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

Abstract

Detection of sub-Hz gravitational waves is of significant importance for astrophysics. It enables the observation of intermediate-mass black hole mergers, the issuance of early alerts for gravitational-wave events, and the exploration of the stochastic gravitational-wave background. The Cryogenic sub-Hz cROss torsion-bar detector with quantum NOn-demolition Speed meter (CHRONOS) is a proposed gravitational-wave detector based on a Sagnac speed-meter topology that uses torsion bars as test masses. Its prototype design aims to achieve a strain sensitivity of $3 \times 10^{-18}~\mathrm{Hz}^{-1/2}$ at 1~Hz and thus enable the detection of $\mathcal{O}(10^4),M_\odot$ intermediate-mass black hole mergers at 100~Mpc with a signal-to-noise ratio of 3. We show that the torsion-bar-based speed meter can suppress noise originating from laser intensity fluctuations by canceling the net torque on the bar and by using a balanced homodyne readout. We further present, for the first time, an analytic intensity-noise model for a gravitational-wave detector employing a torsion-bar Sagnac speed-meter configuration. Using this model, we evaluate the expected performance of a 2.5~m arm-length CHRONOS prototype. The projected laser-intensity noise is $2.9 \times 10^{-20}~\mathrm{Hz}^{-1/2}$ at 1~Hz, which is sufficiently low to allow the detection of binary intermediate-mass black hole mergers.