  We propose using trapped electrons as high-$Q$ resonators for detecting meV
dark photon dark matter. When the rest energy of the dark photon matches the
energy splitting of the two lowest cyclotron levels, the first excited state of
the electron cyclotron will be resonantly excited. A proof-of-principle
measurement, carried out with one electron, demonstrates that the method is
background-free over a 7.4 day search. It sets a limit on dark photon dark
matter at 148 GHz (0.6 meV) that is around 75 times better than previous
constraints. Dark photon dark matter in the 0.1-1 meV mass range (20-200 GHz)
could likely be detected at a similar sensitivity in an apparatus designed for
dark photon detection.
