# Sub-Hertz Optomechanically-Induced Transparency

## TL;DR

This paper demonstrates a long interferometer with suspended mirrors where radiation pressure is strongly enhanced, enabling optomechanically-induced transparency at room temperature with potential for optical buffering and quantum information applications.

## Contribution

The authors realize a high-cooperativity optomechanical system with a meter-long interferometer, achieving transparency windows at room temperature and approaching the delay-bandwidth limit.

## Key findings

- Achieved a cooperativity of 50 in a suspended-mirror interferometer.
- Observed optomechanically-induced transparency with windows as narrow as 100 mHz.
- Demonstrated an optical buffer with seconds-long storage times.

## Abstract

Optical interferometers with suspended mirrors are the archetype of all current audio-frequency gravitational-wave detectors. The radiation pressure interaction between the motion of the mirror and the circulating optical field in such interferometers represents a pristine form of light-matter coupling, largely due to 30 years of effort in developing high quality optical materials with low mechanical dissipation. However, in all current suspended interferometers, the radiation pressure interaction is too weak to be useful as a resource, and too strong to be neglected. Here, we demonstrate a meter-long interferometer with suspended mirrors, of effective mass $~ 125$ g, where the radiation pressure interaction is enhanced by strong optical pumping to realize a cooperativity of $50$. We probe this regime by observing optomechanically-induced transparency of a weak on-resonant probe. The low resonant frequency and high-Q of the mechanical oscillator allows us to demonstrate transparency windows barely $100$ mHz wide at room temperature. Together with a near-unity ($\sim 99.9\%$) out-coupling efficiency, our system saturates the theoretical delay-bandwidth product, rendering it an optical buffer capable of seconds-long storage times.