Nanoscale torsional dissipation dilution for quantum experiments and precision measurement

We show that torsion resonators can experience massive dissipation dilution due to nanoscale strain, and draw a connection to a century-old theory from the torsion balance community which suggests that a simple torsion ribbon is naturally soft-clamped. By disrupting a commonly held belief in the nanomechanics community, our findings invite a rethinking of strategies towards quantum experiments and precision measurement with nanomechanical resonators. For example, we revisit the optical lever technique for monitoring displacement, and find that the rotation of a strained nanobeam can be resolved with an imprecision smaller than the zero-point motion of its fundamental torsional mode, without the use of a cavity or interferometric stability. We also find that a strained torsion ribbon can be mass-loaded without changing its Q factor. We use this strategy to engineer a chip-scale torsion balance whose resonance frequency is sensitive to micro-g fluctuations of the local gravitational field. Enabling both these advances is the fabrication of high-stress Si₃N₄ nanobeams with width-to-thickness ratios of 10⁴ and the recognition that their torsional modes have Q factors scaling as their width-to-thickness ratio squared, yielding Q factors as high as 10⁸ and Q-frequency products as high as 10¹³ Hz.