  We propose a space-based gravitational wave detector consisting of two
spatially separated, drag-free satellites sharing ultra-stable optical laser
light over a single baseline. Each satellite contains an optical lattice atomic
clock, which serves as a sensitive, narrowband detector of the local frequency
of the shared laser light. A synchronized two-clock comparison between the
satellites will be sensitive to the effective Doppler shifts induced by
incident gravitational waves (GWs) at a level competitive with other proposed
space-based GW detectors, while providing complementary features. The detected
signal is a differential frequency shift of the shared laser light due to the
relative velocity of the satellites, and the detection window can be tuned
through the control sequence applied to the atoms' internal states. This scheme
enables the detection of GWs from continuous, spectrally narrow sources, such
as compact binary inspirals, with frequencies ranging from ~3 mHz - 10 Hz
without loss of sensitivity, thereby bridging the detection gap between
space-based and terrestrial optical interferometric GW detectors. Our proposed
GW detector employs just two satellites, is compatible with integration with an
optical interferometric detector, and requires only realistic improvements to
existing ground-based clock and laser technologies.
