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We investigate the effects of the directions of Dzyaloshinskii—Moriya (DM) interaction vector and magnetic field on the quantum discord in the pure DM model. For different directions of DM vector, we find that there are different optimal parameter components of magnetic field. Moreover, we find that the optimal parameter components rules are the same for the Hamiltonian H1 and H2. According to the rules, for a certain axial DM vector, we can get the maximal quantum discord by adjusting the direction of the external magnetic field, which is feasible under the current experimental technology.

It is known that the inevitable interaction of the entangled qubits with their environments may result in the degradation of quantum correlation. We study the decoherence of two remote qubits under general local single- and two-sided amplitude-damping channel (ADC). By using concurrence, quantum discord and Clauser–Horne–Shimony–Holt (CHSH) inequality, we find that the relation between the residual quantum correlations and the initial ones are different. Recently, Wang et al. [Int. J. Theor. Phys. 54 (2015) 5] showed that there exist a set of partially entangled states that are more robust than maximally entangled states in terms of the residual quantum correlation measured by concurrence, fully entangled fraction and quantum discord, respectively. Here we find that both in single- and two-sided ADC, only the evolution of CHSH inequality with the initial parameter is proportional to that of the initial nonlocality. That means the initial state with maximally nonlocality will retain its role in the evolution. It implies that the evolution of nonlocality may reveal the characteristics of quantum state better. Furthermore, we discuss the evolutions of the three different quantum measurements with the initial parameter under generalized amplitude damping channel (GADC) and find that they are all proportional to that of the initial state.

Excitation energy transfer (EET) plays a vital role in many areas of physics and biology processes. Here we address the role of quantum-jump-based feedback control in the efficiency of EET through a chain model. Usually, the decoherence caused by dissipative noise is detrimental to the transfer efficiency. We demonstrate that feedback control can always enhance the efficiency of EET and the dependence of different feedback controls is also discussed in detail. In addition, we investigate the strategy to enhance the efficiency of EET in the Fenna–Matthews–Olson complex as a prototype for larger photosynthetic energy transfer systems.

We study the Bell-nonlocality dynamics of three remote atoms, two of which are trapped in one single-mode cavity and the third atom is trapped in another remote single-mode cavity. The interactions between the atoms and the cavity modes are studied via Tavis Cummings and Jaynes Cummings models. Here, the two single-mode cavities are introduced to simulate two different enviroments of the three atoms. The tripartite nonlocal correlations are studied in terms of the Svetlichny inequality and the WWZB inequality, respectively. The results show that the tripartite Bell-nonlocality sudden death will occur for the W state and GHZ state initial conditions. The detailed results demonstrate that the tripartite nonlocality of GHZ state is more robust than that of W state when suffering from the effect of environments.

In this paper, we present the explicit transformations of the optimal 1 → 3, 4, 5 phase-covariant cloning in 3 dimensions. The cloning fidelities are covered by the theoretical bounds of the optimal 1 → 3k, 3k + 1, 3k + 2 phase-covariant cloning of qutrits, where k ≥ 1 is the integral [Phys. Rev. A 67 (2003) 042306].

We propose an improved architecture for the physical implementation of two-qubit operations between arbitrary pairs of qubits by current biasing of coupled Josephson junctions. This improvement is necessary for the realization of a functional and scalable quantum computer. The proposed architecture is within the reach of current experimental technology. Secondly, we investigate a simple scheme for generating the graph states via the improved architecture. Since the controlled-phase gate between arbitrary pairs of qubits can be implemented easily, we can generate any multi-qubit graph states corresponding to various shape graphs, which meet the expectations of various quantum information processing schemes. It is a simple, scalable and feasible scheme for the generation of various graph states based on the current-controlled superconducting charge qubit network.

On the basis of generations of 1-dimensional and 2-dimensional graph states, we generate a 3-dimensional N3-qubit graph state based on the Josephson charge qubits. Since any two charge qubits can be selectively and effectively coupled by a common inductance, the controlled phase transform between any two-qubit can be performed. Accordingly, we can generate arbitrary multi-qubit graph states corresponding to arbitrary shape graph, which meet the expectations of various quantum information processing schemes. All the devices in the scheme are well within the current technology. It is a simple, scalable and feasible scheme for the generation of various graph states based on the Josephson charge qubits.

A scheme of teleportation of a tripartite state via W state is suggested. The W state serves as quantum channels. Standard Bell-state measurements and Von Neumann measurements are performed. After the sender operates the measurements and informs the receiver her results, he can reconstruct the original state by the corresponding unitary transformation. The probability of the successful teleportation is also obtained.

