Meterscale Strong Coupling between Magnons and Photons

We experimentally realize a meterscale strong coupling effect between magnons and photons at room temperature, with a coherent coupling of $\ensuremath{\sim}20\text{ }\text{ }\mathrm{m}$ and a dissipative coupling of $\ensuremath{\sim}7.6\text{ }\text{ }\mathrm{m}$. To this end, we integrate a saturable gain into a microwave cavity and then couple this active cavity to a magnon mode via a long coaxial cable. The gain compensates for the cavity dissipation, but preserves the cavity radiation that mediates the indirect photon-magnon coupling. It thus enables the long-range strong photon-magnon coupling. With full access to traveling waves, we demonstrate a remote control of photon-magnon coupling by modulating the phase and amplitude of traveling waves, rather than reconfiguring subsystems themselves. Our method for realizing long-range strong coupling in cavity magnonics provides a general idea for other physical systems. Our experimental achievements may promote the construction of information networks based on cavity magnonics.