Quantum noise and torque correlations in a torsion-bar Sagnac speed meter
Abstract
Quantum fluctuations entering through weakly transmitting mirrors can limit a torsion-bar Sagnac speed meter for low-frequency gravitational-wave detection, even when its optical layout suppresses radiation-pressure back action. We show that, in the Cryogenic sub-Hz cROss torsion bar detector with quantum NOn-demolition Speed meter (CHRONOS), vacuum fields entering from behind the end and middle test masses can dominate the low-frequency quantum noise despite cancellation of the torques on opposite ends of each bar. For a reference configuration with mirror power transmissions of only 10−7, these fields contribute 89.7% of the total quantum-noise power spectral density at 0.1 Hz. Omitting them would underestimate the amplitude spectral density by a factor of 3.11. Our input-output model connects the correlations produced by successive optical interactions to the forces on each bar and the measured signal. This connection shows why strong torque cancellation does not, by itself, ensure a corresponding improvement in detector sensitivity. Replacing mirror transmission with absorption at fixed reflectivity leaves the excess quantum noise essentially unchanged for the two operating points studied. The predicted signal response and quantum noise agree with independent FINESSE 3 calculations as recycling detuning and homodyne angle are varied. These results establish rear vacuum fields as a limiting noise source in this torsion-bar speed meter and provide quantitative requirements for its mirror and readout design.
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BibTeX
@misc{inoue2026quantum,
title = {{Quantum noise and torque correlations in a torsion-bar Sagnac speed meter}},
author = {Yuki Inoue and Mario Onglao and Daiki Tanabe and Masashi Hazumi and Vivek Kumar and Apimook Watcharangkool},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.19938},
eprint = {2610.19938},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.19938},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Quantum noise and torque correlations in a torsion-bar Sagnac speed meter
AU - Yuki Inoue
AU - Mario Onglao
AU - Daiki Tanabe
AU - Masashi Hazumi
AU - Vivek Kumar
AU - Apimook Watcharangkool
PY - 2026
DA - 2026/10/14
PB - arXiv
DO - 10.48550/arxiv.2610.19938
UR - https://arxiv.org/abs/2610.19938
AB - Quantum fluctuations entering through weakly transmitting mirrors can limit a torsion-bar Sagnac speed meter for low-frequency gravitational-wave detection, even when its optical layout suppresses radiation-pressure back action. We show that, in the Cryogenic sub-Hz cROss torsion bar detector with quantum NOn-demolition Speed meter (CHRONOS), vacuum fields entering from behind the end and middle test masses can dominate the low-frequency quantum noise despite cancellation of the torques on opposite ends of each bar. For a reference configuration with mirror power transmissions of only 10^−7, these fields contribute 89.7% of the total quantum-noise power spectral density at 0.1 Hz. Omitting them would underestimate the amplitude spectral density by a factor of 3.11. Our input-output model connects the correlations produced by successive optical interactions to the forces on each bar and the measured signal. This connection shows why strong torque cancellation does not, by itself, ensure a corresponding improvement in detector sensitivity. Replacing mirror transmission with absorption at fixed reflectivity leaves the excess quantum noise essentially unchanged for the two operating points studied. The predicted signal response and quantum noise agree with independent FINESSE 3 calculations as recycling detuning and homodyne angle are varied. These results establish rear vacuum fields as a limiting noise source in this torsion-bar speed meter and provide quantitative requirements for its mirror and readout design.
ER -