title: Minimal noise subsystems.
abstract: A system subjected to noise contains a decoherence-free subspace or subsystem (DFS) only if the noise possesses an exact symmetry. Here we consider noise models in which a perturbation breaks a symmetry of the noise, so that if S is a DFS under a given noise process it is no longer so under the new perturbed noise process. We ask whether there is a subspace or subsystem that is more robust to the perturbed noise than S. To answer this question we develop a numerical method that allows us to search for subspaces or subsystems that are maximally robust to arbitrary noise processes. We apply this method to a number of examples, and find that a subsystem that is a DFS is often not the subsystem that experiences minimal noise when the symmetry of the noise is broken by a perturbation. We discuss which classes of noise have this property.
authors.affiliation: Hearne Institute for Theoretical Physics, Louisiana State University, Baton Rouge, Louisiana 70803, USA.|Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.|Department of Physics, University of Massachusetts at Boston, Boston, Massachusetts 02125, USA.
authors.forename: Xiaoting
authors.initials: X
authors.lastname: Wang
authors.affiliation: Physics Department and Computer Science Department, Southern Illinois University, Carbondale, Illinois 62901, USA.
authors.forename: Mark
authors.initials: M
authors.lastname: Byrd
authors.affiliation: Department of Physics, University of Massachusetts at Boston, Boston, Massachusetts 02125, USA.|U.S. Army Research Laboratory, Computational and Information Sciences Directorate, Adelphi, Maryland 20783, USA.
authors.forename: Kurt
authors.initials: K
authors.lastname: Jacobs