  We propose a method for detecting single chiral phonons that will enable
their use as dark-matter detectors. We suggest metal--organic frameworks (MOFs)
as detector materials, as their flexibility yields low-energy chiral phonons
with measurable magnetic moments, and their anisotropy leads to directional
sensitivity, which mitigates background contamination. To demonstrate our
proposal, we calculate the phononic structure of the MOF
InF$_3$($4,4'$-bipyridine), and show that it has highly chiral acoustic
phonons. Detection of such chiral phonons via their magnetic moments would
dramatically lower the excitation energy threshold for dark matter detection to
the energy of a single phonon. We show that single phonon detection in a MOF
would extend detector reach ten or more orders of magnitude below current
limits, enabling exploration of a multitude of as-yet-unprobed dark matter
candidates.
