Tractable Quantification of Entanglement for Multipartite Pure States

We present kth-order entanglement measure and global kth-order entanglement measure for multipartite pure states, and extend Bennett＇s measure of partial entropy for bipartite pure states to a multipaxtite case. These measures are computable and can effectively classify and quantify the entanglement of multipartite pure states.     

Entanglement of Bell diagonal mixed states

A sufficient and necessary condition for separability of Bell diagonal mixed states for bipartite systems in higher dimensions is presented. Moreover, we present a necessary condition for genuine entanglement of Bell diagonal mixed states in higher dimensions for multipartite systems.     

Bipartite entanglement purification with neutral atoms

We theoretically study bipartite entanglement purification with neutral atoms via cavity-assistant interaction and linear optical elements. We focus on entanglement distillation and the recurrence protocol, whose performances under idealized and realistic conditions are discussed. The implementation of these purification protocols has been tested with numerical simulations. We analyze the performance and stability of all required operations and emphasize that all techniques are feasible with current experimental technology.     

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.     

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.     

Remote State Preparation with Genuine Multipartite Entanglement

Enlightened by the work of Yeo and Chua [Phys. Rev. Lett. 96 （2006） 060502] for teleportation and dense coding with genuine multipartite entanglement, we present an explicit protocol for faithful remote state preparation in a real coefficient case by using the same four-particle entangled state which is not reducible to pair of Bell states. It is shown that any complex coefficient case can be changed to a real coefficient case. With this protocol, the state can play an analogous role to Einstein-Podolsky-Rosen pairs in the theory of multipartite entanglement.     

Continuous-Variable Quantum Teleportation of Entangled Coherent States

We propose a quantum teleportation scheme for tripartite entangled coherent state （ECS） with continuous variable. Our scheme is feasible and economical in the sense that we need only linear optical devices such as beam splitters, phase shifters and photon detectors and employ three bipartite maximally ECSs as quantum channels. We also generalize the tripartite scheme into multipartite ease and calculate the minimum average fidelity for the schemes in tripartite and multipartite cases.     

Classification of 4-qubit Entangled Graph States According to Bipartite Entanglement,Multipartite Entanglement and Non-local Properties

We use concurrence to study bipartite entanglement, Meyer-Wallach measure and its generalizations to study multi-partite entanglement and MABK and SASA inequalities to study the non-local properties of the 4-qubit entangled graph states, quantitatively. Then, we present 3 classifications, each one in accordance with one of the aforementioned properties. We also observe that the classification according to multipartite entanglement does exactly coincide with that according to nonlocal properties, but does not match with that according to bipartite entanglement. This observation signifies the fact that non-locality and multipartite entanglement enjoy the same basic underlying principles, while bipartite entanglement may not reveal the non-locality issue in its entirety.     

Quantum information and entanglement transfer for qutrits

We propose a scheme for the transfer of quantum information among distant qutrits. We apply this scheme to the distribution of entanglement of qutrits states among distant nodes and to the generation of multipartite antisymmetric states. We also discuss applications to quantum secret sharing.     

Multipartite entanglement dynamics and decoherence from a quantum-critical environment

We investigate the entanglement dynamics and decoherence of a multipartite system under an environment which can exhibit a quantum phase transition. Our result implies that the entanglement evolution depends not only on the size of the system and the quantum states of concern but also on the environment. In the sense of the linear entropy to measure decoherence induced by the environment, the decoherence-free subspaces have been identified for our model.     

Generation of Multipartite Ionic Entangled States only by Single-Qubit Measurements via Linear Optical Elements

We propose a scheme for generation of multipartite ionic Greenberger-Horne-Zeilinger （GHZ） states only by single-qubit measurements. Our scheme not only does not need joint measurements but also avoids the difficulty of synchronizing the arrival time of the two scattered photons, which is faced by the previous schemes. Therefore our entanglement generation scheme can be implemented more easily than the schemes based on atomic interference in experiment.     

Bohr's complementarity relation and the violation of CP symmetry in the neutral kaon system

We test Bohr's complementary relation, for single and bipartite neutral kaons. They present a system that is oscillating, decaying and breaks the CP symmetry. We discuss the effect of the CP violation and moreover show that the quantity that complements the single partite information for bipartite kaons is an entanglement measure. Which is independent of CP violation, while nonlocality is sensitive to CP violation.     

General Properties of Thermal Entanglement in an Arbitrary-Length Heisenberg Spin Chain

We investigate general properties of thermal entanglement in arbitrary-length 1D Helsenberg spin-1/2 chain based on classifications of its eigenstates. The influences of magnetic field and temperature on entanglement are qualitatively discussed and three features are presented. The conclusions hold for both bipartite and multipartite entanglement, and are in agreement with the results numerically proven by Arnesen et al. [Phys. Rev. Lett. 50 （2001） 017901].     

Entanglement transfer from photon-subtracted and photon-added two-mode squeezed fields to a pair of qubits

We investigate the entanglement transfer from a bipartite non-Gaussian continuous-variable (CV) system to a pair of localized qubits. The non-Gaussian state is obtained by the de-Gaussification process involving subtracting photons from or adding photons to a Gaussian field. It is shown that such de-Gaussification process can improve the entanglement transfer.     

Entanglement evolution in multipartite cavity-reservoir systems under local unitary operations

We analyze the entanglement evolution of two cavity photons being affected by the dissipation of two individual reservoirs. Under an arbitrary local unitary operation on the initial state, it is shown that there is only one parameter which changes the entanglement dynamics. For the bipartite subsystems, we show that the entanglement of the cavity photons is correlated with that of the reservoirs, although the local operation can delay the time at which the photon entanglement disappears and advance the time at which the reservoir entanglement appears. Furthermore, via a new defined four-qubit entanglement measure and two three-qubit entanglement measures, we study the multipartite entanglement evolution in the composite system, which allows us to analyze quantitatively both bipartite and multipartite entanglement within a unified framework. In addition, we also discuss the entanglement evolution with an arbitrary initial state.     

Multiparty Quantum Secret Sharing of Quantum States with Quantum Registers

A quantum secret sharing scheme is proposed by making use of quantum registers. In the proposed scheme, secret message state is encoded into multipartite entangled states. Several identical multi-particle entanglement states are generated and each particle of the entanglement state is filled in different quantum registers which act as shares of the secret message. Two modes, i.e. the detecting mode and the message mode, are employed so that the eavesdropping can be detected easily and the secret message may be recovered. The security analysis shows that the proposed scheme is secure against eavesdropping of eavesdropper and cheating of participants.     

Robust and efficient generator of almost maximal multipartite entanglement

Quantum chaotic maps can efficiently generate pseudorandom states carrying almost maximal multipartite entanglement, as characterized by the probability distribution of bipartite entanglement between all possible bipartitions of the system. We show that such multipartite entanglement is robust, in the sense that, when realistic noise is considered, distillable entanglement of bipartitions remains almost maximal up to a noise strength that drops only polynomially with the number of qubits.     

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.     

The geometry of bipartite qutrits including bound entanglement

We investigate the state space of bipartite qutrits, in particular we construct an analog to the “magic” tetrahedron for bipartite qubits—a magic simplex W. Due to the high symmetry it is enough to consider certain typical slices through W and optimal entanglement witnesses can be calculated. A whole region of bound entangled states is found.