# Quantum expander for gravitational-wave observatories

## TL;DR

This paper proposes a quantum expander technique that uses squeezing inside optical resonators to increase the detection bandwidth of gravitational-wave observatories, enabling better resolution of high-frequency signals.

## Contribution

It introduces a novel quantum expander method that compensates for resonator linewidth limitations, enhancing gravitational-wave detector sensitivity across a broader frequency range.

## Key findings

- Enables detection of high-frequency gravitational waves
- Maintains low-frequency sensitivity
- Potentially applicable to other cavity-enhanced measurements

## Abstract

Quantum uncertainty of laser light limits the sensitivity of gravitational-wave observatories. In the past 30 years, techniques for squeezing the quantum uncertainty as well as for enhancing the gravitational-wave signal with optical resonators were invented. Resonators, however, have finite linewidths; and the high signal frequencies that are produced during the scientifically highly interesting ring-down of astrophysical compact-binary mergers cannot be resolved today. Here, we propose an optical approach for expanding the detection bandwidth. It uses quantum uncertainty squeezing inside one of the optical resonators, compensating for finite resonators' linewidths while maintaining the low-frequency sensitivity unchanged. Introducing the quantum expander for boosting the sensitivity of future gravitational-wave detectors, we envision it to become a new tool in other cavity-enhanced metrological experiments.