Actividades
Topology by dissipation in atomic quantum wires. [S. Diehl, E. Rico, M.A.Baranov, P. Zoller]
Enrique Rico
Innsbruck University
Robust edge states and non-abelian excitations are the trademark of topological states of matter, with promising applications such as topologically protected quantum memory and computing. While so far topological phases have been exclusively discussed in a hamiltonian context, we show that such phases and the associated topological protection and phenomena also emerge in open quantum systems with engineered dissipation. The specific system studied here is a quantum wire of spin-less atomic fermions in an optical lattice coupled to a bath. The key feature of the dissipative dynamics described by a Lindblad master equation is the existence of Majorana edge modes, representing a non-local decoherence free subspace. The isolation of the edge states is enforced by a dissipative gap in the p-wave paired bulk of the wire. We describe dissipative non-abelian braiding operations within the Majorana subspace, and we illustrate the insensitivity to imperfections. Topological protection is granted by a nontrivial winding number of the system density matrix.
Date: 2 November 2011
Time: 14:30 h
Place: 3rd floor seminar lecture room. Dpto. Física Teórica I. Facultad de Ciencias Físicas UCM
From Quantum Spins to Quantum Links: Towards the Quantum Simulation of Gauge Theories
Uwe-Jens Wiese
ITP University Bern
Quantum links are gauge covariant generalizations of quantum spins that replace the classical parallel transporter matrices in Wilson's formulation of lattice gauge theory. Like Wilson's parallel transporters in an SU(N) lattice gauge theory, quantum links are N*N matrices. However, just like the components of a quantum spin vector, the elements of a quantum link matrix are non-commuting operators. In the resulting quantum link models continuous gauge symmetry is implemented on fundamental discrete quantum degrees of freedom. Ordinary gauge theories result from quantum link models by dimensional reduction. A Z(2) quantum link model is equivalent to Kitaev's toric code. If they can be implemented with ultra-cold atoms in optical lattices or with trapped Rydberg ions, quantum link models can serve as quantum simulators of dynamical gauge theories. Some (2+1)-d U(1) quantum link models can be simulated efficiently on a classical computer. In addition, small systems can be solved by exact diagonalization. This would enable us to validate corresponding quantum simulators. It is to be expected that non-Abelian Chern-Simons gauge theories arise from some (2+1)-d quantum link models. This suggests to consider quantum link models as possible realizations of a topological quantum computer.
Date: 3 November 2011
Time: 14:30 h
Place: Dpto. Física Teórica, 3rd floor seminar lecture room. Facultad de Ciencias Físicas UCM
Non-locality, contextuality and graphs
Adan Cabello
Universidad de Sevilla
There is a curious connection between physics and graph theory: for some games and for any Bell and non-contextual inequality, the classical/local/non-contextual bound is given by the so-called independence number of the graph in which vertices represent propositions tested in the experiment and edges link propositions that cannot be simultaneously true. More interestingly, the maximum quantum value is given (or upper bounded) by the so-called Lovasz number of the graph, while the maximum value attainable under the assumption that the sum of probabilities of exclusive propositions cannot be larger than 1 is given by the so-called fractional packing number. We show how to use these connections to identify interesting experiments, and report some recent theoretical and experimental developments.
Date: 24 November 2011
Time: 14:30 h
Place: Dpto. Física Teórica I, 3rd floor Seminar Lecture room. Facultad de Ciencias Físicas UCM
ALGORITHMIC INFORMATION AS A FUNDAMENTAL CONCEPT IN PHYSICS, MATHEMATICS, AND BIOLOGY”
Prof. GREGORY J. CHAITIN
24th-26th October, 2011
at Facultad de Ciencias Físicas UCM
Affiliation: formerly at IBM Watson Research Center.
Currently: CAPES Foreign Visiting Professor,
Universidad Federal de Rio de Janeiro;
Associated Researcher at the Valparaiso Institute of Complex Systems in Chile:
Honorary Professor at the Universidad de Buenos Aires;
Honorary Doctorate from the University of Maine.
We use software models to discuss what is a law of physics, what are the limitations of the axiomatic method, and to analyze Darwin's theory of evolution.
24th October 2011 (Monday)
Title: “Algorithmic information as a fundamental concept in physics, mathematics, and biology”
*Time: 15:30 h
*SALA DE GRADOS DE LA FACULTAD (1st Floor)
‘Blackboard-chalk talk’
25th October 2011 (Tuesday)
Title: “Metabiology: Life as evolving software. Part I: Conceptual”
*Time: 15:30 h
*SALA DE GRADOS DE LA FACULTAD (1st Floor)
‘Blackboard-chalk talk' and computer-internet Access’
26th October 2011 (Wednesday)
Title:“Metabiology: Life as evolving software. Part II: Technical”
*Time: 15:30 h
*SALA DE GRADOS DE LA FACULTAD (1st Floor)
‘Blackboard-chalk talk' and computer-internet access’
Coherent backscattering effect: the case of ultracold matter waves
Nicolás Cherroret
University of Freiburg
In this talk, I will briefly review recent developments concerning the coherent backscattering (CBS) effect, a well-known precursor of Anderson localization in disordered systems. I will then focus on the particular and important case of CBS of ultracold matter waves in random (optical) potentials, and explain how this effect manifests itself in the momentum distribution of the matter wave. A detailed theoretical analysis of this distribution shows that the CBS effect can be used to prove that transport occurs in the phase-coherent regime, and that measuring its time dependence permits monitoring the transition from classical diffusion to Anderson localization.
Date: Thursday 6 de Octubre, 2011
Time: 14:30 h
Place: Seminar room. Dept. Física Teórica I, Planta 3ª, Facultad de Ciencias Físicas UCM
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