Showing papers on "Concurrence published in 2014"

Entanglement Monogamy Relations of Qubit Systems

Abstract: We investigate the monogamy relations related to the concurrence and the entanglement of formation. General monogamy inequalities given by the {\alpha}th power of concurrence and entanglement of formation are presented for N-qubit states. The monogamy relation for entanglement of assistance is also established. Based on these general monogamy relations, the residual entanglement of concurrence and entanglement of formation are studied. Some relations among the residual entanglement, entanglement of assistance, and three tangle are also presented.

Increase of entanglement by local PT -symmetric operations

Abstract: Entanglement plays a central role in the field of quantum information science. It is well known that the degree of entanglement cannot be increased under local operations. Here, we show that the concurrence of a bipartite entangled state can be increased under the local $\mathcal{PT}$-symmetric operation. This violates the property of entanglement monotonicity. We also use the Bell--Clauser-Horne-Shimony-Holt and steering inequalities to explore this phenomenon.

Monogamy of entanglement of formation

Abstract: It is well known that a particle cannot freely share entanglement with two or more particles. This restriction is generally called monogamy. However the formal quantification of such restriction is only known for some measures of entanglement and for two-level systems. The first and broadly known monogamy relation was established by Coffman, Kundu, and Wootters for the square of the concurrence. Since then, it is usually said that the entanglement of formation is not monogamous, as it does not obey the same relation. We show here that despite that, the entanglement of formation cannot be freely shared and therefore should be said to be monogamous. Furthermore, the square of the entanglement of formation does obey the same relation of the squared concurrence, a fact recently noted for three particles and extended here for N particles. Therefore the entanglement of formation is as monogamous as the concurrence. We also numerically study how the entanglement is distributed in pure states of three qubits and the relation between the sum of the bipartite entanglement and the classical correlation.

Pairwise thermal entanglement in the Ising-XYZ diamond chain structure in an external magnetic field

Abstract: Quantum entanglement is one of the most fascinating types of correlation that can be shared only among quantum systems. The Heisenberg chain is one of the simplest quantum chains which exhibits a rich entanglement feature, due to the fact that the Heisenberg interaction is quantum coupling in the spin system. The two particles were coupled trough XYZ coupling or simply called as two-qubit XYZ spin, which are the responsible for the emergence of thermal entanglement. These two-qubit operators are bonded to two nodal Ising spins, and this process is repeated infinitely resulting in a diamond chain structure. We will discuss the two-qubit thermal entanglement effect on the Ising-XYZ diamond chain structure. The concurrence could be obtained straightforwardly in terms of two-qubit density operator elements; using this result we study the thermal entanglement, as well as the threshold temperature where entangled state vanishes. The present model displays a quite unusual concurrence behavior, e.g., the boundary of two entangled regions becomes a disentangled region, and this is intrinsically related to the XY-anisotropy in the Heisenberg coupling. Although a similar property had been found for only two qubits, here we show it for the case of a diamond chain structure, which reasonably represents real materials.

Hierarchical monogamy relations for the squared entanglement of formation in multipartite systems

Abstract: We show exactly that the squared entanglement of formation (SEF) obeys a set of hierarchical monogamy relations for an arbitrary $N$-qubit mixed state. Based on this set of monogamy relations, we are able to construct the set of hierarchical multipartite entanglement indicators for $N$-qubit states, which still work well even when the concurrence-based indicators lose efficacy. As a by-product, an intriguing analytical relation between the entanglement of formation (EOF) and squared concurrence (SC) for an arbitrary mixed state of $2\ensuremath{\bigotimes}d$ systems is derived, making the concurrence calculable via the corresponding EOF. Furthermore, we analyze the multipartite entanglement dynamics in composite cavity-reservoir systems with the present set of hierarchical indicators. Moreover, for multilevel systems, it is illustrated that the SEF can be monogamous even if the SC is polygamous.

