Quantum state transfer via a two-qubit Heisenberg XXZ spin model

Transfer of quantum states through a two-qubit Heisenberg XXZ spin model with a nonuniform magnetic field b is investigated by means of quantum theory. The influences of b, the spin exchange coupling J and the effective transfer time T=Jt on the fidelity have been studied for some different initial states. Results show that fidelity of the transferred state is determined not only by J, T and b but also by the initial state of this quantum system. Ideal information transfer can be realized for some kinds of initial states. We also found that the interactions of the z-component Jz and uniform magnetic field B do not have any contribution to the fidelity. These results may be useful for quantum information processing.     

Effect of Dzyaloshinskill-Moriya anisotropic antisymmetric interaction and intrinsic decoherence on entanglement teleportation

We study the effect of the Dzyaloshinskill-Moriya anisotropic antisymmetric interaction (DM interaction) on entanglement teleportation in a two-qubit Heisenberg model with intrinsic decoherence taken into account. For some initial states of the channel, the DM interaction and intrinsic decoherence have no effect on the teleported entanglement and fidelity. While for other one, the DM interaction and intrinsic decoherence have a pronounced effect.     

Thermal entanglement in the anisotropic Heisenberg XYZ model with different inhomogeneous magnetic fields

Thermal entanglement is investigated in a two-qubit Heisenberg XYZ system with different inhomogeneous magnetic fields. It is found that different magnetic fields have different entanglement and critical values. In addition, according to the relation of spin-spin coupling coefficients, a more efficient control parameter of magnetic field can be obtained by adjusting the direction of external magnetic field.     

Pairwise entanglement of a three-qubit Heisenberg XY chain in a nonuniform magnetic field with intrinsic decoherence

Taking into account the intrinsic decoherence, the concurrence of the nearest and the next-to-nearest neighbor qubits in a three-qubit Heisenberg XY chain are investigated when a nonuniform magnetic field is included. We show that the effects of the external magnetic field, including the uniform and inhomogeneous magnetic fields, on the time evolution of entanglement between the nearest and the next-to-nearest neighbor qubits rely deeply on the initial states. We can moderate the destructive effect of intrinsic decoherence by controlling the uniform and inhomogeneous magnetic fields, so that a proper value of uniform and inhomogeneous magnetic fields can, to a great extent enhance the stationary entanglement.     

Interplay between the Dzyaloshinskii–Moriya anisotropic antisymmetric interaction and the SWAP operation in a two-qubit Heisenberg model

The SWAP operation in a two-qubit Heisenberg model in the presence of Dzyaloshinskii–Moriya (DM) anisotropic antisymmetric interaction is investigated. It is shown that the SWAP operation can be implemented for some kinds of DM coupling and the influence of DM couplings is divided into different cases. The conditions of the DM coupling under which the SWAP operation is feasible are established.     

Thermodynamics of the anisotropic spin-1/2 Heisenberg chain and related quantum chains

The free energy and correlation lengths of the spin-1/2XYZ chain are studied at finite temperature. We use the quantum transfer matrix approach and derive non-linear integral equations for all eigenvalues. Analytic results are presented for the low-temperature asymptotics, in particular for the criticalXXZ chain in an external magnetic field. These results are compared to predictions by conformal field theory. The integral equations are solved numerically for the non-criticalXXZ chain and the related spin-1 biquadratic chain at arbitrary temperature.Work performed within the research program of the Sonderforschungsbereich 341, Köln-Aachen-Jülich     

Thermal Entanglement in the Anisotropic Heisenberg XYZ Model with Different Dzyaloshinskii-Moriya Couplings

Thermal entanglement is investigated in a two-qubit Heisenberg XYZ system with different Dzyaloshinskii Moriya （DM） couplings. It is shown that different DM interaction parameters have different influences on the entanglement and the critical temperature. In addition, according to the relation between the real coupling coefficients Jx and Jz, a more efficient DM control parameter can be obtained by adjusting the direction of DM interaction.     

Effects of Dzyaloshinski-Moriya anisoyropic antisymmetric interaction on entanglement and teleportation in a two-qubit Heisenberg chain with intrinsic decoherence

We studied the effects of Dzyaloshinski-Moriya (DM) anisoyropic antisymmetric interaction on entanglement and teleportation in a two-qubit Heisenberg chain with intrinsic decoherence taken into account. The main result is the systematic analysis of the negativity and fully entangled fraction’s evolution as a function of DM interaction D and time t. The contrast between the case of the maximally entangled initial state and unentangled ones is presented.     

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.     

Boundary Quantum Entanglement of the XXZ Spin Chain with Boundary Impurities

The boundary quantum entanglement for the s = 1/2 X X Z spin chain with boundary impurities is studied via the density matrix renormalization group （DMRG） method. It is shown that the entanglement entropy of the boundary bond （the impurity and the chain spin next to it） behaves differently in different phases. The relationship between the singular points of the boundary entropy and boundary quantum critical points is discussed.     

