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We propose a scheme for teleporting unknown atomic entangled states in cavity QED. With the assistance of a strong classical field, the photon number dependent parts in the effective Hamiltonian are canceled. Thus, the scheme is insensitive to both the cavity decay and the thermal field. In addition, our scheme does not require the Bell-state measurement directly and the success probability can reach 1.0 in our scheme.

A teleportation scheme for unknown atomic states is proposed in cavity QED. The Bell state measurement is not needed in the teleportation process, and the success probability can reach 1.0. In addition, the current scheme is insensitive to the cavity decay and thermal field.

We propose a physical scheme for teleporting an unknown atomic state in cavity QED. Our scheme does not involve the Bell-state measurement. The most distinctive advantage of our scheme is that teleportation and distillation procedure can be realized simultaneously. In addition, the current scheme is insensitive to the cavity decay.

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.

A system comprising Lambda-type three level atoms and the two-mode Fock cavity field is considered in this paper. Under the adiabatical approximation and the large detuning condition, the effective Hamiltonian of the system in the interaction picture can be given out. If the two identical three level atoms pass through the cavity in turn, the entangled state atoms can be generated. When the interaction time is taken to an appropriate value, the maximally entangled states are created. On the same time, the tangles for the multipartite of the system are studied in detail.

In this paper a novel scheme is proposed where with the help of Victor (a state preparer) an unknown EPR pair is copied in a certain probability so that three parties of communication can obtain the EPR state, respectively. The first stage of this scheme is to teleport an unknown EPR state to Carla (one receiver). The second stage of this scheme, Alice (a sender) and Bob (the other receiver) get the same EPR pair or an orthogonal-complement copy of the EPR pair with the help of Victor.

We propose schemes to implement the 1 → 2 phase-covariant quantum cloning machine for input state in the x–y equator or x–z equator.

In this paper, we proposed a physical scheme to concentrate the non-maximally entangled atomic pure states via cavity QED by using atomic collision in a far-off-resonant cavity. The most distinctive advantage of our scheme is that there is no excitation of cavity mode during the distillation procedure. Therefore the requirement on the quality of cavity is greatly loosened., 15 pages, 1 figure

We show that the Hadamard and unitary gates could be implemented by a unitary evolution together with a measurement for any unknown state chosen from a set A = { | Ψ i 〉 , | Ψ ¯ i 〉 } ( i = 1 , 2 ) if and only if | Ψ 1 〉 , | Ψ 2 〉 , | Ψ ¯ 1 〉 , | Ψ ¯ 2 〉 are linearly independent. We also derive the best transformation efficiencies.

A quantum state-dependent quantum deleting machine is constructed. We obtain a upper bound of the global fidelity on N -to- M quantum deleting from a set of K non-orthogonal states. Quantum networks are constructed for the above state-dependent quantum deleting machine when K =2. Our deleting protocol only involves a unitary interaction among the initial copies, with no ancilla. We also present some analogies between quantum cloning and deleting.

In this paper, we propose two physical schemes, which can teleport unknown atomic state from user A (Alice) to user B (Bob) using the W class state as quantum channel. The two schemes are all based on Cavity QED techniques. The fidelity of the teleported state in the two schemes is equal to 1.0, and the successful probability of the schemes is decided by the smaller superposition coefficient of the state used as quantum channel. Each of the schemes has its own advantages.

In this Letter, we present a scheme for quantum teleportation and entanglement purification based on cavity QED if the two parties share a pair of atoms in a non-maximally entangled state. This scheme can be extended to the case of multi-atom entanglement purification.

The behavior of a two-level entangled atom in an optical field is studied in this paper. The interaction of an optical field and one of the entangled atoms is analyzed in detail. A general solution to the Schrodinger equation about the motion of the entangled atom is obtained. Next, the dynamic properties of the two-atom entangled system are arrived at. The properties of the action are dependent on the initial state of the atom. After detecting the entangled atom out of the field, we can obtain the state of the other atom moving in the field. It is shown that the state of the atom out of the field influences the dynamic properties of the split-levels of the atom in the field.

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.