We present a deterministic scheme for generating large scale GHZ states in a cavity-quantum dot system. A singly charged quantum dot is embedded in a double-sided optical microcavity with partially reflective top and bottom mirrors. The GHZ-type Bell spin state can be created and two n-spin GHZ states can be perfectly fused to a 2n-spin GHZ state with the help of n ancilla single-photon pulses. The implementation of the current scheme only depends on the photon detection and its need not to operate multi-qubit gates and multi-qubit measurements. Discussions about the effect of the cavity loss, side leakage and exciton cavity coupling strength for the fidelity of generated states show that the fidelity can remain high enough by controlling system parameters. So the current scheme is simple and feasible in experiment.

Single qubit pure state is a fundamental resource in quantum information and quantum computation. Therefore, it is of great importance to protect the coherence of single qubits against decoherence. In this letter, we demonstrate that decoherence caused by spontaneous emission can be effectively suppressed by adding a universal static external field. In order to have an intuitive view to the protection effects and its physical mechanisms, we study the coherence evolution of a single qubit on a Bloch sphere. We can clearly see that different external resonant drivings can rotate the Bloch vector around different axes, and the steady-state solution of the master equation (under protection) are visualized on the Bloch sphere. Furthermore, the frequency detuning between the qubit system and the driving is taken into account, and the results show that our protection scheme still works fine in the detuned cases and the smaller the detuning is, the better the protection effect is. In addition, this protocol can protect the coherence of single qubit states with a wide range of driving parameters, and help people to design simple coherence protection schemes for qubit states. The simplicity and the abundance of the current scheme may warrant its experimental realization.

Quantum Zeno effect with mixed initial state is studied here. Frequent projective measurements performed on a bipartite joint pure state system will result in the quantum Zeno effect on the subsystem of interest. This shows the existence of Quantum Zeno effect in the system with mixed initial states.

We present two schemes for teleporting an unknown two-particle entangled state from a sender (Alice) to a receiver (Bob) via a four-particle entangled cluster state. As it is shown, the unknown two-particle entangled state can be teleported perfectly, and the successful possibilities and fidelities of the two schemes both reach unit., six pages, no figure

We investigate the effects of pure Dzyaloshinskii–Moriya (DM) interaction with magnetic field on entanglement in intrinsic decoherence, assuming that the system is initially in four Bell states and , respectively. It is found that if the system is initially in the state , the entanglement can obtain its maximum when the DM interaction vector D is in the plane of XOZ and magnetic field B = B y with the infinite time t, moreover the entanglement is independent of By and t when B y is perpendicular to D. In addition, we obtain similar results when the system is initially in the states or . However, we find that if the system is initially in the state , the entanglement can obtain its maximum for infinite t, when the DM vector is in the plane of YOZ, XOZ, or XOY, with the magnetic field parallel to X, Y, or Z axis, respectively. Moreover, when the axial B is perpendicular to D for the initial state ρ4(0), the negativity oscillates with time t and reaches a stable value, the larger the value of B is, the greater the stable value is, and the shorter the oscillation time of the negativity is. Thus we can adjust the direction and value of the external magnetic field to obtain the maximal entanglement, and avoid the adverse effects of external environment in some initial state. This is feasible within the current experimental technology.

The purity and fidelity of quantum operations are of great importance in characterizing the quality of quantum operations. The currently available definitions of the purity and fidelity of quantum operations are based on the average over all possible input pure quantum states, i.e. they are state-dependent (SD). In this paper, without resorting to quantum states, we define the state-independent (SI) purity and fidelity of a general quantum operation (evolution) in virtue of a new density matrix formalism for quantum operations, which is extended from the quantum state level to quantum operation level. The SI purity and fidelity gain more intrinsic physical properties of quantum operations than state-dependent ones, such as the purity of a one-qubit amplitude damping channel (with damping rate 1) is 1/2, which is in line with the fact that the channel is still a nonunitary operation described by two Kraus operators rather than a unitary one. But the state-dependent Haar average purity is 1 in this case. So the SI purity and fidelity proposed here can help the experimentalists to exactly quantify the implementation quality of an operation. As a byproduct, a new measure of the operator entanglement is proposed for a quantum evolution (unitary or nonunitary) in terms of the linear entropy of its density matrix on the orthonormal operator bases (OOBs) in Hilbert–Schmidt space.

