  Theories of dark energy require a screening mechanism to explain why the
associated scalar fields do not mediate observable long range fifth forces. The
archetype of this is the chameleon field. Here we show that individual atoms
are too small to screen the chameleon field inside a large high-vacuum chamber,
and therefore can detect the field with high sensitivity. We derive new limits
on the chameleon parameters from existing experiments, and show that most of
the remaining chameleon parameter space is readily accessible using atom
interferometry.
