  We propose a technique to design control algorithms for a class of finite
dimensional quantum systems so that the control law does not present
discontinuities. The class of models considered admits a group of symmetries
which allows us to reduce the problem of control to
  a quotient space where the control system is `fully actuated'. As a result we
can prescribe a desired trajectory which is, to some extent, arbitrary and
derive the corresponding control. We discuss the application to the
simultaneous control of two non-interacting spin 1/2 particles with different
gyromagnetic ratios in zero field NMR in detail. Our method provides a flexible
toolbox for the design of control algorithms to drive the state of finite
dimensional quantum systems to any desired final configuration with smooth
controls.
