Sub-hertz optomechanically induced transparency with a kilogram-scale mechanical oscillator

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 mirrors 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. In conjunction with modest resolved-sideband operation, this regime is efficiently probed via 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 (approximate to 99.9%) Mout-coupling efficiency, our system saturates the theoretical delay-bandwidth product, rendering it an optical buffer capable of seconds-long storage times.