Sensitivity below the standard quantum limit in gravitational wave detectors with Michelson-Fabry-Perot readout

We calculate the quantum noise limited displacement sensitivity of a Michelson-Fabry-Perot (MFP) interferometer with detuned cavities, followed by phase-sensitive homodyne detection. We show that the standard quantum limit can be surpassed even with resonant cavities and without any signal-recycling mirror nor additional cavities. Indeed, thanks to the homodyne detection, the output field quadrature can be chosen in such a way to cancel the effect of input amplitude fluctuations, i.e., eliminating the force noise. With detuned cavities, the modified opto-mechanical susceptivity allows to reach unlimited sensitivity for large enough (yet finite) optical power. Our expressions include mirror losses and cavity delay effect, for a realistic comparison with experiments. Our study is particularly devoted to gravitational wave detectors and we consider both an interferometer with free-falling mirrors, and a MFP interferometer as readout for a massive detector. In the latter case, the sensitivity curve of the recently conceived ‘‘DUAL’’ detector, based on two acoustic modes, is obtained.