Experimental constraint on dark matter detection with optical atomic clocks

The total mass density of the Universe appears to be dominated by dark matter. However, beyond its gravitational interactions at the galactic scale, little is known about its nature 1 . Several proposals have been advanced in recent years for the detection of dark matter 2 – 4 . In particular, a network of atomic clocks could be used to search for transient indicators of hypothetical dark matter 5 in the form of stable topological defects; for example, monopoles, strings or domain walls 6 . The clocks become desynchronized when a dark-matter object sweeps through the network. This pioneering approach 5 requires a comparison between at least two distant optical atomic clocks 7 – 9 . Here, by exploiting differences in the susceptibilities of the atoms and the cavity to the fine-structure constant 10 , 11 , we show that a single optical atomic clock 12 is already sensitive to dark-matter events. This implies that existing optical atomic clocks 13 , 14 can serve as a global topological-defect dark-matter observatory, without any further developments in experimental apparatus or the need for long phase-noise-compensated optical-fibre links 15 . Using optical atomic clocks, we explored a new dimension of astrophysical observations by constraining the strength of atomic coupling to hypothetical dark-matter cosmic objects. Under the conditions of our experiments, the degree of constraint was found to exceed the previously reported limits 16 by more than three orders of magnitude. An experimental apparatus using a single optical atomic clock to detect dark matter topological defects (like strings) is proposed. Tests show it can constrain the dark matter–Standard Model coupling strength ~3 orders of magnitude better than previous limits.