arXiv subjects

Otfried Guehne

Publications and source records attributed to Otfried Guehne.

12 recordsLinked to original sources

arXiv2006

Two-setting Bell Inequalities for Graph States

We present Bell inequalities for graph states with high violation of local realism. In particular, we show that there is a two-setting Bell inequality for every nontrivial graph state which is violated by the state at least by a factor of two. These inequalities are facets of the convex polytope containing the many-body correlations consistent with local hidden variable models. We first present a method which assigns a Bell inequality for each graph vertex. Then for some families of graph states composite Bell inequalities can be constructed with a violation of local realism increasing exponentially with the number of qubits. We also suggest a systematic way for obtaining Bell inequalities with a high violation of local realism for arbitrary graphs.

quant-ph↗

arXiv2005

Multipartite entanglement in spin chains

We investigate the presence of multipartite entanglement in macroscopic spin chains. We discuss the Heisenberg and the XY model and derive bounds on the internal energy for systems without multipartite entanglement. Based on this we show that in thermal equilibrium the above mentioned spin systems contain genuine multipartite entanglement, even at finite modest temperatures.

quant-ph↗

arXiv2005

Experimental Analysis of a 4-Qubit Cluster State

Linear optics quantum logic operations enabled the observation of a four-photon cluster state. We prove genuine four-partite entanglement and study its persistency, demonstrating remarkable differences to the usual GHZ state. Efficient analysis tools are introduced in the experiment, which will be of great importance in further studies on multi-particle entangled states.

quant-ph↗

arXiv2005

Bell Inequalities for Graph States

We investigate the non-local properties of graph states. To this aim, we derive a family of Bell inequalities which require three measurement settings for each party and are maximally violated by graph states. In turn, for each graph state there is an inequality maximally violated only by that state. We show that for certain types of graph states the violation of these inequalities increases exponentially with the number of qubits. We also discuss connections to other entanglement properties such as the positivity of the partial transpose or the geometric measure of entanglement.

quant-ph↗

arXiv2005

Addendum to "Sufficient conditions for three-particle entanglement and their tests in recent experiments"

A recent paper [M. Seevinck and J. Uffink, Phys. Rev. A 65, 012107 (2002)] presented a bound for the three-qubit Mermin inequality such that the violation of this bound indicates genuine three-qubit entanglement. We show that this bound can be improved for a specific choice of observables. In particular, if spin observables corresponding to orthogonal directions are measured at the qubits (e.g., X and Y spin coordinates) then the bound is the same as the bound for states with a local hidden variable model. As a consequence, it can straightforwardly be shown that in the experiment described by J.-W. Pan et al. [Nature 403, 515 (2000)] genuine three-qubit entanglement was detected.

quant-ph↗

arXiv2005

Entanglement Detection in the Stabilizer Formalism

We investigate how stabilizer theory can be used for constructing sufficient conditions for entanglement. First, we show how entanglement witnesses can be derived for a given state, provided some stabilizing operators of the state are known. These witnesses require only a small effort for an experimental implementation and are robust against noise. Second, we demonstrate that also nonlinear criteria based on uncertainty relations can be derived from stabilizing operators. These criteria can sometimes improve the witnesses by adding nonlinear correction terms. All our criteria detect states close to Greenberger-Horne-Zeilinger states, cluster and graph states. We show that similar ideas can be used to derive entanglement conditions for states which do not fit the stabilizer formalism, such as the three-qubit W state. We also discuss connections between the witnesses and some Bell inequalities.

quant-ph↗

arXiv2005

Detecting Genuine Multipartite Entanglement with Two Local Measurements

We present entanglement witness operators for detecting genuine multipartite entanglement. These witnesses are robust against noise and require only two local measurement settings when used in an experiment, independent from the number of qubits. This allows detection of entanglement for an increasing number of parties without a corresponding increase in effort. The witnesses presented detect states close to GHZ, cluster and graph states. Connections to Bell inequalities are also discussed.

quant-ph↗

arXiv2004

Separability Criteria from Uncertainty Relations

We explain several separability criteria which rely on uncertainty relations. For the derivation of these criteria uncertainty relations in terms of variances or entropies can be used. We investigate the strength of the separability conditions for the case of two qubits and show how they can improve entanglement witnesses.

quant-ph↗

arXiv2004

Two measurement settings can suffice to verify multipartite entanglement

We present entanglement witnesses for detecting genuine multi-qubit entanglement. Our constructions are robust against noise and require only two local measurement settings, independent of the number of qubits. Thus they allow to verify entanglement of many qubits in experiments while requiring only a small effort. In contrast, usual methods need an effort which increases exponentially with the number of qubits. The witnesses detect states close to GHZ states and cluster states.

quant-ph↗

arXiv2004

Entropic uncertainty relations and entanglement

We discuss the relationship between entropic uncertainty relations and entanglement. We present two methods for deriving separability criteria in terms of entropic uncertainty relations. Especially we show how any entropic uncertainty relation on one part of the system results in a separability condition on the composite system. We investigate the resulting criteria using the Tsallis entropy for two and three qubits.

quant-ph↗

arXiv2004

Witnessing multipartite entanglement

We present the experimental detection of genuine multipartite entanglement using entanglement witness operators. To this aim we introduce a canonical way of constructing and decomposing witness operators so that they can be directly implemented with present technology. We apply this method to three- and four-qubit entangled states of polarized photons, giving experimental evidence that the considered states contain true multipartite entanglement.

quant-ph↗

arXiv2004

Characterizing Entanglement via Uncertainty Relations

We derive a family of necessary separability criteria for finite-dimensional systems based on inequalities for variances of observables. We show that every pure bipartite entangled state violates some of these inequalities. Furthermore, a family of bound entangled states and true multipartite entangled states can be detected. The inequalities also allow to distinguish between different classes of true tripartite entanglement for qubits. We formulate an equivalent criterion in terms of covariance matrices. This allows us to apply criteria known from the regime of continuous variables to finite-dimensional systems.

quant-ph↗