The cosmological applications of atomic clocks so far have been limited to searches for the uniform-in-time drift of fundamental constants. We point out that a transient-in-time change of fundamental constants can be induced by dark-matter objects that have large spatial extent, such as stable topological defects built from light non-Standard Model fields. Networks of correlated atomic clocks, some of them already in existence, such as the Global Positioning System, can be used as a powerful tool to search for topological defect dark matter, thus providing another important fundamental physics application for the ever-improving accuracy of atomic clocks. During the encounter with an extended dark-matter object, as it sweeps through the network, initially synchronized clocks will become desynchronized. Time discrepancies between spatially separated clocks are expected to exhibit a distinct signature, encoding the defect' s space structure and its interaction strength with atoms.
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We would like to thank N. Fortson, P. Graham, J. Hall, M. Murphy, J. Sherman, J. Weinstein and I. Yavin for discussions. This work was supported by the US National Science Foundation, the Natural Sciences and Engineering Research Council of Canada, and the province of Ontario.