  We investigate the use of external time-dependent magnetic field for the
control of the quantum states in a two-electron quantum ring. The hyperfine
interaction of the confined electrons with surrounding nuclei couples the
singlet state with the three triplet states. When the external magnetic field
is changed, the singlet ground state becomes degenerate with the triplet states
allowing singlet-triplet transitions. By choosing different speeds for the
magnetic field switching the final quantum state of the system can be
manipulated. We evaluate suitable magnetic field values and time scales for
efficient quantum ring control.
