  We experimentally demonstrate a cancellation of back-action noise in optical
measurements. Back-action cancellation was first proposed within the framework
of gravitational-wave detection by dual resonators as a way to drastically
improve their sensitivity. We have developed an experiment based on a
high-finesse Fabry-Perot cavity to study radiation-pressure effects in
ultra-sensitive displacement measurements. Using an intensity-modulated
intracavity field to mimic the quantum radiation-pressure noise, we report the
first observation of back-action cancellation due to a destructive interference
between radiation-pressure effects on both mirrors of the cavity. We have
observed a sensitivity improvement by a factor larger than 20 both in
displacement and weak force measurements.
