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Trotzky S.,Ludwig Maximilians University of Munich | Trotzky S.,Max Planck Institute of Quantum Optics | Trotzky S.,Johannes Gutenberg University Mainz | Chen Y.-A.,Ludwig Maximilians University of Munich | And 12 more authors.
Nature Physics | Year: 2012

The problem of how complex quantum systems eventually come to rest lies at the heart of statistical mechanics. The maximum-entropy principle describes which quantum states can be expected in equilibrium, but not how closed quantum many-body systems dynamically equilibrate. Here, we report the experimental observation of the non-equilibrium dynamics of a density wave of ultracold bosonic atoms in an optical lattice in the regime of strong correlations. Using an optical superlattice, we follow its dynamics in terms of quasi-local densities, currents and coherences-all showing a fast relaxation towards equilibrium values. Numerical calculations based on matrix-product states are in an excellent quantitative agreement with the experimental data. The system fulfills the promise of being a dynamical quantum simulator, in that the controlled dynamics runs for longer times than present classical algorithms can keep track of. © 2012 Macmillan Publishers Limited. All rights reserved. Source

Gross D.,Leibniz University of Hanover | Eisert J.,University of Potsdam | Eisert J.,Institute for Advanced Study Berlin
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2010

We discuss the notion of quantum computational webs: These are quantum states universal for measurement-based computation, which can be built up from a collection of simple primitives. The primitive elements-reminiscent of building blocks in a construction kit-are (i) one-dimensional states (computational quantum wires) with the power to process one logical qubit and (ii) suitable couplings, which connect the wires to a computationally universal web. All elements are preparable by nearest-neighbor interactions in a single pass, of the kind accessible in a number of physical architectures. We provide a complete classification of qubit wires, a physically well-motivated class of universal resources that can be fully understood. Finally, we sketch possible realizations in superlattices and explore the power of coupling mechanisms based on Ising or exchange interactions. © 2010 The American Physical Society. Source

Wolf H.,Institute for Advanced Study Berlin | Wolf H.,University of Ulm
Journal of Experimental Biology | Year: 2011

Animals have needed to find their way about almost since a free-living life style evolved. Particularly, if an animal has a home - shelter or nesting site - true navigation becomes necessary to shuttle between this home and areas of other activities, such as feeding. As old as navigation is in the animal kingdom, as diverse are its mechanisms and implementations, depending on an organism's ecology and its endowment with sensors and actuators. The use of landmarks for piloting or the use of trail pheromones for route following have been examined in great detail and in a variety of animal species. The same is true for senses of direction - the compasses for navigation - and the construction of vectors for navigation from compass and distance cues. The measurement of distance itself - odometry - has received much less attention. The present review addresses some recent progress in the understanding of odometers in invertebrates, after outlining general principles of navigation to put odometry in its proper context. Finally, a number of refinements that increase navigation accuracy and safety are addressed. © 2011. Source

Ohliger M.,University of Potsdam | Kieling K.,University of Potsdam | Eisert J.,University of Potsdam | Eisert J.,Institute for Advanced Study Berlin
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2010

We discuss the potential and limitations of Gaussian cluster states for measurement-based quantum computing. Using a framework of Gaussian-projected entangled pair states, we show that no matter what Gaussian local measurements are performed on systems distributed on a general graph, transport and processing of quantum information are not possible beyond a certain influence region, except for exponentially suppressed corrections. We also demonstrate that even under arbitrary non-Gaussian local measurements, slabs of Gaussian cluster states of a finite width cannot carry logical quantum information, even if sophisticated encodings of qubits in continuous-variable systems are allowed for. This is proven by suitably contracting tensor networks representing infinite-dimensional quantum systems. The result can be seen as sharpening the requirements for quantum error correction and fault tolerance for Gaussian cluster states and points toward the necessity of non-Gaussian resource states for measurement-based quantum computing. The results can equally be viewed as referring to Gaussian quantum repeater networks. © 2010 The American Physical Society. Source

Kieling K.,University of Potsdam | O'Brien J.L.,University of Bristol | Eisert J.,University of Potsdam | Eisert J.,Institute for Advanced Study Berlin
New Journal of Physics | Year: 2010

As primitives for entanglement generation, controlled phase gates have a central role in quantum computing. Especially in ideas realizing instances of quantum computation in linear optical gate arrays, a closer look can be rewarding. In such architectures, all effective nonlinearities are induced by measurements. Hence the probability of success is a crucial parameter of such quantum gates. In this paper, we discuss this question for controlled phase gates that implement an arbitrary phase with one and two control qubits. Within the class of post-selected gates in dual-rail encoding with vacuum ancillas, we identify the optimal success probabilities. We construct networks that allow for implementation using current experimental capabilities in detail. The methods employed here appear specifically useful with the advent of integrated linear optical circuits, providing stable interferometers on monolithic structures. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft. Source

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