Lithographically defined Si3N4 torsional pendulum

Abstract

Torsion pendulums provide an opportunity to trap large masses in a potential weak enough to explore two-body gravitation. Cooled to, and then released from, a ground state, weak quantum effects, including those from gravity, might reveal themselves in the evolving decoherence of a torsion pendulum, if its baseline dissipation were sufficiently dilute for quantum coherent oscillation. Monolithic ribbon-like or multi-filar suspension geometries provide a key to such dilution in torsion, but are challenging to make. As a solution, we introduce a lithographically defined Si3N4 ribbon suspension in a wafer-scale approach to pendulum fabrication that is conducive to such 2D geometries, making extreme aspect ratios and even multi-filar designs a possibility. A monofilar, monolithic, centimeter scale torsion pendulum is fabricated and released in a first proof of concept. Mounted in a vacuum, it is optically excited and cooled using measurement-based feedback. Though only 37 mg, the device displays a fundamental frequency of 162 mHz and an undiluted Q of 12 000, demonstrating a foundational step toward ultracoherent, ultralow frequency torsion pendulums.