Conceptual design and science cases of a juggled interferometer for gravitational wave detection

The juggled interferometer (JIFO) is an Earth-based gravitational wave detector using repeatedly freefalling test masses. With no worries of seismic noise and suspension thermal noise, the JIFO can have much better sensitivity at lower frequencies than the current Earth-based gravitational wave detectors. The data readout method of a JIFO could be challenging if one adopts the fringe-locking method. We present a phase reconstruction method in this paper by building up a complex function which has a fringe-independent signal-to-noise ratio. Considering the displacement noise budget of the Einstein Telescope (ET), we show that the juggled test masses significantly improve the sensitivity at 0.1-2.5 Hz even with discontinuous data. The science cases brought with the improved sensitivity would include detecting quasinormal modes of black holes with 104-105 M circle dot, testing Brans-Dicke theory with black-hole and neutron-star inspirals, and detecting primordial-black-hole-related gravitational waves.