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Improving the sensitivity of advanced gravitational-wave detectors using discrete sampling variation measurement technique

We propose a new measurement concept that can be implemented in the third generation of gravitational-wave (GW) detectors: the discrete sampling variation measurement (DSVM). We demonstrate that this method allows to overcome the standard quantum limit (SQL) for free body and improve the sensitivity of interferometric gravitational wave detectors in wide frequency band. DSVM technique is based on a well known method of quantum variation measurement that suggests to modulate in the optimal way the local oscillator phase in homodyne readout scheme of interferometric detector in order to measure such quadrature of the output light wave that has minimal uncertainty at the moment and therefore can be measured with precision, higher than SQL. The main disadvantage of variation measurement is the necessity to know "a priori" the form of detected signal in order to choose optimally the modulation function. It can be overcome if one modulates the local oscillator phase periodically with period less than sampling period required by Nyquist-Kotel'nikov-Shannon sampling theorem, and then filters the output data using optimally chosen filter functions.