Population relaxation effects on entanglement dynamics of the two-qubit spin chains

By analytically solving the master equation, we examine entanglement dynamics of the two-qubit spin system coupled via the XY interaction and in the presence of population relaxation. We found that the entanglement dynamical behaviors are very sensitive to different initialized states. Particularly, for initial states ?=(|01〉〈01|+|10〉〈10|)/2 and ?=(|00〉〈00|+|11〉〈11|)/2, a combination of the XY coupling with the population relaxation can even be used to enhance the entanglement. Moreover, we show that among different initially separable states, ?=|01〉〈01| and ?=|10〉〈10| are more robust in generating pairwise entanglement.     

Entanglement sudden death in qubit-qutrit systems

We demonstrate the existence of entanglement sudden death (ESD), the complete loss of entanglement in finite time, in qubit-qutrit systems. In particular, ESD is shown to occur in such systems initially prepared in a one-parameter class of entangled mixed states and then subjected to local dephasing noise. Together with previous results, this proves the existence of ESD for some states in all quantum systems for which rigorously defined mixed-state entanglement measures have been identified. We conjecture that ESD exists in all quantum systems prepared in appropriate bipartite states.     

Effects of Purity on Dynamics of Multipartite Entanglement

We study the effects of purity on entanglement dynamics of a multipartite system in cavity QED. Three two-level atoms A, B and C are initially prepared in an entangled state and locally coupled with independent cavities a, b and c, respectively. We consider the effects of purity of atomic initial state on the entanglement evolution of both the systems of atoms and cavities. It is found that depending on the purity of atomic initial state, the entanglement of atoms （cavities） may or may not exhibit the sudden death （birth） phenomenon.     

Dynamics of entanglement of bosonic modes on symmetric graphs

We investigate the dynamics of an initially disentangled Gaussian state on a general finite symmetric graph. As concrete examples we obtain properties of this dynamics on mean field graphs (also called fully connected or complete graphs) of arbitrary sizes. In the same way that chains can be used for transmitting entanglement by their natural dynamics, these graphs can be used to store entanglement. We also consider two kinds of regular polyhedron which show interesting features of entanglement sharing.     

Sudden death and birth of entanglement beyond the Markovian approximation

We investigate the entanglement dynamics of two initially entangled qubits interacting independently with two uncorrelated reservoirs beyond the Markovian approximation. Quite different from the Markovian reservoirs [C.E. López, et al., Phys. Rev. Lett. 101 (2008) 080503], we find that entanglement sudden birth (ESB) of the two reservoirs occurs without certain symmetry with respect to the entanglement sudden death (ESD) of the two qubits. A phenomenological interpretation of entanglement revival is also given.     

Thermal and phase decoherence effects on entanglement dynamics of the quantum spin systems

Entanglement dynamics of the N-qubit XY model in thermal and dephasing environments are investigated by solving the Lindblad form of the master equation. Analytical solutions for the two-qubit case and numerical solutions for the multi-qubit case are obtained. For the two-qubit case, our results revealed two main features for entanglement evolution from different initial states. First, the thermal reservoir always induces degradation of the entanglement, and the entanglement may undergo sudden death during certain intervals of the evolution time. Second, the dephasing environment induces damped oscillation of the entanglement for initially separable states and mixed states with relative large values of Δ or J; however, it always induces exponentially decay of the entanglement for the initial Bell states. For the multi-qubit case, our results show that the entanglement decreases monotonically as the time evolves for the initial W state, and behaves as damped oscillation for the initial “one-particle” state. Particularly, for system with large number of qubits, the curves of the concurrence C₁₂ with different N are almost overlapped in dephasing environment.     

