Coherent coupling between multiple ferrimagnetic spheres and a microwave cavity at millikelvin temperatures

The spin resonance of electrons can be coupled to a microwave cavity mode obtain photon-magnon hybrid system. These quantum systems are widely studied for both fundamental physics and technological applications. In this article, the behavior large number ferrimagnetic spheres single is put under test. We use second-quantization modeling harmonic oscillators theoretically describe our experimental setup understand influence several parameters. magnon-polariton dispersion relation used characterize system, with particular focus on vacuum Rabi splitting due multiple spheres. combine results obtained simple analyze more complex one, show that it devised in such way minimize degrees freedom needed completely it. By studying single-sphere coupling two possible size-effects related sample diameter have been identified, while multiple-spheres configurations reveal how upscale This characterization useful implementation an axion-to-electromagnetic field transducer ferromagnetic haloscope dark matter searches. Our dedicated setup, consisting ten 2 mm-diameter YIG copper cavity, aim at mK temperatures. Moreover, we novel applications optimally-controlled foreseen setups embedding samples.