Remote entanglement will inevitably decrease due to the interactions between quantum systems and their environments. Therefore protecting remote entanglement against decoherence is of great importance in realizing quantum communication and quantum comp utation. In this paper, we demonstrate that decoherence caused by weak-measurement-induced damping can be effectively suppressed by adding local unitary operation series on each qubit. The results show that the entanglement of the output state can approach that of the state before amplitude damping. The most distinct advantage of this entanglement protection scheme is that any unitary operation (except the identity operation) has this entanglement reversal effect on the amplitude-damped states. Furthermore, in each local unitary operation series, all the operations can be different from one other, and all the time intervals between any two adjacent operations can be different too. In addition, there is no need for the two remote users to synchronize their operations. Unlike most of the previous schemes, we do not assume the instantaneous local unitary operation, and each operation has a duration. All these advantages suggest that this remote entanglement protection scheme is much simpler and feasible than the previous ones, and we hope it can be implemented in the near future. Recently, Y. S. Kim et al. [Nat. Phys. 8, 117 (2012)] pointed out that ``pre-weak measurement $+$ amplitude damping $+$ bit-flipping operation $+$ post-weak measurement'' can actively combat specific decoherence. By combing our proposal with Kim et al.'s scheme, the protection performance can be greatly improved by replacing the bit-flipping operation with an arbitrary rotational operation around the $x$ axis, the number of the operations as well as the interval between any two adjacent operations all can be different, and the application range of the scheme can be greatly broadened.

In this work, we study how to distill two typical multipartite entangled noisy states, the amplitude-damped W state and the amplitude-damped Greenberger-Horne-Zeilinger state. Unlike the currently existing multipartite entanglement purification schemes, our scheme is a single-copy-based scheme, i.e., only one noisy state is needed in each distillation round, and the complicated multilateral controlled-not (cnot) operations are replaced by measurements of local positive-operator-valued measures (POVMs). The results show that local POVM measurements on one copy of the noisy state can project the initial state onto a new state with higher entanglement and nonlocality. In this scheme, although the final state can violate the multipartite Bell inequality, the corresponding initial noisy state does not violate it, which means that this distillation scheme can reveal the hidden genuine multipartite nonlocality of these initial states. For the case of the amplitude-damped W state, if the POVM parameters are appropriately chosen, the fidelity of the output state can approach $1.0$ for all the initial states with fidelity bigger than zero. So this single-copy-based multipartite entanglement distillation scheme has a wide range of distillable fidelities, and it is much simpler than the two-copy- or multiple-copy-based schemes. Furthermore, if the POVM measurement is carried out only on some member qubits rather than all of them, the distillation scheme can succeed too, which will greatly decrease the implementation complexity of the scheme. So this multipartite entanglement distillation scheme may be implementable in some physical system.

We present a scheme for directly measuring the concurrence of atomic two-qubit states (including pure states and mixed states) by detecting the photons leaking from the cavities where the atoms are located. We found that it is possible to encode the concurrence of two-atom entangled states in the photon detection results on the cavities. Cavity decay usually plays a passive role in quantum information processing, but in our scheme the decay of cavity mode plays a constructive role, and the state detection for trapped atoms were replaced by the photon detection. Our scheme is more implementable and may be feasible within the current technology.

We propose a scheme for bipartite electron-spin entanglement concentration via a double-quantum-dot system. The current scheme only requires spin-parity measurements and single-qubit measurements combined with one additional qubit rather than the routine Bell state measurement. The spin-parity measurement will make the distinction of qubit parity nondestructively.

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 present three protocols for directly measuring the concurrence of two-photon polarization-entangled states, including pure states and mixed states. Two signal photon pairs in the same polarization-entangled states are needed in each detection round, and the signal photons will pass through polarization-independent beam splitters simultaneously. The concurrence of the photon pairs will be encoded in the total probability of picking up the balanced states in the two output modes of the beam splitter, which can be directly measured by single-photon detectors. The photon detection efficiency can be further enhanced with the help of the weak cross-Kerr nonlinearity, and the signal photons will not be absorbed. The third scheme using the bilateral nondemolition measurements can measure the concurrence of both pure and mixed states with high efficiency and high fidelity and is robust against the copy error of the state to be measured. The sophisticated controlled-not operation of the previous schemes is not required here, so our protocols are more feasible within the current experimental technology.

Two qubits in pure entangled states going through separate paths and interacting with their own individual environments will gradually lose their entanglement. Here we show that the entanglement change of a two-qubit state due to amplitude damping noises can be recovered by entanglement swapping. Some initial states can be asymptotically purified into maximally entangled states by iteratively using our protocol., Comment: 5 pages, 6 figures, any comments are welcome

An optical scheme for simulating nonlocality distillation is proposed in post-quantum regime. The nonlocal boxes are simulated by measurements on appropriately pre- and post-selected polarization entangled photon pairs, i.e. post-quantum nonlocality is simulated by exploiting fair-sampling loophole in a Bell test. Mod 2 addition on the outputs of two nonlocal boxes combined with pre- and post-selection operations constitutes the key operation of simulating nonlocality distillation. This scheme provides a possible tool for the experimental study on the nonlocality in post-quantum regime and the exact physical principle precisely distinguishing physically realizable correlations from nonphysical ones.