Sudden Death, Birth and Stable Entanglement in a Two-Qubit Heisenberg XY Spin Chain

Taking the decoherence effect due to population relaxation into account, we investigate the entanglement properties for two qubits in the Heisenberg XY interaction and subject to an external magnetic field. It is found that the phenomenon of entanglement sudden death (ESD) as well as sudden birth (ESB) appear during the evolution process for particular initial states. The influence of the external magnetic field and the spin environment on ESD and ESB are addressed in detail. It is shown that the concurrence, a measure of entanglement, can be controlled by tuning the parameters of the spin chain, such as the anisotropic parameter, external magnetic field, and the coupling strength with their environment. In particular, we find that a critical anisotropy constant exists, above which ESB vanishes while ESD appears. It is also notable that stable entanglement, which is independent of different initial states of the qubits, occurs even in the presence of decoherence.     

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].     

Thermal Entanglement of Anisotropic XY Chains in a Transverse Field

By using the concept of concurrence, we numerically investigate the thermal entanglement between any two nearest-neighbour spins in uniform and periodic anisotropic XY chains in a transverse field at finite temperature T. It is found that the entanglement has more than one critical temperatures on some parameter regions for uniform and periodic chains. We also discuss the behaviour of the thermal entanglement at the vicinity of quantum phase transition of periodic anisotropic XY chains and find that a11 the derivatives aλC have similar behaviour as that of the uniform chain.     

Multiparty Quantum Remote Secret Conference

We present two schemes for multiparty quantum remote secret conference in which each legitimate conferee can read out securely the secret message announced by another, but a vicious eavesdropper can get nothing about it. The first one is based on the same key shared efficiently and securely by all the parties with Greenberger-Horne- Zeilinger （GHZ） states, and each conferee sends his secret message to the others with one-time pad crypto-system. The other one is based on quantum encryption with a quantum key~ a sequence of GHZ states shared among all the conferees and used repeatedly after confirming their security. Both these schemes are optimal as their intrinsic efficiency for qubits approaches the maximal value.     

Cryptanalysis of Multiparty Quantum Secret Sharing of Quantum State Using Entangled States

Security of a quantum secret sharing of quantum state protocol proposed by Guo et al. [Chin. Phys. Lett. 25 （2008） 16] is reexamined. It is shown that an eavesdropper can obtain some of the transmitted secret information by monitoring the classical channel or the entire secret by intercepting the quantum states, and moreover, the eavesdropper can even maliciously replace the secret message with an arbitrary message without being detected. Finally, the deep reasons why an eavesdropper can attack this protocol are discussed and the modified protocol is presented to amend the security loopholes.     

Entanglement in a spin-one spin chain

The thermal entanglement in a two-spin-qutrit system with two spins coupled by exchange interaction is investigated in terms of the measure of entanglement called ‘negativity’. We strictly show that for any temperature the entanglement is symmetric with respect to zero magnetic field. The behavior of negativity is presented for four different cases. We find that the entanglement may be enhanced under a nonuniform magnetic field. Because there is not any necessary and sufficient condition for quantum separability in systems of dimension 3⊗3, our results are qualitative, not quantitative.     

The dynamics of a central spin in quantum Heisenberg XY model

In the thermodynamic limit, we present an exact calculation of the time dynamics of a central spin coupling with its environment at finite temperatures. The interactions belong to the Heisenberg XY type. The case of an environment with finite number of spins is also discussed. To get the reduced density matrix, we use a novel operator technique which is mathematically simple and physically clear, and allows us to treat systems and environments that could all be strongly coupled mutually and internally. The expectation value of the central spin and the von Neumann entropy are obtained.     

A New Quantum Secure Direct Communication Protocol Using Decoherence-Free Subspace

A new quantum secure direct communication （QSDC） protocol is proposed by using decoherence free subspace （DFS） to avoid insecurity of the present QSDC protocols in a quantum noise channel. This protocol makes it easily for Bob and Alice to find eavesdropping in channel because the collective dephasing noise disappears in DFS. The probability of successful attack by Eve in this protocol is smaller than in BB84 protocol. Thus this protocol realizes secure QSDC and is feasible with present-day technology.     

Threshold Quantum Secret Sharing of Secure Direct Communication

We propose a （L, n）-threshold quantum secret sharing protocol of secure direct communication following some ideas of Zhang＇s protocol [Phys. Lett. A 342 （2005） 60] and Tokunaga et al.＇s protocol [Phys. Rev. A 71 （2005） 012314]. The sender distributes the classical secret shares to his or her n agents and each agent owns a secret share in advance. The sender＇s secure direct communication message can be extracted by an agent subset by collaboration in such a way that at least t or more agents can obtain the secret message with the mutual assistances but any t - 1 or fewer agents cannot. In contrast to the previous multiparty quantum secret sharing protocols in which the sender＇s secret message can be recovered only if all the agents collaborate, our protocol is more practical and more flexible.     

Secure Quantum Secret Sharing Based on Reusable GHZ States as Secure Carriers

We show that a potential eavesdropper can eavesdrop whole secret information when the legitimate users use secure carrier to encode and decode classical information repeatedly in the protocol proposed by Bagherinezhad S and Karimipour V [Phys. Rev. A 67（2003）044302]. Then we present a revised quantum secret sharing protocol by using the Greenberger-Horne-Zeilinger state as secure carrier. Our protocol can resist Eve＇s attack.