  The axion is a light pseudoscalar particle which suppresses CP-violating
effects in strong interactions and also happens to be an excellent dark matter
candidate. Axions constituting the dark matter halo of our galaxy may be
detected by their resonant conversion to photons in a microwave cavity
permeated by a magnetic field. The current generation of the microwave cavity
experiment has demonstrated sensitivity to plausible axion models, and upgrades
in progress should achieve the sensitivity required for a definitive search, at
least for low mass axions. However, a comprehensive strategy for scanning the
entire mass range, from 1-1000 $\mu$eV, will require significant technological
advances to maintain the needed sensitivity at higher frequencies. Such
advances could include sub-quantum-limited amplifiers based on squeezed vacuum
states, bolometers, and/or superconducting microwave cavities. The Axion Dark
Matter eXperiment at High Frequencies (ADMX-HF) represents both a pathfinder
for first data in the 20-100 $\mu$eV range ($\sim$5-25 GHz), and an innovation
test-bed for these concepts.
