  We report on the first earth-scale quantum sensor network based on optical
atomic clocks aimed at dark matter (DM) detection. Exploiting differences in
the susceptibilities to the fine-structure constant of essential parts of an
optical atomic clock, i.e. the cold atoms and the optical reference cavity, we
can perform sensitive searches for dark matter signatures without the need of
real-time comparisons of the clocks. We report a two orders of magnitude
improvement in constraints on transient variations of the fine-structure
constant, which considerably improves the detection limit for the standard
model (SM) - DM coupling. We use Yb and Sr optical atomic clocks at four
laboratories on three continents to search for both topological defect (TD) and
massive scalar field candidates. No signal consistent with a dark-matter
coupling is identified, leading to significantly improved constraints on the
DM-SM couplings.
