Tremendous advances in the study of magnetic light-matter interactions have recentlybeen achieved using man-made nanostructures that exhibit and exploit an optical magneticresponse. However, naturally occurring emitters can also exhibit magnetic resonances in theform of optical-frequency magnetic-dipole transitions. Here we quantify the magnetic natureof light emission using energy- and momentum-resolved spectroscopy, and leverage a pair ofspectrally close electric- and magnetic-dipole transitions in trivalent europium to probe vacuumfluctuations in the electric and magnetic fields at the nanometre scale. These results reveal anew tool for nano-optics: an atomic-size quantum emitter that interacts with the magneticcomponent of light.
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