SpaceQ -- Direct Detection of Ultralight Dark Matter with Space Quantum
  Sensors
  Recent advances in quantum sensors, including atomic clocks, enable searches
for a broad range of dark matter candidates. The question of the dark matter
distribution in the Solar system critically affects the reach of dark matter
direct detection experiments. Partly motivated by the NASA Deep Space Atomic
Clock (DSAC), we show that space quantum sensors present new opportunities for
ultralight dark matter searches, especially for dark matter states bound to the
Sun. We show that space quantum sensors can probe unexplored parameter space of
ultralight dark matter, covering theoretical relaxion targets motivated by
naturalness and Higgs mixing. If an atomic clock were able to make measurements
on the interior of the solar system, it could probe this highly sensitive
region directly and set very strong constraints on the existence of such a
bound-state halo in our solar system. We present sensitivity projections for
space-based probes of ultralight dark matter which couples to electron, photon,
and gluon fields, based on current and future atomic, molecular, and nuclear
clocks.
