| Summary: | The quest for ultrahigh-Q nanomechanical resonators has driven intense study of strain-induced dissipation dilution, an effect whereby vibrations of a tensioned plate are effectively trapped in a lossless potential. Here, we show for the first time that torsion modes of nanostructures can experience dissipation dilution, yielding a new class of ultrahigh-Q nanomechanical resonators with broad applications to quantum experiments and precision measurement. Specifically, we show that torsion modes of strained nanoribbons have Q factors scaling as their width-to-thickness ratio squared (characteristic of “soft clamping”), yielding Q factors as high as 10^{8} and Q-frequency products as high as 10^{13} Hz for devices made of Si_{3}N_{4}. Using an optical lever, we show that the rotation of one such nanoribbon can be resolved with an imprecision 100 times smaller than the zero-point motion of its fundamental torsion mode, without the use of a cavity or interferometric stability. We also show that a strained nanoribbon can be mass loaded without changing its torsional Q. We use this strategy to engineer a chip-scale torsion pendulum with an ultralow damping rate of 7 μHz and show how it can be used to sense micro-g fluctuations of the local gravitational field. Our findings signal the potential for a new field of imaging-based quantum optomechanics, demonstrate that the utility of strained nanomechanics extends beyond force microscopy to inertial sensing, and hint that the landscape for dissipation dilution remains largely unexplored.
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