Entanglement universality of two-qubit X-states

Abstract: We demonstrate that for every two-qubit state there is a X-counterpart, i.e., a corresponding two-qubit X-state of same spectrum and entanglement, as measured by concurrence, negativity or relative entropy of entanglement. By parametrizing the set of two-qubit X-states and a family of unitary transformations that preserve the sparse structure of a two-qubit X-state density matrix, we obtain the parametric form of a unitary transformation that converts arbitrary two-qubit states into their X-counterparts. Moreover, we provide a semi-analytic prescription on how to set the parameters of this unitary transformation in order to preserve concurrence or negativity. We also explicitly construct a set of X-state density matrices, parametrized by their purity and concurrence, whose elements are in one-to-one correspondence with the points of the concurrence versus purity (CP) diagram for generic two-qubit states.

Lower bound of concurrence for qubit systems

Abstract: We study the concurrence of four-qubit quantum states and provide analytical lower bounds of concurrence in terms of the monogamy inequality of concurrence for qubit systems. It is shown that these lower bounds are able to improve the existing bounds and detect entanglement better. The approach is generalized to arbitrary qubit systems.

Lower Bound of Concurrence for Qubit Systems

Abstract: We study the concurrence of four-qubit quantum states and provide analytical lower bounds of concurrence in terms of the monogamy inequality of concurrence for qubit systems. It is shown that these lower bounds are able to improve the existing bounds and detect entanglement better. The approach is generalized to arbitrary qubit systems.

Entangled spin states in geodesic motion around massive body

Abstract: We discuss the effect of the gravitational field of a massive body on the spin entanglement of a two-qubit system in the singlet and triplet spin states in circular geodesic motion. We study the entanglement transport using Wootters concurrence, which depends on the momentum state of the system. We describe the behavior of the concurrence as a function of the orbital radius and show that the spin entanglement is more robust against changes caused by motion in the singlet state than in the triplet state. Furthermore, for the singlet (triplet) state, momentum correlation increases (decreases) the concurrence.

Spin-echo entanglement protection from random telegraph noise

Abstract: We analyze local spin-echo procedures to protect entanglement between two non-interacting qubits, each subject to pure-dephasing random telegraph noise. For superconducting qubits this simple model captures characteristic features of the effect of bistable impurities coupled to the device. An analytic expression for the entanglement dynamics is reported. Peculiar features related to the non-Gaussian nature of the noise already observed in the single qubit dynamics also occur in the entanglement dynamics for proper values of the ratio $g=v/\gamma$, between the qubit-impurity coupling strength and the switching rate of the random telegraph process, and of the separation between the pulses $\Delta t$. We find that the echo procedure may delay the disappearance of entanglement, cancel the dynamical structure of entanglement revivals and dark periods, and induce peculiar plateau-like behaviors of the concurrence.

Entanglement of a 2-qubit system coupled to a bath of quantum spin glass

Abstract: We study the quantum entanglement (concurrence) of a 2-qubit system coupled to a small spin glass bath of 2 to n ≥ 4 qubits. The bath is described by the quantum X X Heisenberg model with random J coupling and varying magnetic field h . We look at the dynamics of the steady state average concurrence for the system and obtain a general formula to describe the concurrence with J = 0 and h = 0 for n bath sites. The physics of 2-qubit system coupled with n bath sites for J = 0 is analytically described for small n . The result for large n was numerically found to be qualitatively similar. For small fluctuation in J , a mean steady state average concurrence of about 0.5 is obtained.

Multipartite entanglement dynamics in a cavity

Abstract: We study the dynamics of two kinds of entanglement, and there interplay. On one hand, the intrinsic entanglement within a central system composed by three two level atoms, and measured by multipartite concurrence, on the other, the entanglement between the central system and a cavity, acting as an environment, and measured with purity. Using dipole-dipole and Ising interactions between atoms we propose two Hamiltonians, a homogeneous and a quasi-homogeneous one. We find an upper bound for concurrence as a function of purity, associated to the evolution of the $W$ state. A lower bound is also observed for the homogeneous case. In both situations, we show the existence of critical values of the interaction, for which the dynamics of entanglement seem complex.

