In order to expand the astrophysical reach of gravitational wave detectors, several interferometer topologies have been proposed to evade the thermodynamic and quantum mechanical limits in future detectors. In this work, we make a systematic comparis
on among them by considering their sensitivities and complexities. We numerically optimize their sensitivities by introducing a cost function that tries to maximize the broadband improvement over the sensitivity of current detectors. We find that frequency-dependent squeezed-light injection with a hundred-meter scale filter cavity yields a good broadband sensitivity, with low complexity, and good robustness against optical loss. This study gives us a guideline for the near-term experimental research programs in enhancing the performance of future gravitational-wave detectors.
Using a network of seismometers and sets of optimal filters, we implemented a feed-forward control technique to minimize the seismic contribution to multiple interferometric degrees of freedom of the LIGO interferometers. The filters are constructed
by using the Levinson-Durbin recursion relation to approximate the optimal Wiener filter. By reducing the RMS of the interferometer feedback signals below sim10 Hz, we have improved the stability and duty cycle of the joint network of gravitational wave detectors. By suppressing the large control forces and mirror motions, we have dramatically reduced the rate of non-Gaussian transients in the gravitational wave signal stream.
