  We survey the prospective sensitivities of terrestrial and space-borne atom
interferometers (AIs) to gravitational waves (GWs) generated by cosmological
and astrophysical sources, and to ultralight dark matter. We discuss the
backgrounds from gravitational gradient noise (GGN) in terrestrial detectors,
and also binary pulsar and asteroid backgrounds in space-borne detectors. We
compare the sensitivities of LIGO and LISA with those of the 100m and 1km
stages of the AION terrestrial AI project, as well as two options for the
proposed AEDGE AI space mission with cold atom clouds either inside or outside
the spacecraft, considering as possible sources the mergers of black holes and
neutron stars, supernovae, phase transitions in the early Universe, cosmic
strings and quantum fluctuations in the early Universe that could have
generated primordial black holes. We also review the capabilities of AION and
AEDGE for detecting coherent waves of ultralight scalar dark matter.
