Muonium Spectroscopy as a Quantum Sensor for Axion Dark Matter.

High-intensity muon beams could enable a muonium-based axion search through resonant quantum transitions between hyperfine states. Combining theoretical calculations with simulation results, we demonstrate that such a muonium-based experimental approach could complement and tighten constraints on the axion-muon coupling beyond existing limits from the muon g-2 measurement over the axion mass range of 18-130  μeV. These results establish a new spectroscopic channel in muonium that enables searches for axion and axionlike particle dark matter.