Control of a velocity-sensitive audio-band quantum non-demolition interferometer

TL;DR

This paper proposes a method to improve low-frequency sensitivity in a Sagnac speed meter interferometer by extracting a displacement signal, enhancing gravitational wave detection capabilities without compromising quantum non-demolition properties.

Contribution

It introduces a novel technique to extract a displacement signal for low-frequency control in a Sagnac interferometer, maintaining its quantum non-demolition advantage.

Findings

Displacement signal extraction improves low-frequency sensitivity.

Optimal filters enable effective signal combination.

Quantum non-demolition properties are preserved.

Abstract

The Sagnac speed meter interferometer topology can potentially provide enhanced sensitivity to gravitational waves in the audio-band compared to equivalent Michelson interferometers. A challenge with the Sagnac speed meter interferometer arises from the intrinsic lack of sensitivity at low frequencies where the velocity-proportional signal is smaller than the noise associated with the sensing of the signal. Using as an example the on-going proof-of-concept Sagnac speed meter experiment in Glasgow, we quantify the problem and present a solution involving the extraction of a small displacement-proportional signal. This displacement signal can be combined with the existing velocity signal to enhance low frequency sensitivity, and we derive optimal filters to accomplish this for different signal strengths. We show that the extraction of the displacement signal for low frequency control purposes can be performed without reducing significantly the quantum non-demolition character of this type of interferometer.