Title: Spin Precession Experiments for Light Axionic Dark Matter,
Abstract: Axion-like particles are promising candidates to make up the dark matter of
the universe, but it is challenging to design experiments that can detect them
over their entire allowed mass range. Dark matter in general, and in particular
axion-like particles and hidden photons, can be as light as roughly $10^{-22}
\;\rm{eV}$ ($\sim 10^{-8} \;\rm{Hz}$), with astrophysical anomalies providing
motivation for the lightest masses ("fuzzy dark matter"). We propose
experimental techniques for direct detection of axion-like dark matter in the
mass range from roughly $10^{-13} \;\rm{eV}$ ($\sim 10^2 \;\rm{Hz}$) down to
the lowest possible masses. In this range, these axion-like particles act as a
time-oscillating magnetic field coupling only to spin, inducing effects such as
a time-oscillating torque and periodic variations in the spin-precession
frequency with the frequency and direction set by fundamental physics. We show
how these signals can be measured using existing experimental technology,
including torsion pendulums, atomic magnetometers, and atom interferometry.
These experiments demonstrate a strong discovery capability, with future
iterations of these experiments capable of pushing several orders of magnitude
past current astrophysical bounds.