Yuki Inoue, Hsiang-Yu Huang, Vivek Kumar, Mario Juvenal S. Onglao, Daiki Tanabe, Ta-Chun Yu

The frequency band between $0.1$ and $10\mathrm{Hz}$ remains largely unexplored in gravitational-wave astronomy due to strong seismic, Newtonian, and suspension thermal noise that limit ground-based detectors. The Cryogenic sub-Hz cROss torsion-bar detector with quantum NOn-demolition Speed meter (CHRONOS) is a novel detector concept designed to access this frequency range from the ground. CHRONOS combines cryogenic torsion-bar test masses with a triangular Sagnac interferometer implementing a speed-meter readout, which suppresses quantum radiation-pressure noise and enables quantum non-demolition measurements in the sub-Hz regime. The detector targets a strain sensitivity of $h \sim 10^{-18}\mathrm{Hz^{-1/2}}$ around $2\mathrm{Hz}$ and stochastic gravitational wave background of $Ω_{GW} \sim 2\times 10^{-3}$ at $2\mathrm{Hz}$. This sensitivity opens a new observational window between space-based detectors such as LISA and ground-based interferometers, enabling observations of intermediate-mass black hole binaries, searches for stochastic gravitational-wave backgrounds, and tests of macroscopic quantum measurements.