The axion is an hypothetical beyond the Standard Model particle, first introduced in the seventies as a consequence of the strong CP problem of QCD. Axions can be the main constituents of the galactic Dark Matter halos. Their experimental search can be carried out with Earth-based instruments immersed in the Milky Way's halo, which are therefore called ``haloscopes''. Nowadays haloscopes rely on the inverse Primakoff effect to detect axion-induced excesses photons in a microwave cavity under a static magnetic field.
A ferromagnetic axion haloscope does not exploit the axion-to-photon conversion but its interaction with the electron spin. It consists in an axionic-to-electromagnetic signal transducer, which is then measured by a suitable rf detector. The transducer is an hybrid system formed by a magnetic material coupled to a microwave cavity through a static magnetic field, while the detector is a quantum-limited Josephson parametric amplifier. As it measures variation in the magnetization of a sample, the ferromagnetic haloscope is configured as a spin-magnetometer.
We report on the design and operation of such haloscope. Our prototype reached the sensitivity limit imposed by quantum mechanics, the Standard Quantum Limit, and can be improved only by quantum technologies like single photon counters.