Magnetically induced pumping and memory storage in quantum rings

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Nanoscopic rings pierced by external magnetic fields and asymmetrically connected to wires behave in sharp contrast with classical expectations. By studying the real-time evolution of tight-binding models in different geometries, we show that the creation of a magnetic dipole by a bias-induced current is a process that can be reversed: connected rings excited by an internal ac flux produce ballistic currents in the external wires. In particular we point out that, by employing suitable flux protocols, single-parameter nonadiabatic pumping can be achieved, and an arbitrary amount of charge can be transferred from one side to the other. We also propose a set up that could serve a memory device, in which both the operations of writing and erasing can be efficiently performed.