  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.
