Gravitational-wave observations have opened a new window onto the Universe, providing direct access to compact-object binaries, strong-field gravity, and populations of black holes and neutron stars that are otherwise inaccessible. Ground-based detectors such as Advanced LIGO, Virgo, and KAGRA have achieved remarkable sensitivity above roughly 10 Hz, while future observatories including Cosmic Explorer and the Einstein Telescope aim to extend this reach even further. At much lower frequencies, space-based missions such as LISA will explore the millihertz band.

Despite this rapid progress, the intermediate frequency range between about 0.1 and 10 Hz remains largely unexplored. This so-called sub-Hz band is scientifically crucial. It contains early inspiral signals of stellar-mass binaries that will later merge in ground-based detectors, potential mergers of intermediate-mass black holes, and stochastic gravitational-wave backgrounds originating from astrophysical populations or from the early Universe. Observations in this band would enable true multi-band gravitational-wave astronomy and offer new opportunities to test fundamental physics.

The main challenge in accessing the sub-Hz band from the ground is noise. Seismic motion, gravity-gradient (Newtonian) noise, suspension thermal noise, and quantum radiation-pressure noise all become increasingly dominant at low frequencies. Conventional translational interferometers face fundamental limitations in this regime, motivating the development of new detector concepts that are intrinsically less sensitive to these noise sources.

CHRONOS (Cryogenic sub-Hz cROss torsion-bar detector with quantum NOn-demolition Speed meter) is a next-generation ground-based gravitational-wave detector concept designed specifically to overcome these challenges. CHRONOS combines cryogenic torsion-bar test masses with a ring-cavity Sagnac interferometer operated as a quantum non-demolition speed meter. By measuring rotational motion rather than translational displacement, the detector naturally suppresses seismic and suspension noise. In addition, the speed-meter readout coherently cancels quantum radiation-pressure noise, enabling improved low-frequency sensitivity without requiring extremely high optical power.

With a realistic optical and mechanical design, CHRONOS is expected to achieve strain sensitivities around 10⁻¹⁹ per square root hertz in the sub-Hz band. This performance opens a new observational window between space-based and conventional ground-based detectors. CHRONOS therefore provides a unique platform for exploring low-frequency gravitational waves, studying stochastic backgrounds, and probing the fundamental physics of the early Universe.