  We devise and demonstrate a method to search for non-gravitational couplings
of ultralight dark matter to standard model particles using space-time
separated atomic clocks and cavity-stabilized lasers. By making use of
space-time separated sensors, which probe different values of an oscillating
dark matter field, we can search for couplings that cancel in typical local
experiments. We demonstrate this method using existing data from a frequency
comparison of lasers stabilized to two optical cavities connected via a 2220 km
fiber link [Nat. Commun. 13, 212 (2022)]. The absence of significant
oscillations in the data results in constraints on the coupling of scalar dark
matter to electrons, d_me, for masses between 1e-19 eV and 2e-15 eV. These are
the first constraints on d_me alone in this mass range, and improve the dark
matter constraints on any scalar-Fermion coupling by up to two orders of
magnitude.
