At the time the Peccei–Quinn (PQ) solution [1555] to the strong CP problem [1553] was proposed it seemed perfectly reasonable to identify the PQ symmetry breaking scale with the electroweak scale. As the mass and the couplings scale inversely with the symmetry breaking scale, the expectation was that the mass of the axion would be hundreds of keV with couplings strong enough to be seen in accelerator and reactor experiments. The experimental situation quickly began to disfavour a massive axion as null results began to be amassed from a variety of particle and nuclear physics experiments. Tacitly acknowledging that there was no strong theoretical guidance for such a choice, new models soon appeared with arbitrarily large fa, resulting in couplings so extraordinarily weak as to render the axion effectively invisible. Frustrating as this might have been to experimentalists, an unintended consequence of such a small coupling would be the prospect for the axion to be the dark matter pervading the Universe (since) [1600].

Still, it was unsatisfying that axion dark matter could, in principle, completely escape detection. Fortunately, in 1983 Pierre Sikivie made a proposal that could make ‘invisible’ axions ‘visible’ again [1277; 1754; 1755]. To overcome the dreadfully small couplings and excessively long decay natural lifetimes, a technique was borrowed from the gravitational wave community. Specifically, Sikivie proposed that axion decay into two photons could be stimulated with a high-Q oscillator.