Magnetically induced pumping and memory storage in quantum rings

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 {\it writing} and {\it erasing} can be efficiently performed.