Control Scheme for Polarization Circulation Speed Meter Using a Dual-Retardation Waveplate

TL;DR

This paper introduces a novel control scheme using a dual-retardation waveplate for the polarization circulation speed meter in gravitational wave detectors, aiming to improve cavity control and reduce quantum noise.

Contribution

It proposes the Dual-Retardance Control (DRC) scheme for cavity length and alignment control in polarization circulation speed meters, a novel approach in this context.

Findings

The DRC scheme effectively maintains the cavity's phase condition.

Comparison shows DRC has comparable shot noise performance to simpler schemes.

Designs for experimental demonstration are proposed with expected transfer function results.

Abstract

In interferometric gravitational wave detectors, quantum radiation pressure noise, which is a back action of the measurement, will limit their sensitivities at low frequencies. Speed meters are one of the solutions to reduce the back action noise and improve the sensitivities, and furthermore, they can surpass the standard quantum limit over a wide range of frequencies. The Polarization Circulation Speed Meter is the latest incarnation of the speed meter concept in the sense that it requires a slight modification in the conventional interferometer designs; however, its control scheme has not been developed. The main difficulty is the length and alignment control of the cavity formed by the polarization circulation mirror and the input test masses, whose round-trip phase shift should be kept to $\pi$. In this article, we propose a new control scheme using a dual-retardation waveplate, called Dual-Retardance Control (DRC). In addition, we compare the shot noise level of the DRC to another simpler scheme by dithering. Finally, we design the experimental setup for the demonstration of the DRC and show the expected results through the transfer function measurement.