Broadband and Resonant Approaches to Axion Dark Matter Detection
  When ultralight axion dark matter encounters a static magnetic field, it
sources an effective electric current that follows the magnetic field lines and
oscillates at the axion Compton frequency. We propose a new experiment to
detect this axion effective current. In the presence of axion dark matter, a
large toroidal magnet will act like an oscillating current ring, whose induced
magnetic flux can be measured by an external pickup loop inductively coupled to
a SQUID magnetometer. We consider both resonant and broadband readout circuits
and show that a broadband approach has advantages at small axion masses. We
estimate the reach of this design, taking into account the irreducible sources
of noise, and demonstrate potential sensitivity to axion-like dark matter with
masses in the range of 10^{-14}-10^{-6} eV. In particular, both the broadband
and resonant strategies can probe the QCD axion with a GUT-scale decay
constant.
