Controllable Subspaces of Open Quantum Dynamical Systems

This paper discusses the concept of controllable subspace for open quantum dynamical systems.It is constructively demonstrated that combining structuralfeatures of decoherence-free subspaces with the ability to performopen-loop coherent control on open quantum systems will allowdecoherence-free subspaces to be controllable. This is in contrastto the observation that open quantum dynamical systems are notopen-loop controllable. To a certain extent, this paper gives analternative control theoretical interpretation on whydecoherence-free subspaces can be useful for quantum computation.