A Low-Frequency Torsion Pendulum with Interferometric Readout

We describe a torsion pendulum with a large mass-quadrupole moment and a resonant frequency of 2.8 mHz, whose angle is measured using a modified Michelson interferometer. The system achieved noise levels of $\sim200
$\text{prad}$/\sqrt{$\text{Hz}$}$between0.2-30Hzand$\sim10
$\text{prad}$/\sqrt{$\text{Hz}$}$ above 100 Hz. Such a system can be applied to a broad range of fields from the study of rotational seismic motion and elastogravity signals to gravitational wave observation and tests of gravity.