Performance of multiple filter-cavity schemes for frequency-dependent squeezing in gravitational-wave detectors

TL;DR

This paper compares two filter-cavity configurations for frequency-dependent squeezing in gravitational-wave detectors, analyzing their performance, resilience to imperfections, and proposing a practical implementation scheme for future detectors like Einstein Telescope.

Contribution

It provides a theoretical and numerical comparison of two filter-cavity schemes, highlighting the coupled cavity's advantages and proposing a feasible implementation approach.

Findings

Coupled cavity scheme offers similar or better performance than two-cavity option.

Resilience to optical losses and mismatching is comparable or improved in the coupled scheme.

Mode-mismatch phases significantly limit squeezing performance.

Abstract

Gravitational-wave detectors use state-of-the-art quantum technologies to reduce the noise induced by vacuum fluctuations, via injection of squeezed states of light. Future detectors, such as Einstein Telescope, may require the use of two filter cavities or a 3-mirror coupled filter cavity to achieve a complex rotation of the squeezing ellipse, in order to reduce the quantum noise over the whole detector bandwidth. In this work, we compare the theoretical feasibility and performances of these two optical layouts and their resilience with respect to different degradation sources (optical losses, mismatching, locking precision), analytically and numerically. We extend previous analysis on squeezing degradation and find that the coupled cavity scheme provides similar or better performances than the two-cavity option, in terms of resilience with respect to imperfections and optical losses. We further highlight the role of mode-mismatch phases in limiting squeezing. Finally, we propose a possible two-step implementation scheme for Einstein Telescope using a single filter cavity that can be possibly upgraded into a coupled filter cavity.