Entanglement and Sudden Death of a Nonlinear Two-Level-Atom Pair Interacting with a Radiation Field

Abstract: We study the entanglement between a pair of two-level atoms simultaneously interacting via a singlemode thermal field. The Hamiltonian also describes a two-photon process. The entanglement between a nonlinear atom-field interacting system is also studied using atomic and field entropy changes. We use concurrence to detect the sudden death phenomenon. Furthermore, we discuss the relationship between entropy changes and concurrence entanglement. Our results show that the behavior of the entropy change is in agreement with the behavior of the concurrence when we measure the entanglement between the two-subsystem structure.

Average entanglement dynamics in open two-qubit systems with continuous monitoring

Abstract: We present a comprehensive implementation of the quantum trajectory theory for the description of the entanglement dynamics in a Markovian open quantum system made of two qubits. We introduce the average concurrence to characterize the entanglement in the system and derive a deterministic evolution equation for it that depends on the ways in which information is read from the environment. This buildt-in flexibility of the method is used to address two actual issues in quantum information: entanglement protection and entanglement estimation. We identify general physical situations in which an entanglement protection protocol based on local monitoring of the environment can be implemented. Additionally, we methodically find unravelings of the system dynamics providing analytical tight bounds for the unmonitored entanglement in the system at all times. We conclude by showing the independence of the method from the choice of entanglement measure.

Fidelity and Entanglement with Environment Effect

Abstract: A quantum entangling gate is investigated in dissipative system by considering input initial entangled two qubits. By analyzing the relation between the fidelity and the concurrence, we find that the fidelity of gate is strongly correlated to the entanglement of the qubits, where the center positions of entangled death is corresponding to the turning points of the fidelity. The results are helpful for one to implement quantum computation with dissipative effects.

Quantum correlations in a two-qubit anisotropic Heisenberg XYZ chain with uniform magnetic field

Abstract: Quantum correlations in an anisotropic Heisenberg XYZ chain is investigated by use of concurrence C and measurement-induced disturbance (MID). We show that the behaviors of the MID are remarkably different from the concurrence. Firstly, it is shown that there is a revival phenomenon in the concurrence but not in the MID, which is suitable for both the ground state case and the finite temperature case. Based on the analysis of the ground-state C and MID structures, we illustrate the reason why the ground-state MID does not show a revival phenomenon in detail. Then we explore different effects of the external and self parameters on entanglement and MID behaviors. It can be shown that the region of MID is evidently larger than the case of concurrence, and that the concurrence signals a quantum phase transition even at finite T while MID does not. Cases where the concurrence finally maintains one nonzero constant value regardless of the value of the variable B for a constant Jz, while MID decreases monotonously to zero with increasing B. We also show that if B can take a proper range of values, the concurrence decreases with the improvement of the anisotropic parameter γ, whereas an opposite effect for MID behaviors is presented.

Le rôle de la concurrence dans l'essor des écoles supérieures de commerce

Abstract: Cet article analyse les transformations qu’ont connues les ecoles superieures de commerce, en France, depuis la fin des annees 1990. A travers l’etude de trois processus (internationalisation, developpement de la recherche et diversification de l’offre de formation et du recrutement) il s’agit de mettre en evidence les logiques qui president a ces transformations. L’enjeu est notamment de montrer que si les demandes externes (demandes des familles ou des entreprises) peuvent jouer un role, celui-ci peut apparaitre secondaire face aux logiques d’imitation et de concurrence a l’œuvre dans l’espace ou evoluent ces etablissements.

Concurrence analysis method and device

Abstract: The invention discloses a concurrence analysis method and device. The concurrence analysis method comprises the steps of obtaining a program control flow diagram, wherein each thread establishing node in the program control flow diagram is a direct precursor node of a starting node of a thread established by the thread establishing node; calculating an initial concurrence node set of the starting nodes of all threads in the program control flow diagram, an initial concurrence node set of direct successor nodes of the nodes established by the threads and an initial non-concurrence node set of the direct successor nodes of terminal nodes of the threads; initializing the concurrence node set of the starting nodes of the program control flow diagram, beginning to perform downward traversal from the starting nodes of the program control flow diagram and calculating the concurrence node set of the traversed nodes according to an inheritance rule. Due to the fact that the concurrence node set of each node is obtained through one-time traversal, operation is simple, operation amount is low, and the purpose of improving concurrence analysis efficiency is achieved.