An effective scheme is proposed for preparing large-scale maximally entangled W states by fusing arbitrary-size polarization entangled W or W-like states via polarization-dependent beam splitter (PDBS). In our scheme, W m+n−1 state can be generated from an n-qubit W state and an m-qubit W state, and W m+n+t−2 state can be generated from W m , W n and W t states. The fusion process can start with Bell states too. Because PDBS has selective transmission rates for different polarization states, our fusion has a distinct property, i.e. a large-scale maximally entangled W state can be generated by fusing small-size W-like states, and the cost analyses indicate that our fusion scheme is more efficient than the previous one in fusing W-like states. In addition, there is no need for any controlled quantum gate and any ancillary photon in our scheme, so it is simple and feasible.

In this letter we propose a scheme of entanglement concentration via entanglement swapping in the quantum dot (QD) plus double-sided cavity system with the help of an input photon. With our scheme, bipartite maximally entangled states among two spatially separated particles can be generated without direct interaction and initial entanglement. Moreover, four-particle maximally entangled states can be created too. Contrary to previous work investigating entanglement concentration in QD systems, the non-maximally entangled state to be concentrated can be unknown for us; furthermore, the complicated Bell-state measurement of entanglement swapping is replaced by single-photon detection, which will greatly reduce the realisation complexity of the current scheme.

We investigate the dynamics of entanglement in the excitation transfer through a chain of interacting molecules. In the case of two-molecule coupled to noisy environments we show that entanglement can be further enhanced if the distance between the molecules is oscillating. Our results demonstrate that motional effect plays a constructive role on quantum entanglement in the dynamics of excitation transfer. This mechanism might provide useful guideline for designing artificial systems to battle against decoherence., Comment: 5 pages, 5 figures

We study the entanglement dynamics between two spatially separated atoms trapped in two separate optical cavities. Based on cavity-assisted interactions between the atoms and separated photonic pulses, we propose a scheme for the implemention of a controlled-phase-flip gate (CPF gate) between each of the atoms and the photonic pulse to protect the remote atomic quantum entanglement against the decaying caused by spontaneous emission. What we need to do is to shoot the horizontally polarized photons onto the cavity mirror successively, plus a train of Hadamard operations on the atoms, and no measurement is needed here. It is shown that the quantum entanglement of the two remote atoms can be protected effectively in this way. We also extend our scheme to the case of weak coupling and low Q cavity cases. The simplicity of the current scheme may warrant its experimental realization.

We investigate the effects of the directions of Dzyaloshinskii-Moriya (DM) interaction and magnetic field on the thermal entanglement in the pure DM model. It is found that when the Hamiltonian is H1 = D (σ1 ⊗ σ2)+ B σ1, the entanglement can reach its maximum if the directions of the magnetic field and the DM vector are parallel. In addition, when the Hamiltonian is H2 = D (σ1 ⊗ σ2) + B (σ1 + σ2), if the directions of the magnetic field and the DM vector are perpendicular in a high magnetic field, or their directions are parallel in a weak magnetic field, the entanglement can also reach its maximum. Thus the entanglement can be enhanced by adjusting the direction of the external magnetic field, and this is feasible within the current experimental technology.

In this paper, we derive the explicit transformations of the optimal 1 → 3, 4, 5 phase-covariant cloning in three dimensions, and then generalize them to the cases of 1 → M = 3n, 3n + 1, 3n + 2 (n ≥ 1 integer) cloning. The clone fidelities are coincident with the theoretical bounds found.

The thermal entanglement in the spin-S Heisenberg XYZ model is studied in detail by using the entanglement measure of negativity. The effects of spin on the thermal entanglement, the threshold temperature, the critical uniform external magnetic field, the nonuniform external magnetic field and the entanglement extremum are discussed, respectively. It is shown that with increasing spin, the entanglement will increase, and then decrease slowly. In addition, we find that entanglement will approach a constant Nc with the increase of DM interaction, the constant increases with the increase of spin, and both the threshold temperature Tc and critical uniform external magnetic field Bc will increase with the increasing spin. Thus high-spin system can inhibit the influence of the external environment better.

Probabilistic quantum cloning (PQC) cannot copy a set of linearly dependent quantum states. In this paper, we show that if incorrect copies are allowed to be produced, linearly dependent quantum states may also be cloned by the PQC. By exploiting this kind of PQC to clone a special set of three linearly dependent quantum states, we derive the upper bound of the maximum confidence measure of a set. An explicit transformation of the maximum confidence measure is presented.