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technical paper
Travelling Photons Mediated Coherent and Dissipative Couplings in Long
Engineering coupling between systems is essential for many phenomena of quantum physics and technology, especially in long distance without field overlap. We construct a long-distance cavity magnonic system to realize indirect coupling mediated by travelling photons. A high-quality dielectric resonator (cavity photon mode) and an Yttrium Iron Garnet sphere (YIG, magnon mode) are placed along the transmission line with a longitudinal distance of over 1 metre to avoid direct coupling. By tuning the longitudinal distance given by phase and the YIG sphere's transverse position, we realized indirect coupling in transmission: in the reciprocal case with the YIG sphere in the center of the transmission line, we have coherent and dissipative couplings at the longitudinal phase Φ=(2n+1)π and Φ=(2n+2)π; while in the nonreciprocal case with the YIG sphere on the side of the transmission line, we have coherent and dissipative couplings for the left- and right-going travelling photons at the longitudinal phase Φ=(2n+0.5)π and Φ=(2n+1.5)π. The coupled-mode theory modified by a higher-order coupling term is adopted to explain our results and enhances our understanding of long-distance coupling and nonreciprocity. Our illustration of switching long-distance coherent and dissipative couplings, gives access to designing nonreciprocal links and constructing advanced remote-control strategies among spatially separated systems to build networks.