  Indirect controllability of an arbitrary finite dimensional quantum system
(N-dimensional qudit) through a quantum accessor is investigated. Here, The
qudit is coupled to a quantum accessor which is modeled as a fully controllable
spin chain with nearest neighbor (anisotropic) XY-coupling. The complete
controllability of such indirect control system is investigated in detail. The
general approach is applied to the indirect controllability of two and three
dimensional quantum systems. For two and three dimensional systems, a simpler
indirect control scheme is also presented.
