Quadrature-witness readout for backscatter mitigation in gravitational-wave detectors limited by back-action

TL;DR

This paper introduces a quadrature witness technique compatible with frequency-dependent squeezing to detect and subtract back-scattering noise in gravitational-wave detectors, enhancing their sensitivity.

Contribution

The paper proposes a novel quadrature witness method that enables back-scattering noise mitigation while integrating with existing frequency-dependent squeezing techniques.

Findings

The quadrature witness can identify non-stationary back-scattering noise.

Implementation could significantly improve detector sensitivity.

Method is compatible with current squeezing technologies.

Abstract

Disturbances in gravitational wave (GW) observational data are often caused by non-stationary noise in the detector itself, such as back-scattering of laser stray light into the signal field. Unlike GW signals, non-stationary noise can appear in both the GW-signal quadrature and the orthogonal quadrature, which is usually not measured. Simultaneous sensing of this orthogonal quadrature provides a witness channel that can be used to reconstruct the disturbance in the signal quadrature enabling a subtraction of non-stationary noise. Here, we present the concept of quadrature witness that is compatible with frequency-dependent squeezing, which is already used to simultaneously reduce photon shot noise and photon radiation pressure noise. We demonstrate that implementing this approach in a GW detector could reduce noise caused by loud back-scatter events, thereby improving the overall sensitivity and robustness of GW observatories.