We present the development of a high-$Q$ monolithic silica pendulum weighing 7 milligram. The measured $Q$ value for the pendulum mode at 2.2 Hz was $2.0\ifmmode\times\else\texttimes\fi{}{10}^{6}$. To the best of our knowledge this is the lowest dissipative milligram-scale mechanical oscillator to date. By employing this suspension system, the optomechanical displacement sensor for gravity measurements we recently reported in Matsumoto et al. [Phys. Rev. Lett. 122, 071101 (2019)] can be improved to realize quantum-noise-limited sensing at several hundred hertz. 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 milligram-scale oscillators in quantum states, as well as improving the sensitivity of gravitational-wave detectors.
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