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.

We consider a model of two identical atoms coupled to a single-mode cavity. When in atom-field strong coupling regime, the entanglement of the two atoms with spontaneous emission should be investigated beyond rotating-wave approximation (RWA). In order to improve the entanglement of the two atoms, some typical feedback based on quantum-jump are attempted to impose on the atoms. The result of numerical simulations shows that an appropriate feedback control can improve the entanglement.

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.

Two known nonorthogonal quantum states can be cloned either deterministically or probabilistically. In this paper, we investigate quantum cloning by combining these two extreme cases, i.e., a trade-off between the copy fidelity and the success probability. For a special set of two known nonorthogonal quantum states, we start with an explicit unitary transformation and derive the copy fidelity as a function of the success probability. The result shows that the higher is the copy fidelity, the less is the success probability. This quantum cloning may have important applications in quantum cryptography.

Operator entanglement of two-qubit joint unitary operations is revisited. Schmidt number is an important attribute of a two-qubit unitary operation, and may have connection with the entanglement measure of the unitary operator. We found the entanglement measure of two-qubit unitary operators is classified by the Schmidt number of the unitary operators. The exact relation between the operator entanglement and the parameters of the unitary operator is clarified too., To appear in the Brazilian Journal of Physics

We investigate the geometric picture of the level surfaces of quantum entanglement and geometric measure of quantum discord (GMQD) of a class of X-states, respectively. This pictorial approach provides us a direct understanding of the structure of entanglement and GMQD. The dynamic evolution of GMQD under two typical kinds of quantum decoherence channels is also investigated. It is shown that there exists a class of initial states for which the GMQD is not destroyed by decoherence in a finite time interval. Furthermore, we establish a factorization law between the initial and final GMQD, which allows us to infer the evolution of entanglement under the influences of the environment., 10 pages, 4 figures, comments are welcome

We present a proposal for storing and retrieving a continuous-variable quadripartite polarization-entangled cluster state, using macroscopic atomic ensembles in a magnetic field. The Larmor precession of the atomic spins leads to a symmetry between the atomic canonical operators. In this scheme, each of the four spatially separated pulses passes twice through the respective ensemble in order to map the polarization-entangled cluster state onto the long-lived atomic ensembles. The stored state can then be retrieved by another four read-out pulses, each crossing the respective ensemble twice. By calculating the variances, we analyzed the fidelities of the storage and retrieval, and our scheme is feasible under realistic experimental conditions., Comment: 6 pages, 4 figures

We investigate the dynamics of quantum discord during the purification process. In the case of Werner states, it is shown that quantum discord is increased after a round of purification protocol. Furthermore, quantum mutual information and classical correlation is also increased during this process.Wealso give an analytic expression for a class of higher dimensional states which have additive quantum discord., Comment: 4 pages, 3 figures

Based on mutually unbiased measurements, an optimal tomographic scheme for the multiqutrit states is presented explicitly. Because the reconstruction process of states based on mutually unbiased states is free of information waste, we refer to our scheme as the optimal scheme. By optimal we mean that the number of the required conditional operations reaches the minimum in this tomographic scheme for the states of qutrit systems. Special attention will be paid to how those different mutually unbiased measurements are realized; that is, how to decompose each transformation that connects each mutually unbiased basis with the standard computational basis. It is found that all those transformations can be decomposed into several basic implementable single- and two-qutrit unitary operations. For the three-qutrit system, there exist five different mutually unbiased-bases structures with different entanglement properties, so we introduce the concept of physical complexity to minimize the number of nonlocal operations needed over the five different structures. This scheme is helpful for experimental scientists to realize the most economical reconstruction of quantum states in qutrit systems.

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 present a physical scheme for implementing the discrete quantum Fourier transform in a coupled semiconductor double quantum dot system. The main controlled-R gate operation can be decomposed into many simple and feasible unitary transformations. The current scheme would be a useful step towards the realization of complex quantum algorithms in the quantum dot system., 3 pages,2 figures

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.

