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W Dür

Publications and source records attributed to W Dür.

At least 19 recordsLinked to original sources
PubMed2006

Ground-state approximation for strongly interacting spin systems in arbitrary spatial dimension.

We introduce a variational method for the approximation of ground states of strongly interacting spin systems in arbitrary geometries and spatial dimensions. The approach is based on weighted graph states and superpositions thereof. These states allow for the efficient computation of all local observables (e.g., energy) and include states with diverging correlation length and unbounded multiparticle entanglement. As a demonstration, we apply our approach to the Ising model on 1D, 2D, and 3D square lattices. We also present generalizations to higher spins and continuous-variable systems, which allows for the investigation of lattice field theories.

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PubMed2005

Scalable multiparticle entanglement of trapped ions.

The generation, manipulation and fundamental understanding of entanglement lies at the very heart of quantum mechanics. Entangled particles are non-interacting but are described by a common wavefunction; consequently, individual particles are not independent of each other and their quantum properties are inextricably interwoven. The intriguing features of entanglement become particularly evident if the particles can be individually controlled and physically separated. However, both the experimental realization and characterization of entanglement become exceedingly difficult for systems with many particles. The main difficulty is to manipulate and detect the quantum state of individual particles as well as to control the interaction between them. So far, entanglement of four ions or five photons has been demonstrated experimentally. The creation of scalable multiparticle entanglement demands a non-exponential scaling of resources with particle number. Among the various kinds of entangled states, the 'W state' plays an important role as its entanglement is maximally persistent and robust even under particle loss. Such states are central as a resource in quantum information processing and multiparty quantum communication. Here we report the scalable and deterministic generation of four-, five-, six-, seven- and eight-particle entangled states of the W type with trapped ions. We obtain the maximum possible information on these states by performing full characterization via state tomography, using individual control and detection of the ions. A detailed analysis proves that the entanglement is genuine. The availability of such multiparticle entangled states, together with full information in the form of their density matrices, creates a test-bed for theoretical studies of multiparticle entanglement. Independently, 'Greenberger-Horne-Zeilinger' entangled states with up to six ions have been created and analysed in Boulder.

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PubMed2005

Spin gases: quantum entanglement driven by classical kinematics.

A spin gas is a natural extension of a classical gas. It consists of a large number of particles whose (random) motion is described classically, but, in addition, have internal (quantum mechanical) degrees of freedom that interact during collisions. For specific types of quantum interactions we determine the entanglement that occurs naturally in such systems. We analyze how the evolution of the quantum state is determined by the underlying classical kinematics of the gas. For the Boltzmann gas, we calculate the rate at which entanglement is produced and characterize the entanglement properties of the equilibrium state.

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PubMed2005

Solid-state circuit for spin entanglement generation and purification.

We show how realistic charge manipulation and measurement techniques, combined with the exchange interaction, allow for the robust generation and purification of four-particle spin entangled states in electrically controlled semiconductor quantum dots. The generated states are immunized to the dominant sources of noise via a dynamical decoherence-free subspace; all additional errors are corrected by a purification protocol. This approach may find application in quantum computation, communication, and metrology.

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PubMed2005

Entanglement in spin chains and lattices with long-range Ising-type interactions.

We consider N initially disentangled spins, embedded in a ring or d-dimensional lattice of arbitrary geometry, which interact via some long-range Ising-type interaction. We investigate relations between entanglement properties of the resulting states and the distance dependence of the interaction in the limit N-->infinity. We provide a sufficient condition when bipartite entanglement between blocks of L neighboring spins and the remaining system saturates and determine S(L) analytically for special configurations. We find an unbounded increase of S(L) as well as diverging correlation and entanglement length under certain circumstances. For arbitrarily large N, we can efficiently calculate all quantities associated with reduced density operators of up to ten particles.

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PubMed2004

Nonadditivity of quantum capacity for multiparty communication channels.

We investigate multiparty communication scenarios where information is sent from several senders to several receivers. We establish a relation between the quantum capacity of multiparty communication channels and their distillability properties, which enables us to show that the quantum capacity of such channels is not additive.

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PubMed2004

Stability of macroscopic entanglement under decoherence.

We investigate the lifetime of macroscopic entanglement under the influence of decoherence. For Greenberger-Horne-Zeilinger-type superposition states, we find that the lifetime decreases with the size of the system (i.e., the number of independent degrees of freedom), and the effective number of subsystems that remain entangled decreases with time. For a class of other states (e.g., cluster states), however, we show that the lifetime of entanglement is independent of the size of the system.

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PubMed2003

Multiparticle entanglement purification for graph states.

We introduce a class of multiparticle entanglement purification protocols that allow us to distill a large class of entangled states. These include cluster states, Greenberger-Horne-Zeilinger states, and various error correction codes all of which belong to the class of two-colorable graph states. We analyze these schemes under realistic conditions and observe that they are scalable; i.e., the threshold value for imperfect local operations does not depend on the number of parties for many of these states. When compared to schemes based on bipartite entanglement purification, the protocol is more efficient and the achievable quality of the purified states is larger. As an application we discuss an experimental realization of the protocol in optical lattices which allows one to purify cluster states.

