Branching spin chain dynamics

We study the dynamics of various branched spin chain systems. In such systems entanglement can be generated and distributed, providing an essential resource for teleportation or distributed quantum processing. We show in detail how simple operations can be employed at chosen times to change the subsequent dynamics of the branched spin chains, rendering the distributed entanglement more accessible.     

A Scheme for Atomic Entangled States and Quantum Gate Operations in Cavity QED

We propose a scheme for controllably entangling the ground states of five-state W-type atoms confined in a cavity and realizing swap gate and phase gate operations. In this scheme the cavity is only virtually excited and the atomic excited states are almost not occupied, so the produced entangled states and quantum logic operations are very robust against the cavity decay and atomic spontaneous emission.     

Maximizing Thermal Entanglement of XY Model Using Site-Dependent Magnetic Fields in Specific Directions

We study the thermal entanglement by means of concurrence in a two-qubit isotropic XY model in the presence of site-dependent external magnetic fields in arbitrary directions. We find that at a given temperature and magnetic field strength, the mirror symmetry of the two fields about the x-y plane is a necessary condition for maximum entanglement. However, if there is no constraint on the field strengths, then the necessary condition for maximum entanglement reduces to the configuration that the two fields are vertical, anti-parallel and with the same strength. We also investigate the anisotropic XY model and find that the above conclusion more or less holds.     

Quantification of continuous variable entanglement with only two types of simple measurements

Here we propose an experimental set-up in which it is possible to obtain the entanglement of a two-mode Gaussian state, be it pure or mixed, using only simple linear optical measurement devices. After a proper unitary manipulation of the two-mode Gaussian state only number and purity measurements of just one of the modes suffice to give us a complete and exact knowledge of the state’s entanglement.     

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.     

Optimal dense coding with thermal entangled states

We study optimal dense coding with thermal entangled states of a two-qubit Heisenberg XX chain and a two-qutrit system. For a two-qubit Heisenberg XX chain, the dense coding capacity is a function of temperature and external magnetic field. Only in the case of an external magnetic field being less than the coupling constant, the optimal dense coding can be realized with thermal entangled states. For a two-qutrit system, we consider the dense coding capacity taking into account of nonlinear coupling constant and an external magnetic field. We find that the nonlinear coupling constant must be less than 0 for dense coding. For the two models, we give the conditions that the parameters of the models have to satisfy a valid dense coding.     

Deterministic exact teleportation via two partially entangled pairs of particles

We present an explicit protocol for deterministic exact teleportation via two partially entangled pairs of particles. The protocol consists of a local generalized measurement described by a positive operator-valued measure, one-way classical communication, and a corresponding local unitary operation. We find the required generalized measurement, which can experimentally be realized by performing a unitary operation in the extended space and a conventional orthogonal measurement. A simple protocol for deterministic entanglement concentration is also obtained.     

Thermal entanglement of a two-qutrit XX spin chain with Dzialoshinski-Moriya interaction

The effect of Dzialoshinski-Moriya (DM) interaction on thermal entanglement of a two-qutrit XX spin chain in a homogenous magnetic field is investigated. Our results imply that DM interaction and the magnetic field play competing roles in enhancing thermal entanglement.     

Optical Bell state and Greenberger-Horne-Zeilinger-state analyzers through the cavity input-output process

With the ancillary one-sided cavities each trapping an alkali atom, the schemes for the analyzers of two-photon Bell states and three-photon Greenberger-Horne-Zeilinger (GHZ) states are proposed, respectively. Moreover, all of two-photon Bell states and three-photon GHZ states can be nondestructively distinguished. The influence of atomic spontaneous emission and output coupling inefficiency are discussed.     

Fermion to boson mapping

The enveloping algebra, D_n, of fermions is extended on the lattice to include the discrete space invariance. This extended algebra, denoted X, has the space symmetry as a factor: X/D_n = space group.     

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.     

On the partial trace over collective spin degrees of freedom

We derive analytical properties for the degeneracy ν(N,j) occurring in the decomposition of the state space C2⊗N. We also investigate the dynamics of two qubits coupled via Ising interactions to separate spin baths, and we study the thermodynamic limit.     

Reexamining the security of the improved quantum secret sharing scheme

We find that, in the improvement [S.J. Qin et al., Phys. Lett. A 357 (2006) 101] of the multiparty quantum secret sharing [Z.J. Zhang et al., Phys. Rev. A 71 (2005) 044301], Charlie can solely obtain Alice’s secret messages without Bob’s helps. In other words, the improved secret sharing scheme is still insecure. In the end, we further modify Qin et al. improved three-party quantum secret sharing scheme and make it really secure.     

Remote preparation of a class of three-qubit states

We propose a remote state preparation (RSP) scheme of a three-particle Greenberger-Horne-Zeilinger (GHZ) class state by using three 2-qubit maximally entangled states as the quantum channel. The success probability of preparation and classical communication cost are calculated. Generally speaking, conditioned on Alice’s classical message, with probability 1/4 Bob can successfully prepare the original state by performing one appropriate unitary operation. However, for four kinds of special states, the success probability of preparation can be increased to 1/2 or even 1 after consuming some extra bits.     

Generation of Multi-Photon Cluster States through the Cavity Input--Output Process

We propose a scheme to generate the multi-photon cluster states via the cavity input-output process and the single-bit rotations. The method can be generalized to construct a series of multi-photon graph states, and the successful probability is close to unity in the ideal condition.     

Preparation of Entanglement of Atoms and of Photons via Cavity Input-Output Process

We propose a method to prepare multipartite entangled states such as cluster states and graph states based on the cavity input-output process and single photon measurement. Two quantum gates, a controlled phase gate and a fusion gate between two atoms trapped in respective cavities, are proposed to prepare atomic cluster states and graph states with one and two dimensions. We also introduce a scheme that can generate an arbitrary multipartite photon duster state which uses two coherent states as a qubit basis.     

Multiparty secret sharing of quantum information via cavity QED

The scheme on multiparty secret sharing of an atomic quantum state information via entanglement swapping in cavity QED [Y.Q. Zhang, X.R. Jin, S. Zhang, Phys. Lett. A 341 (2005) 380] is revisited and accordingly an improved version is proposed. The possible decoherence effect in the original version can be avoided after revision of assuming the prior distribution of entanglement. Moreover, the success probability of teleportation has been raised from 6.25% in the original version to 100% in the present version.     

Isotopically engineered Si and Ge for spintronics and quantum computation

Si, Ge as well as SiGe structures are the promising materials for spintronics and quantum computation due to the fact that in both crystals only one isotope (²⁹Si and ⁷³Ge) has nuclear spin. As a result, isotope engineering of Si and Ge permits to control the density of nuclear spins and vary the spin coherence time, a crucial parameter in spintronics. In the first part we discuss the NMR study of nuclear spin decoherence in Ge single crystals with different abundance of the ⁷³Ge isotope. It was observed that the slow component of the dephasing process is elongated with depletion of Ge crystal with isotope ⁷³Ge. The second part is devoted to the development of the Kane's model of nuclear spin-based quantum computer, which uses the nuclear spin of ³¹P impurity atoms in a ²⁸Si matrix as quantum bits (qubits). We discuss a new method of placing ³¹P atoms in a ²⁸Si based on neutron-transmutation-doping of isotopically engineered Si and Ge. In the proposed structure, interqubit coupling is due to indirect hyperfine interaction of ³¹P nuclear spins with electrons localized in a ²⁸Si quasi-one-dimensional nanowire, which allows one to control the coupling between distant qubits.