The entanglement dynamics of two identical atoms coupled to a single-mode cavity is considered. Based on the reduced non-perturbative quantum master equation method, the entanglement evolution of the two atoms with spontaneous emission under the quantum-jump-based feedback control is investigated beyond the conventional rotating-wave approximation (RWA). The research of entanglement evolution beyond RWA is a significant problem because of its importance to the field of strong coupling. The result of numerical simulations shows that an appropriate feedback control can prolong the entanglement time and increase the entanglement amplitude. The effect of synchronous control on two atoms is superior to that of one control.

A scheme for implementing 2-qubit quantum controlled phase gate (QCPG) is proposed with two superconducting quantum interference devices (SQUIDs) in a cavity. The gate operations are realized within the two lower flux states of the SQUIDs by using a quantized cavity field and classical microwave pulses. Our scheme is achieved without any type of measurement, does not use the cavity mode as the data bus and only requires a very short resonant interaction of the SQUID-cavity system. As an application of the QCPG operation, we also propose a scheme for generating the cluster states of many SQUIDs., 4 pages, resumitted

We propose two schemes for the generation of the cluster states. One is based on cavity quantum electrodynamics (QED) techniques. The scheme only requires resonant interactions between two atoms and a single-mode cavity. The interaction time is very short, which is important in view of decoherence. Furthermore, we also discuss the cavity decay and atomic spontaneous emission case. The other is based on atomic ensembles. The scheme has inherent fault tolerance function and is robust to realistic noise and imperfections. All the facilities used in our schemes are well within the current technology., Comment: Complete rewite version, adding the main results of quant-ph/0511045. 7 pages and 3 figures

We present a physical scheme for entanglement concentration of unknown atomic entangled states via cavity decay. In the scheme, the atomic state is used as stationary qubit and photonic state as flying qubit, and a close maximally entangled state can be obtained from pairs of partially entangled states probabilistically., Comment: Three pages, Two figures

A scheme for the generation of the cluster states based on the Josephson charge qubit is proposed. The two-qubit generating case is first introduced, and then generalized to multi-qubit case. The scheme is simple and easily manipulated, because any two charge qubits can be selectively and effectively coupled by a common inductance. More manipulations can be realized before decoherence sets in. All the devices in the scheme are well within the current technology., Comment: 3 figures, 4 pages

Using the highly detuned interaction between three-level $\Lambda$-type atoms and coherent optical fields, we can realize the C-NOT gates from atoms to atoms, optical fields to optical fields, atoms to optical fields and optical fields to atoms. Based on the realization of the C-NOT gates we propose an entanglement purification scheme to purify a mixed entangled states of coherent optical fields. The simplicity of the current scheme makes it possible that it will be implemented in experiment in the near future., Comment: 5 pages, no figure

We investigate schemes for quantum secret sharing and quantum dense coding via tripartite entangled states. We present a scheme for sharing classical information via entanglement swapping using two tripartite entangled GHZ states. In order to throw light upon the security affairs of the quantum dense coding protocol, we also suggest a secure quantum dense coding scheme via W state in analogy with the theory of sharing information among involved users., Comment: 4 pages, no figure. A complete rewrritten vession, accepted for publication in Chinese Physics

Experimentally feasible scheme for teleportation of atomic entangled state via entanglement swapping is proposed in cavity quantum electrodynamics (QED) without joint Bell-state measurement (BSM). In the teleportation processes the interaction between atoms and a single-mode nonresonant cavity with the assistance of a strong classical driving field substitute the joint measurements. The discussion of the scheme indicates that it can be realized by current technologies., Comment: 5 pages, no figure

We propose a scheme for implementing quantum algorithms with resonant interactions. Our scheme only requires resonant interactions between two atoms and a cavity mode, which is simple and feasible. Moreover, the implementation would be an important step towards the fabrication of quantum computers in cavity QED system., Comment: 4 pages, 3 figures

We propose an entanglement swapping scheme in cavity QED. In the scheme, the previously used joint measurement is not needed. The entanglement swapping in our proposal is a non-post-selection one, i.e., after the swapping is done, the swapped entanglement is still there., Three pages, one figure

A scheme for teleporting an unknown two-particle entangled state via W class states is proposed. In this scheme, the W class entangled states are considered as quantum channels. It is shown that by means of optimal discrimination between two nonorthogonal quantum states, probabilistic teleportation of the two-particle entangled state can be achieved., Comment: 8 pages, no figures