  The astrophysical reach of current and future ground-based gravitational-wave
detectors is mostly limited by quantum noise, induced by vacuum fluctuations
entering the detector output port. The replacement of this ordinary vacuum
field with a squeezed vacuum field has proven to be an effective strategy to
mitigate such quantum noise and it is currently used in advanced detectors.
However, current squeezing cannot improve the noise across the whole spectrum
because of the Heisenberg uncertainty principle: when shot noise at high
frequencies is reduced, radiation pressure at low frequencies is increased. A
broadband quantum noise reduction is possible by using a more complex squeezing
source, obtained by reflecting the squeezed vacuum off a Fabry-Perot cavity,
known as filter cavity. Here we report the first demonstration of a
frequency-dependent squeezed vacuum source able to reduce quantum noise of
advanced gravitational-wave detectors in their whole observation bandwidth. The
experiment uses a suspended 300-m-long filter cavity, similar to the one
planned for KAGRA, Advanced Virgo and Advanced LIGO, and capable of inducing a
rotation of the squeezing ellipse below 100 Hz.
