Macroscopic quantum mechanics in gravitational-wave observatories and beyond

TL;DR

This paper reviews how quantum correlations manifest in macroscopic systems like gravitational-wave detectors, highlighting recent observations and exploring implications for fundamental physics and future mesoscopic optomechanical research.

Contribution

It provides a comprehensive review of quantum correlations in macroscopic systems, focusing on gravitational-wave observatories and potential future research directions.

Findings

Quantum uncertainty in GW detectors affects correlated systems.

Observations confirm quantum effects in large-scale optomechanical systems.

Links to fundamental physics research with mesoscopic quantum states.

Abstract

The existence of quantum correlations affects both microscopic and macroscopic systems. On macroscopic systems they are difficult to observe and usually irrelevant for the system's evolution due to the frequent energy exchange with the environment. The world-wide network of gravitational-wave (GW) observatories exploits optical as well as mechanical systems that are highly macroscopic and largely decoupled from the environment. The quasi-monochromatic light fields in the kilometre-scale arm resonators have photon excitation numbers larger than $10^{19}$, and the mirrors that are quasi-free falling in propagation direction of the light fields have masses of around 40 kg. Recent observations on the GW observatories LIGO and Virgo clearly showed that the quantum uncertainty of one system affected the uncertainty of the other. Here, we review these observations and provide links to research goals targeted with mesoscopic optomechanical systems in other fields of fundamental physical research. These may have Gaussian quantum uncertainties as the ones in GW observatories or even non-Gaussian ones, such as Schr\"odinger cat states.