Travelling photons-mediated coherent and dissipative couplings in long-distance cavity magnonics

Engineering strong coupling between systems is essential for many phenomena of quantum physics and technology, especially in long distance without field overlap. We demonstrate the indirect coherent and dissipative couplings in a spatially separated cavity magnonic system mediated by travelling photons. The system is constructed by placing a dielectric resonator (cavity photon mode) and an Yttrium Iron Garnet sphere (magnon mode) along the transmission line that provides travelling photons, with a distance much larger than the resonance wavelength to avoid direct coupling. The long-distance coupling between cavity and magnon modes is highly tunable with position and could be manipulated to be nonreciprocal. At a fixed distance of (n+1/2)*wavelength, we observe coherent coupling for the left-going travelling photons and dissipative coupling for the right-going travelling photons. A general model of indirect magnetic dipole-dipole interaction is derived to explain our results, which enhances our understanding of long-distance coupling and nonreciprocity. Our engineered approach to indirect coherent and dissipative coupling through travelling photons provides a way to establish nonreciprocal links among spatially separated systems to build networks of oscillators, which opens a range of opportunities for remote control that can be easily switched between coherent and dissipative coupling mechanisms.