  We propose two distinct atom interferometer gravitational wave detectors, one
terrestrial and another satellite-based, utilizing the core technology of the
Stanford $10 \text{m}$ atom interferometer presently under construction. The
terrestrial experiment can operate with strain sensitivity $ \sim
\frac{10^{-19}}{\sqrt{\text{Hz}}}$ in the 1 Hz - 10 Hz band, inaccessible to
LIGO, and can detect gravitational waves from solar mass binaries out to
megaparsec distances. The satellite experiment probes the same frequency
spectrum as LISA with better strain sensitivity $ \sim
\frac{10^{-20}}{\sqrt{\text{Hz}}}$. Each configuration compares two widely
separated atom interferometers run using common lasers. The effect of the
gravitational waves on the propagating laser field produces the main effect in
this configuration and enables a large enhancement in the gravitational wave
signal while significantly suppressing many backgrounds. The use of ballistic
atoms (instead of mirrors) as inertial test masses improves systematics coming
from vibrations and acceleration noise, and reduces spacecraft control
requirements.
