Gravitational-Wave Detector for Postmerger Neutron Stars: Beyond the Quantum Loss Limit of the Fabry-Perot-Michelson Interferometer

Advanced gravitational-wave detectors that have made groundbreaking discoveries are Michelson interferometers with resonating optical cavities as their arms. As light travels at a finite speed, these cavities are optimal for enhancing signals at frequencies within the bandwidth, beyond which, however, a small amount of optical loss will significantly impact the high-frequency signals. We find an elegant interferometer configuration with an "L resonator" as the core, significantly surpassing the loss-limited sensitivity of dual-recycled Fabry-Perot-Michelson interferometers at high frequencies. Following this concept, we provide a broadband design of a 25-km detector with outstanding sensitivity between 2 and 4 kHz. We perform Monte Carlo population studies of binary neutron-star mergers, given the most recent merger rate from the GWTC-3 catalog and several representative neutron-star equations of state. We find that the new interferometer configuration significantly outperforms other third-generation detectors by a factor of 1.7 to 4 in the signal-to-noise ratio of the postmerger signal. Assuming a detection threshold with signal-to-noise ratio > 5 and for the cases we explore, the new design is the only detector that robustly achieves a detection rate of the neutron-star postmerger larger than one per year, with the expected rate between O(1) and O(10) events per year.