High Q mg-scale monolithic pendulum for quantum-limited gravity measurements

We present the development of a high $Q$ monolithic silica pendulum weighing 7 mg. The measured $Q$ value for the pendulum mode at 2.2 Hz was $2.0\times10^6$. To the best of our knowledge this is the lowest dissipative mg-scale mechanical oscillator to date. By employing this suspension system, the optomechanical displacement sensor for gravity measurements we recently reported in Phys. Rev. Lett. 122, 071101 (2019) can be improved to realize quantum-noise-limited sensing at several hundred Hz. In combination with the optical spring effect, the amount of intrinsic dissipation measured in the pendulum mode is enough to satisfy requirements for measurement-based quantum control of a massive pendulum confined in an optical potential. This paves the way for not only testing dark matter via quantum-limited force sensors, but also Newtonian interaction in quantum regimes, namely, between two mg-scale oscillators in quantum states, as well as improving the sensitivity of gravitational-wave detectors.