Lithographically Defined Si$_3$N$_4$ 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 silicon nitride (Si$_3$N$_4$) ribbon suspension in a wafer-scale approach to pendulum fabrication that is conducive to such 2-D 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 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 12000, demonstrating a foundational step towards ultra-coherent, ultra-low frequency torsion pendulums.