The entanglement dynamics of two coupled qubits in different environment

In this Letter, the entanglement dynamics of two interacting qubits in a common bath and in two independent baths, at finite and zero temperature are investigated. Entanglement sudden death (ESD) and entanglement sudden birth (ESB) are observed when the two qubits are embedded in two independent baths at finite temperature. At zero temperature, the entanglement of the two qubits may evolve to a steady state with non-zero value when the two qubits are embedded in a common bath, the entanglement sudden birth does not occur when the qubits are embedded in two independent baths.     

Unconditional continuous variable entanglement cascading

We demonstrate that the Gaussian entanglement cascading can be realized unconditionally for any nonzero squeezing in all of the entangled sources. We obtain the upper bound of the Gaussian cascaded entanglement, which cannot be exceeded by any entanglement cascading protocol based on Gaussian local operations and classical communications. We then propose an unconditional entanglement cascading protocol that can produce the cascaded entanglement reaching the upper bound. The protocol, as a generalization of the standard continuous variable teleportation, includes two steps: (i) each repeater site locally implements the Bell measurement and classically sends the results to Bob; (ii) Bob optimally displaces his own mode based on these results.     

Control of the entanglement of a two-level atom in a dissipative cavity via a classical field

We investigate the entanglement dynamics and purity of a two-level atom, which is additionally driven by a classical field, interacting with a coherent field in a dissipative environment. It is shown that the amount of entanglement and the purity of the system can be improved by controlling the classical field.     

Entanglement dynamics for two interacting spins

We study the dynamical generation of entanglement for a very simple system: a pair of interacting spins, s₁ and s₂, in a constant magnetic field. Two different situations are considered: (a) s₁ → ∞, s₂ = 1/2 and (b) s₁ = s₂ → ∞, corresponding, respectively, to a quantum degree of freedom coupled to a semiclassical one (a qubit in contact with an environment) and a fully semiclassical system, which in this case displays chaotic behavior. Relations between quantum entanglement and classical dynamics are investigated.     

Entanglement between two atoms in two distant cavities connected by an optical fiber beyond strong fiber-cavity coupling

This paper investigates entanglement between two atoms in two distant cavities, which are connected by an optical fiber. We give an exact expression of the evolution of the whole system, and study the entanglement between the two atoms. We find that even the fiber-cavity coupling constant is smaller than the atom-cavity coupling constant, high degree entanglement between the two atoms is obtainable. This result gives a new prospect for experimental realization.     

Tripartite Entanglement via Microwave Driven Atomic Coherence

We propose a new scheme to achieve the tripartite entanglement based on the standard criteria [Phys. Rev. A 67（2003） 052315] in a inverse-tripod atomic system. In our scheme, the atomic coherence is introduced by two microwave fields which drive the upper three levels of atom. By numerically simulating the dynamics of system, we investigate the generation and evolution of entanglement in the presence of atom and cavity decay. As a result, the present research provides an efficient approach to achieve fully tripartite entanglement with different frequencies and initial states for each entangled mode, which may have impact on the progress of multicolored multi-notes quantum information networks.     

Entanglement dynamics of a three-qubit system interacting with a quantum spin environment

We investigate the entanglement dynamics and decoherence of a three-qubit system under a quantum spin environment at a finite temperature in the thermodynamics limit. For the case under study, we find the evolution of pairwise entanglement depends not only on the initial states but also on the parameters related to the system and the spin environment. In addition, an undesirable entanglement sudden death occurs in the process of entanglement evolution, and this effect can be controlled by the coupling constant between two qubits, external magnetic field, and the interaction between the system and the environment.     

Monogamy and entanglement in tripartite quantum states

We present an interesting monogamy equation for (2⊗2⊗n)-dimensional pure states, by which a quantity is found to characterize the tripartite entanglement with the GHZ type and W type entanglements as a whole. In particular, we, for the first time, reveals that for any quantum state of a pair of qubits, the difference between the two remarkable entanglement measures, concurrence and negativity, characterizes the W type entanglement of tripartite pure states with the two-qubit state as reduced density.     

