  Long-range interactions are a key resource in many quantum phenomena and
technologies. Free-space photons mediate power-law interactions but lack
tunability and suffer from decoherence processes due to their omnidirectional
emission. Engineered dielectrics can yield tunable and coherent interactions,
but typically at the expense of making them both shorter-ranged and sensitive
to material disorder and photon loss. Here, we propose a platform that can
circumvent all these limitations based on three-dimensional subwavelength
atomic arrays subjected to magnetic fields. Our key result is to show how to
design the polaritonic bands of these atomic metamaterials to feature a pair of
frequency-isolated Weyl points. These Weyl excitations can thus mediate
interactions that are simultaneously long-range, due to their gapless nature;
robust, due to the topological protection of Weyl points; and decoherence-free,
due to their subradiant character. We demonstrate the robustness of these
isolated Weyl points for a large regime of interatomic distances and magnetic
field values and characterize the emergence of their corresponding Fermi arcs
surface states. The latter can as well lead to two-dimensional, non-reciprocal
atomic interactions with no analogue in other chiral quantum optical setups.
