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We report on the cancellation of quantum backaction noise in an optomechanical cavity. We perform measurements of the displacement of the microresonator, one in reflection of the cavity and one in transmission of the cavity. We show that measuring the amplitude quadrature of the light transmitted by the optomechanical cavity allows us to cancel the backaction noise between 2 and 50 kHz as a consequence of the strong optical spring present in the detuned cavity. This cancellation yields a more sensitive measurement of the microresonator's position with a 2 dB increase in sensitivity. To confirm that the backaction is eliminated, we measure the noise in the transmission signal as a function of circulating power and use a correlation technique between two photodetectors to remove shot noise. Remaining backaction noise would be observable as a power-dependent noise floor, which is not observed. Eliminating the effects of backaction in this frequency regime is an important demonstration of a technique that could be used to mitigate the effects of backaction in interferometric gravitational wave detectors such as Advanced LIGO, VIRGO, and KAGRA.

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