Entanglement and quantum spin glass

Abstract: The paper reviews the entanglement behavior of a 2-qubit system in a quantum spin glass using the Heisenberg XX model and the interaction of the system with a spin glass bath environment. In the first part, we study the entanglement (concurrence) for a 3- and 4-qubit with nearest neighbor interaction. With a fixed mean and varying standard deviation for the J coupling, the concurrence is numerically plotted with temperature for the different configurations. A general formula for the concurrence is given for n qubits at low temperature. In the second part, we study the concurrence of a 2-qubit system coupled to a spin glass bath environment with n = 2 to ≥ 4 qubits. The bath sites are coupled with random J coupling and varying applied magnetic field. A general formula for concurrence is given for mean J = 0 and B = 0 for n bath sites. For small random J and magnetic field B, a steady state is obtained with an approximate concurrence of 0.5, showing that the entanglement is preserved.

Orbital entanglement and electron localization in quantum wires

Abstract: We study the signatures of disorder in the production of orbital electron entanglement in quantum wires. Disordered entanglers suffer the effects of localization of the electron wave function and random fluctuations in entanglement production. This manifests in the statistics of the concurrence, a measure of the produced two-qubit entanglement. We calculate the concurrence distribution as a function of the disorder strength within a random-matrix approach. We also identify significant constraints on the entanglement production as a consequence of the breaking/preservation of timereversal symmetry. Additionally, our theoretical results are independently supported by simulations of disordered quantum wires based on a tight-binding model. PACS numbers: 03.67.Bg, 73.23.-b, 72.15.Rn, 73.63.Nm

Lower Bound of Concurrence Based on Generalized Positive Maps

Abstract: We study the concurrence of arbitrary dimensional bipartite quantum systems. By using a positive but not completely positive map, we present an analytical lower bound of concurrence. Detailed examples are used to show that our bound can detect entanglement better and can improve the well known existing lower bounds.

A Study on Entanglement Sudden Death in the Open Bipartite Systems

Abstract: Examining the relation between concurrence and energy in the open bipartite systems, we give an enlightening discussion about reason which causes the entanglement sudden death. We consider two two-level atoms A and B initially entangled to some extent and coupled individually to two cavities which are initially in their vacuum states. We analyze the dynamics of entanglement and energy for two atoms after tracing over the cavity degrees of freedom. By comparing with concurrence and energy, we obtain a conclusion, i.e., there is a critical value U C =0, when the energy U

Hacia una universidad nueva

Abstract: This article deals with the New University and the main conceptions it ought to comply with, through ideas development and shaping, based on systemic thought and following a concurrence of methodologies of the Checkland, Forrester and Senge's Soft Systems.

Quantum Correlation Properties Between Two Spatially Separated Atoms in Free Space Reinforced by Incoherent Pumping

Abstract: Quantum correlation dynamics between two identical and spatially separated atoms in free space is investigated by the use of concurrence C and quantum discord (QD). The behaviors of QD differs in many unexpected ways from the entanglement in this system. Firstly, it shows the situations which the concurrence and QD can behave very differently with a “sudden birth” phenomenon of the former but not of the latter, and QD is only oscillating decays with time and the interqubit distance. We also verify the cases which QD is always greater than the concurrence and the region where the concurrence is vanished but with nonzero values for QD. Meanwhile an unexpected situation which the concurrence is greater than QD under the initial state |eg〉 is analyzed. It is revealed that the quantum correlation based only on QD is expected to be more robust than entanglement which is not suitable for all the initial states under the decoherence environment. Then, by introducing the incoherent pumping, we also study the different properties of the steady-state entanglement and QD about this atomic subsystem. It is shown that the incoherent pumping can overcome the decay of the atoms and the influences about the interqubit distance r 12/λ on the steady-state correlation can make the decay of the concurrence obviously quicker than QD, the life of the steady-state QD is evidently larger than the steady-state entanglement.

Measuring qubit entanglement using Concurrence and Negativity in Jaynes-Cummings model

Abstract: In this paper we investigate entanglement dynamics of two isolated atoms, each in its own Jaynes-Cummings Cavity. We use the concurrence and the negativity for measuring entanglement.