CHRONOS: Cryogenic sub-Hz cROss torsion bar detector with quantum NOn-demolition Speed meter

Abstract: We propose a next-generation ground-based gravitational-wave detector, the Cryogenic sub-Hz cROss torsion-bar detector with quantum NOn-demolition Speed meter (CHRONOS), optimized for the unexplored 0.1-10Hz band bridging the gap between the space-based 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 cancels quantum radiation-pressure noise and enables sub-Hz observations. We show that detuned power recycling and cavity-length optimization can simultaneously relax technical requirements on both torsion bars and speed meters. Assuming a realistic optical design with a 1m torsion bar, we estimate strain sensitivities of $h \simeq 1 \times 10^{-18}\mathrm{Hz}^{-1/2}$ at 1Hz for detectors with arm lengths of 2.5m, 40m, and 300m. These sensitivities enable (i) direct detection of intermediate-mass black hole binaries up to 380Mpc with ${\rm SNR}=3$, (ii) probing stochastic gravitational-wave backgrounds down to $\Omega_{\rm GW} \sim 3.2 \times 10^{-9}$ at 0.2Hz with 5yr accumulation. Furthermore, CHRONOS enables prompt detection of gravity-gradient signals from magnitude-5.5 earthquakes even with a 2.5m prototype. CHRONOS thus opens new opportunities for quantum-limited geophysical observation and multi-band, multi-messenger gravitational-wave astronomy.