Torque cancellation effect of intensity noise for Cryogenic sub-Hz cROss torsion bar detector with quantum NOn-demolition speed meter (CHRONOS)

Abstract The detection of sub-hertz gravitational waves is of significant importance in various aspects of astrophysics. This enables us to detect intermediate-mass black hole (IMBH) mergers, issue early alerts for gravitational-wave (GW) events, and search the unexplored frequency region of the stochastic background. Cryogenic sub-Hz cROss torsion bar detector with quantum NOn-demolition Speed meter (CHRONOS) is a proposed GW detector based on a speed meter that uses torsion bars as end test masses. It aims to achieve a strain sensitivity of 3 × 10 − 18 Hz − 1 / 2 at 1 Hz with its prototype design to detect O ( 10 4 ) M ⊙ IMBH binary mergers at 100 Mpc with a signal-to-noise ratio of 3. We point out that the torsion-bar-based speed-meter interferometer can suppress noise originating from laser intensity fluctuation by torque cancellation of the bar and by a balanced-homodyne readout. For the first time, we propose an intensity-noise formula for a GW detector employing a Sagnac speed-meter configuration with torsion-bar test masses. Based on our intensity-noise model, we evaluate the expected intensity noise for a 2.5-m-arm prototype of CHRONOS. The projected intensity noise was 2.9 × 10 − 20 Hz − 1 / 2 , which was sufficiently low to detect an IMBH binary merger.