  We show that gravitational wave detectors based on a type of atom
interferometry are sensitive to ultralight scalar dark matter. Such dark matter
can cause temporal oscillations in fundamental constants with a frequency set
by the dark matter mass, and amplitude determined by the local dark matter
density. The result is a modulation of atomic transition energies. This signal
is ideally suited to a type of gravitational wave detector that compares two
spatially separated atom interferometers referenced by a common laser. Such a
detector can improve on current searches for electron-mass or electric-charge
modulus dark matter by up to 10 orders of magnitude in coupling, in a frequency
band complementary to that of other proposals. It demonstrates that this class
of atomic sensors is qualitatively different from other gravitational wave
detectors, including those based on laser interferometry. By using
atomic-clock-like interferometers, laser noise is mitigated with only a single
baseline. These atomic sensors can thus detect scalar signals in addition to
tensor signals.
