We study a new approach to the control of a quantum system S, which uses the coupling of S with a quantum
probe P. The external control affects only P, and the accessibility and controllability properties describe to what
extent it is possible to drive the state of S by varying the initial state of P and using the interaction between
the two systems. In particular, we study the case of two-dimensional system and probe. Two situations are
considered: either the total system S + P is a closed one, or it is surrounded by a bath of decoupled harmonic
oscillators. We give results on the controllability and accessibility properties of this scheme, and we discuss the
relation of these properties with the entangling capability of the interaction between S and P. In particular, we
show that the SWAP operator plays a special role.