Nano-g optomechanical accelerometry with a chip-scale torsion pendulum

Measuring gravity with small test masses is a key program for both tabletop fundamental physics and commercial sensor technology. Here, we explore inertial sensing with a 10 Hz, 0.1 mg torsion pendulum with an ultralow damping rate of 10 µHz and a thermal acceleration sensitivity of ∼ 1 ng₀/^√Hz (g₀ = 9.8 ms⁻²), formed by suspending a Si microchip from a high-stress (σ ≈ 1GPa) Si₃N₄ nanoribbon. Previous studies of this device focused on its operation as a semi-absolute clock gravimeter, demonstrating bias stabilities as low as 500ng₀ at 1000s by frequency tracking. In this work, we operate it as spring-mass accelerometer read out with an optical lever. We demonstrate ∼ 100ng₀/^√Hz sensitivity between 0.1 and 100 Hz (as little as 40ng₀/^√Hz near resonance), limited by vibration isolation and thermal drift—sufficient to observe the ocean wave microseism from our laboratory in Tucson. We present progress towards improved sensitivity based on cross-spectral analysis with a commercial seismometer, measurement-based feedback cooling, and active vibration isolation. Longer term goals include measuring the Earth’s tides and integration with a source mass for short-range gravity measurements.