  We report on the broadband measurement of quantum radiation pressure noise
(QRPN) in an optomechanical cavity at room temperature over a broad range of
frequencies relevant to gravitational-wave detectors. We show that QRPN drives
the motion of a high-reflectivity single-crystal microresonator, which serves
as one mirror of a Fabry-Perot cavity. In our measurements QRPN dominates over
all other noise between 10 kHz and 50 kHz and scales as expected with the
circulating power inside the cavity. The thermal noise of the microresonator,
the largest noise source next to the QRPN, is measured and shown to agree with
a structural damping model from 200 Hz to 30 kHz. By observing the effects of
QRPN in the audio-band, we now have a testbed for studying techniques to
mitigate back-action, such as variational readout and squeezed light injection,
that could be used to improve the sensitivity of gravitational-wave detectors.
