Abstract: Improved spin-microwave coupling is provided using a microwave cavity having a galvanic element connecting two points on its walls. This configuration can provide an unusual cavity mode having a highly concentrated magnetic field (to provide good coupling to a spin system) and a spread-out electric field (to reduce loss, since the electric field is almost entirely in vacuum). The galvanic element preferably includes a nano-scale current concentration feature (i.e., sub 100 nm lateral dimensions) to concentrate the magnetic field. The spin system of interest is preferably disposed in proximity to this current concentration feature. Applications include coherent quantum optical-microwave transduction and quantum memories.