Dark Matter Interferometry
  The next generation of ultralight dark matter (DM) direct detection
experiments, which could confirm sub-eV bosons as the dominant source of DM,
will feature multiple detectors operating at various terrestrial locations. As
a result of the wave-like nature of ultralight DM, spatially separated
detectors will each measure a unique DM phase. When the separation between
experiments is comparable to the DM coherence length, the spatially-varying
phase contains information beyond that which is accessible at a single
detector. We introduce a formalism to extract this information, which performs
interferometry directly on the DM wave. In particular, we develop a
likelihood-based framework that combines data from multiple experiments to
constrain directional information about the DM phase space distribution. We
show that the signal in multiple detectors is subject to a daily modulation
effect unique to wave-like DM. Leveraging daily modulation, we illustrate that
within days of an initial discovery multiple detectors acting in unison could
localize directional parameters of the DM velocity distribution such as the
direction of the solar velocity to sub-degree accuracy, or the direction of a
putative cold DM stream to the sub-arcminute level. We outline how to optimize
the locations of multiple detectors with either resonant cavity (such as ADMX
or HAYSTAC) or quasistatic (such as ABRACADABRA or DM-Radio) readouts to have
maximal sensitivity to the full 3-dimensional DM velocity distribution.
