  Long-standing efforts to detect axions are driven by two compelling
prospects, naturally accounting for the absence of charge-conjugation and
parity symmetry breaking in quantum chromodynamics, and for the elusive dark
matter at ultralight mass scale. Many experiments use advanced cavity resonator
setups to probe the magnetic-field-mediated conversion of axions to photons.
Here, we show how to search for axion matter without relying on such a cavity
setup, which opens a new path for the detection of ultralight axions, where
cavity based setups are infeasible. When applied to Rydberg atoms, which
feature particularly large transition dipole elements, this effect promises an
outstanding sensitivity for detecting ultralight dark matter. Our estimates
show that it can provide laboratory constraints in parameter space that so far
had only been probed astrophysically, and cover new unprobed regions of
parameter space. The Rydberg atomic gases offer a flexible and inexpensive
experimental platform that can operate at room temperature. We project the
sensitivity by quantizing the axion-modified Maxwell equations to accurately
describe atoms and molecules as quantum sensors wherever axion dark matter is
present.
