  We introduce the concept of impedance matching to axion dark matter by posing
the question of why axion detection is difficult, even though there is enough
power in each square meter of incident dark-matter flux to energize a LED light
bulb. By quantifying backreaction on the axion field, we show that a small
axion-photon coupling does not by itself prevent an order-unity fraction of the
dark matter from being absorbed through optimal impedance match. We further
show, in contrast, that the electromagnetic charges and the self-impedance of
their coupling to photons provide the principal constraint on power absorption
integrated across a search band. Using the equations of axion electrodynamics,
we demonstrate stringent limitations on absorbed power in linear,
time-invariant, passive receivers. Our results yield fundamental constraints,
arising from the photon-electron interaction, on improving integrated power
absorption beyond the cavity haloscope technique. The analysis also has
significant practical implications, showing apparent tension with the
sensitivity projections for a number of planned axion searches. We additionally
provide a basis for more accurate signal power calculations and calibration
models, especially for receivers using multi-wavelength open configurations
such as dish antennas and dielectric haloscopes.
