  Atom interferometers measure quantum interference patterns in the wave
functions of cold atoms that follow superpositions of different space-time
trajectories. These can be sensitive to phase shifts induced by fundamental
physics processes such as interactions with ultralight dark matter or the
passage of gravitational waves. The capabilities of large-scale atom
interferometers are illustrated by their estimated sensitivities to the
possible interactions of ultralight dark matter with electrons and photons, and
to gravitational waves in the frequency range around 1 Hz, intermediate between
the peak sensitivities of the LIGO and LISA experiments. Atom interferometers
can probe ultralight scalar couplings with much greater sensitivity than is
currently available from probes of the Equivalence Principle. Their sensitivity
to mid-frequency gravitational waves may open a window on mergers of masses
intermediate between those discovered by the LIGO and Virgo experiments and the
supermassive black holes present in the cores of galaxies, as well as
fundamental physics processes in the early Universe such as first-order phase
transitions and the evolution of networks of cosmic strings.
