near the quantum ground state.