  Quantum vacuum fluctuations impose strict limits on precision displacement
measurements, those of interferometric gravitational-wave detectors among them.
Introducing squeezed states into an interferometer's readout port can improve
the sensitivity of the instrument, leading to richer astrophysical
observations. However, optomechanical interactions dictate that the vacuum's
squeezed quadrature must rotate by 90 degrees around 50Hz. Here we use a
2-m-long, high-finesse optical resonator to produce frequency-dependent
rotation around 1.2kHz. This demonstration of audio-band frequency-dependent
squeezing uses technology and methods that are scalable to the required
rotation frequency, heralding application of the technique in future
gravitational-wave detectors.