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PubMed2003

Separable States can be used to distribute entanglement.

We show that no entanglement is necessary to distribute entanglement; that is, two distant particles can be entangled by sending a third particle that is never entangled with the other two. Similarly, two particles can become entangled by continuous interaction with a highly mixed mediating particle that never itself becomes entangled. We also consider analogous properties of completely positive maps, in which the composition of two separable maps can create entanglement.

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PubMed2003

Entanglement purification for quantum computation.

We show that thresholds for fault-tolerant quantum computation are solely determined by the quality of single-system operations if one allows for d-dimensional systems with 8 < or = d < or = 32. Each system serves to store one logical qubit and additional auxiliary dimensions are used to create and purify entanglement between systems. Physical, possibly probabilistic two-system operations with error rates up to 2/3 are still tolerable to realize deterministic high-quality two-qubit gates on the logical qubits. The achievable error rate is of the same order of magnitude as of the single-system operations. We investigate possible implementations of our scheme for several physical setups.

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PubMed2002

Optimal conversion of nonlocal unitary operations.

We study when a nonlocal unitary operation acting on two d-level systems can probabilistically simulate another one when arbitrary local operations and classical communication are allowed. We provide necessary and sufficient conditions for the simulation to be possible. Probabilistic interconvertability is used to define an equivalence relation between gates. We show that this relation induces a finite number of classes that we identify. In the case of two qubits, two classes of nonlocal operations exist. We show how the representatives of these classes, CNOT and SWAP, can be deterministically converted into any operation of its class and calculate the optimal probability of the reverse process.

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PubMed2002

Entanglement cost of bipartite mixed states.

We compute the entanglement cost of several families of bipartite mixed states, including arbitrary mixtures of two Bell states. This is achieved by developing a technique that allows us to ascertain the additivity of the entanglement of formation for any state supported on specific subspaces. As a side result, the proof of the irreversibility in asymptotic local manipulations of entanglement is extended to two-qubit systems.

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PubMed2001

Multipartite bound entangled states that violate Bell's inequality.

We study the relation between distillability of multipartite states and violation of Bell's inequality. We prove that there exist multipartite bound entangled states (i.e., nonseparable, nondistillable states) that violate a multipartite Bell inequality. This implies that (i) violation of Bell's inequality is not a sufficient condition for distillability and (ii) some bound entangled states cannot be described by a local hidden variable model.

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PubMed2001

Entanglement capabilities of nonlocal Hamiltonians.

We quantify the capability of creating entanglement for a general physical interaction acting on two qubits. We give a procedure for optimizing the generation of entanglement. We also show that a Hamiltonian can create more entanglement if one uses auxiliary systems.

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PubMed2001

Quality of life in children and adolescents: a European public health perspective.

OBJECTIVES: The measurement of health-related quality of life (HRQOL) is increasingly important as a means of monitoring population health status over time, of detecting sub-groups within the general population with poor HRQOL, and of assessing the impact of public health interventions within a given population. At present, no standardised instrument exists which can be applied with equal relevance in pediatric populations in different European populations. The collaborative European KIDSCREEN project aims to develop a standardised screening instrument for children's quality of life which will be used in representative national and European health surveys. Participants of the project are centres from Austria, France, Germany, Netherlands, Spain, Switzerland, and United Kingdom. By including the instrument in health services research and health reporting, it also aims at identifying children at risk in terms of their subjective health, thereby allowing the possibility of early intervention. METHODS: Instrument development will be based on constructing a psychometrically sound HRQOL instrument taking into account the existing state of the art. Development will centre on literature searches, expert consultation (Delphi Methods) and focus groups with children and adolescents (8-17 years). According to international guidelines, items will be translated into the languages of the seven participating countries for a pilot test with 2,100 children and their parents in Europe. The final instrument will be used in representative mail and telephone surveys of HRQOL in 1,800 children and their parents per country (total n = 25,200) and normative data will be produced. The potential for implementing the measurement tool in health services and health reporting will also be evaluated in several different research and public health settings. The final analysis will involve national and cross cultural-analysis of the instrument. RESULTS: The international, collaborative nature of the KIDSCREEN project means it is likely to provide many challenges in terms of producing an instrument which is conceptually and linguistically appropriate for use in many different countries, but it will also provide the opportunity to develop, test and implement the first truly cross-national HRQOL instrument developed for use in children and adolescents. This will help to contribute to a better understanding of perceived health in children and adolescents and to identify populations at risk.

Adolescent↗
PubMed2000

Reversible combination of inequivalent kinds of multipartite entanglement.

We present a family of tripartite entangled states that, in an asymptotical sense, can be reversibly converted into Einstein-Podolsky-Rosen (EPR) states, shared by parties B and C, and tripartite Greenberger-Horne-Zeilinger (GHZ) states. Thus we show that a bipartite and a genuine tripartite entanglement can be reversibly combined in a tripartite state. For such states the corresponding fractions of GHZ and EPR states represent a complete quantification of their (asymptotical) entanglement resources. More generally, we show that AB, AC, and BC EPR entanglement and GHZ entanglement can be reversibly combined in a single tripartite state. Finally, we generalize this result to any number of parties.

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