Searching for topological defect dark matter 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. Extensions of the quantum electrodynamics Lagrangian with dark-matter coupling terms may result in changes to Standard Model parameters. Recently, it was proposed that a network of atomic clocks could be used to search for transient signals of a hypothetical dark matter in the form of stable topological defects. The clocks become desynchronized when a dark-matter object sweeps through the network. This pioneering approach, which is applicable only for distant clocks, is limited by the quality of the fibre links. Here, we present an alternative experimental approach that is applicable to both closely spaced and distant optical atomic clocks and benefits from their individual susceptibilities to dark matter, hence not requiring fibre links. We explore a new dimension of astrophysical observations by constraining the strength of atomic coupling to the hypothetical dark-matter cosmic objects. Our experimental constraint exceeds the previous limits; in fact, it not only reaches the ultimate level expected to be achievable with a constellation of GPS atomic clocks but also has a large potential for improvement.