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 on the large system (``upload access'') as well as the inverse (``download access''). We study sufficient conditions of the coupling Hamiltonian and give lower bounds on the fidelities for downloading and uploading.
@article{arxiv.0704.3027,
title = {Full control by locally induced relaxation},
author = {Daniel Burgarth and Vittorio Giovannetti},
journal= {arXiv preprint arXiv:0704.3027},
year = {2007}
}