  Electrons and ions trapped with electromagnetic fields have long served as
important high-precision metrological instruments, and more recently have also
been proposed as a platform for quantum information processing. Here we point
out that these systems can also be used as highly sensitive detectors of
passing charged particles, due to the combination of their extreme
charge-to-mass ratio and low-noise quantum readout and control. In particular,
these systems can be used to detect energy depositions many orders of magnitude
below typical ionization scales. As an illustration, we show that current
devices can be used to provide competitive sensitivity to models where ambient
dark matter particles carry small electric millicharges $\ll e$. Our
calculations may also be useful in the characterization of noise in quantum
computers coming from backgrounds of charged particles.