Entanglement dynamics in Coriolis coupled rovibrational states of formaldehyde

The entanglement dynamics of two vibrational modes of a polyatomic molecule coupled by Coriolis interaction to overall molecular rotation is studied in terms of two negativities, N(t) and N_s(t), respectively, defined by the minimum of the eigenvalues and by the sum of the negative eigenvalues of the partial transpose of a density matrix. Various initial states are the products of Dicke states and the products of coherent states of vibrations and rotations. Formaldehyde is taken as an example, and the von Neumann entropy s(t) is simulated for the comparison with both negativities. It is shown that negativity N_s(t) is positively correlated with entropy s(t), and the correlated behavior between negativity N(t) and entropy s(t) strongly depends on initial states. However, these three indicators of entanglement display a dominantly positive correlation for the coherent states with small or large parameters. In addition, for the latter state two quantities N(t) and s(t) are nearly unchanged for a long time. This time can be further increased by the increasing of vibrational quantum number so that molecular information processing and quantum computing is allowed. These results are useful in quantum information theory.     

Quantum entanglement in a molecular magnet with itinerant electrons

We study the behaviors of pairwise and multipartite entanglement in a molecular magnet with itinerant electrons. In different ground states, the ratio of pairwise to multipartite entanglement is different. The monogamy of quantum entanglement is shown. Both charge correlation and spin correlation play important roles in the entanglement. The entanglements are generally suppressed by the on-site repulsion U and are mainly determined by spin correlation for large U and by charge correlation for small U. At finite temperature, in general, the thermal fluctuation suppresses the entanglements. However, in some cases, the multipartite entanglement can be enhanced by increasing temperature. Comparing the Heisenberg model with the Hubbard model, it is found that thermal entanglement in the itinerant electron system is more robust because charge correlation can survive at much higher temperature than spin correlation.     

Impurity-assisted enhancement of entanglement in dissipative environments

We propose a scheme to enhance the entanglement of a dissipative system, and the enhancement is happened during certain intervals of the evolution time. Our method is obtained by introducing one impurity into a Heisenberg XY chain and then solving the Lindblad form of the master equation with different initial states. The dynamics of entanglement reveal that one can significantly enhance the entanglement. This result will be helpful for the generation and transportation of entanglement in real solid-state systems.     

Dynamical creation of entanglement versus disentanglement in a system of three-level atoms with vacuum-induced coherences

The dynamics of entanglement between three-level atoms coupled to the common vacuum is investigated. We show that the collective effects such as collective damping, dipole-dipole interaction and the cross coupling between orthogonal dipoles, play a crucial role in the process of creation of entanglement. In particular, the additional cross coupling enhances the production of entanglement. For the specific initial states we find that the effect of delayed sudden birth of entanglement, recently invented by Ficek and Tana? [Ficek, R. Tana?, Phys. Rev. A 77 (2008) 054301] in the case of two-level atoms, can also be observed in the system. When the initial state is entangled, the process of spontaneous emission causes destruction of correlations and its disentanglement. We show that the robustness of initial entanglement against the noise can be changed by local operations performed on the state.     

Non-local dynamics of Bell states in separate cavities

We present non-local dynamics of Bell states in separate cavities. It is demonstrated that (i) the entanglement damping speed will saturate when the cavity leakage rate γ?0.4; (ii) the synchronism relationship between the fidelity and the concurrence depends on the initial state; (iii) if the initial state is , the dynamics of entropy is opposite to that of fidelity.     

Entanglement in Weisskopf-Wigner theory of atomic decay in free space

In this paper, we use the Weisskopf-Wigner theory to study the entanglement in the state of the free-space radiation field produced from vacuum due to atomic decay. We show how bipartite entanglement is shared between different partitions of the radiation modes. We investigate the role played by the size of the partitions and their detuning with the decaying atom. The dynamics of the atom-field entanglement during the atomic decay is also briefly discussed. From this dynamics, we assert that such entanglement is the physical quantity that fix the statistical atomic decay time.