We present a new class of ultra-low-loss torsion micropendula based on strain-engineered nanomechanics, and explore their application as parametric (clock) gravimeters. Specifically, by suspending a 0.1 mg Si paddle from a strained silicon nitride nanoribbon, we realize a 30 Hz torsion micropendulum with a damping rate of 20 micro-Hz, a parametric acceleration sensitivity of 6 Hz/g, and thermal acceleration noise of 3 ng/rtHz. By driving the pendulum into self-oscillation, we realize a clock gravimeter with a frequency stability as low as 50 parts-per-billion, corresponding to an acceleration resolution of 300 ng. Currently the limitation is the intrinsic nonlinearity of the pendulum, which transduces amplitude drift into frequency drift. We demonstrate how the duffing nonlinearity of the suspension can be used to cancel this nonlinearity, paving the way towards a fully isochronous, high-Q micromechanical clock.
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