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Abstract:

We propose a next-generation international ground-based gravitational-wave detector in Taiwan, the Cryogenic sub-Hz cROss torsion-bar detector with quantum NOn-demolition Speed meter (CHRONOS), optimized for the unexplored 0.1–10 Hz frequency band between the space-based detector LISA and future ground-based detectors such as Cosmic Explorer and the Einstein Telescope. CHRONOS combines a ring-cavity Sagnac interferometer with torsion-bar test masses to realize the first quantum nondemolition (QND) measurement of angular momentum in a macroscopic system. By implementing a speed-meter readout in the rotational degree of freedom, CHRONOS coherently suppresses quantum radiation-pressure noise, enabling observations in the sub-Hz regime. We have also proposed a novel detuned power-recycling scheme to further optimize the detector sensitivity. A distinctive feature of CHRONOS is its compact design. Unlike kilometer-scale gravitational-wave observatories, the experiment can be implemented within a footprint of approximately 10 m × 10 m. This compact configuration makes it possible to develop and operate a sub-Hz gravitational-wave detector in a conventional underground laboratory while retaining sensitivity to a wide range of astrophysical, cosmological, and geophysical signals. Based on a realistic optical design incorporating torsion-bar test masses, we estimate a strain sensitivity of approximately (h ~ 1 x 10^{-18} 1/√Hz) at 2 Hz with 2.5 m arm lengths. This sensitivity enables (i) the direct detection of intermediate-mass black hole binaries, (ii) searches for stochastic gravitational-wave backgrounds, (iii) tests of Yukawa-type deviations from gravity, and (iv) searches for dark matter. Furthermore, even the 2.5 m prototype is expected to enable the prompt detection of gravity-gradient signals from earthquakes with magnitudes of approximately M5.5. CHRONOS therefore opens new opportunities for quantum-limited geophysical observations as well as multi-band and multi-messenger gravitational-wave astronomy. In this talk, we will present the current status of the CHRONOS project and recent progress in its international collaboration.