  We propose the use of trapped ions for detection of millicharged dark matter.
Millicharged particles will scatter off the ions, giving a signal either in
individual events or in the overall heating rate of the ions. Ion traps have
several properties which make them ideal detectors for such a signal. First,
ion traps have demonstrated significant isolation of the ions from the
environment, greatly reducing the background heating and event rates. Second,
ion traps can have low thresholds for detection of energy deposition, down to
$\sim \text{neV}$. Third, since the ions are charged, they naturally have large
cross sections for scattering with the millicharged particles, further enhanced
by the low velocities of the thermalized millicharges. Despite ion-trap setups
being optimized for other goals, we find that existing measurements put new
constraints on millicharged dark matter which are many orders of magnitude
beyond previous bounds. For example, for a millicharge dark matter mass
$m_Q=10~\textrm{GeV}$ and charge $10^{-3}$ of the electron charge, ion traps
limit the local density to be $n_Q \lesssim 1 \, \textrm{cm}^{-3}$, a factor
$\sim 10^8$ better than current constraints. Future dedicated ion trap
experiments could reach even further into unexplored parameter space.
