Full control by locally induced relaxation

We demonstrate a scheme for controlling a large quantum system by acting

on a small subsystem only. The local control is mediated to the larger

system by some fixed coupling Hamiltonian. The scheme allows to transfer

arbitrary and unknown quantum states from a memory to the large system

("upload access") as well as the inverse ("download access").

We give sufficient conditions of the coupling Hamiltonian for the

controllability

of the system which can be checked efficiently by a colour-infection game on

the graph

that describes the couplings.