Demonstration of an amplitude filter cavity at gravitational-wave frequencies

TL;DR

This paper demonstrates an amplitude filter cavity that reduces quantum noise at low frequencies in gravitational-wave detectors, improving sensitivity by mitigating anti-squeezing effects and backscatter.

Contribution

It introduces a critically-coupled optical cavity as an amplitude filter to improve low-frequency quantum noise reduction in gravitational-wave measurements.

Findings

Successfully demonstrated amplitude filtering at gravitational-wave frequencies

Reduced anti-squeezing effects below 90Hz

Mitigated backscatter and technical noise

Abstract

Quantum vacuum fluctuations fundamentally limit the precision of optical measurements, such as those in gravitational-wave detectors. Injection of conventional squeezed vacuum can be used to reduce quantum noise in the readout quadrature, but this reduction is at the cost of increasing noise in the orthogonal quadrature. For detectors near the limits imposed by quantum radiation pressure noise (QRPN), both quadratures impact the measurement, and the benefits of conventional squeezing are limited. In this paper, we demonstrate the use of a critically-coupled 16m optical cavity to diminish anti-squeezing at frequencies below 90Hz where it exacerbates QRPN, while preserving beneficial squeezing at higher frequencies. This is called an amplitude filter cavity, and it is useful for avoiding degradation of detector sensitivity at low frequencies. The attenuation from the cavity also provides technical advantages such as mitigating backscatter.