CHRONOS: Cryogenic sub-Hz cROss torsion bar detector with quantum NOn-demolition Speed meter

TL;DR

CHRONOS addresses the gap between LISA and ground-based detectors by targeting the 0.1-10 Hz band. It uses a quantum-non-demolition speed-meter readout with a torsion-bar based Sagnac interferometer to suppress quantum back-action in the rotational degree of freedom. The paper predicts sub-Hz strain sensitivity around $h ~ 1e-18$ for arm lengths up to hundreds of meters and demonstrates strong science potential, including IMBH mergers up to 380 Mpc and a stochastic background reach of $\Omega_{GW} ~ 3.2e-9$ at 0.2 Hz with 5 years of data, as well as prompt gravity-gradient signals from large earthquakes. If realized, CHRONOS would enable multi-band gravitational-wave astronomy and rapid geophysical sensing across a broad scale.

Abstract

We propose a next-generation ground-based gravitational-wave detector, the Cryogenic sub-Hz cROss torsion-bar detector with quantum NOn-demolition Speed meter (CHRONOS), optimized for the unexplored 0.1-10Hz band bridging the gap between the space-based LISA and future ground-based detectors such as Cosmic Explorer and the Einstein Telescope. CHRONOS combines a ring-cavity Sagnac interferometer with torsion-bar test masses to realize the first quantum nondemolition (QND) measurement of angular momentum in a macroscopic system. By implementing a speed-meter readout in the rotational degree of freedom, CHRONOS coherently cancels quantum radiation-pressure noise and enables sub-Hz observations. We show that detuned power recycling and cavity-length optimization can simultaneously relax technical requirements on both torsion bars and speed meters. Assuming a realistic optical design with a 1m torsion bar, we estimate strain sensitivities of $h \simeq 1 \times 10^{-18}\mathrm{Hz}^{-1/2}$ at 1Hz for detectors with arm lengths of 2.5m, 40m, and 300m. These sensitivities enable (i) direct detection of intermediate-mass black hole binaries up to 380Mpc with ${\rm SNR}=3$, (ii) probing stochastic gravitational-wave backgrounds down to $Ω_{\rm GW} \sim 3.2 \times 10^{-9}$ at 0.2Hz with 5yr accumulation. Furthermore, CHRONOS enables prompt detection of gravity-gradient signals from magnitude-5.5 earthquakes even with a 2.5m prototype. CHRONOS thus opens new opportunities for quantum-limited geophysical observation and multi-band, multi-messenger gravitational-wave astronomy.

Figures (5)

• Figure 1: Optical configuration of CHRONOS. The Sagnac interferometer integrates: (i) torsion-bar test masses at the center, (ii) Ring cavity with Sagnac interferometer at both arm, (iii) dual-recycled cavities with SRM and PRM.
• Figure 2: Predicted CHRONOS strain sensitivity (black) and dominant noise contributions. Quantum noise is calculated with sum of shot noise and radiation noise.
• Figure 3: Estimated signal-to-noise ratios for IMBH binary mergers with CHRONOS.
• Figure 4: Expected sensitivity of CHRONOS to the SGWB. LIGO O3 Abbott:2021xxi, LISA pathfinder Boileau:2022figures,Sachs Wolfe effect Ng:2022prdRedshiftSGWB,CMB B-mode($r<0.06$), CMB Lensing, CMB Temperature Namikawa:2019tax, show the current observational limit. Binary blackhole and Binary Neutron star (BBH+BNS) Abbott2019IsotropicStochasticO2, Super massive blackhole (SMBH) Campeti2021Measuring, Primordial Blackhole (PBH, $\chi^* = 100$) Kohri:2018awvDomenech:2021ztg, Nambu-Goto type Cosmic string ($G\mu\sim 10^{-8}$, $\sigma=0.1$) Siemens:2006vkBlanco-Pillado:2017oxoAuclair:2019wcv, with Primordial Gravitational Wave (PGW) Campeti2021Measuring are theoretical predictions.
• Figure 5: Predicted sensitivity to prompt gravity signals from large earthquakes. By detecting gravitational perturbations at the speed of light, CHRONOS enables warnings before surface-wave arrival.