Current-driven quantum switch*

As a key component of a ballistic quantum switch proposed by Mel'nikov and Vinokur [Nature {\bf 415}, 60 (2002)], we realize the current-driven {\it giant-vortex splitting} in a mesoscopic superconducting square, in a given perpendicular magnetic field. We also demonstrate the controllable {\it current-induced transitions} between the states with different angular momenta, which provide the stepwise behavior of the up-down sample conductance as a function of applied current (not field). Theoretical simulations using time-dependent Ginzburg-Landau theory are fully corroborated by transport measurements, where vortex states are monitored using the small-tunnel-junction technique.