Quantum entanglement and quantum nonlocality for N-photon entangled states

Quantum entanglement and quantum nonlocality of N-photon entangled states |\psi_{N m}\rangle =C_m[\cos\gamma|N-m\rangle₁|m\rangle₂ +\e^{\i\θ_m}\sin\gamma|m\rangle₁|N-m\rangle₂] and their superpositions are studied. We point out that the relative phase θ_m affects the quantum nonlocality but not the quantum entanglement for the state |\psi_{N m}\rangle. We show that quantum nonlocality can be controlled and manipulated by adjusting the state parameters of |\psi_{N m}\rangle, superposition coefficients, and the azimuthal angles of the Bell operator. We also show that the violation of the Bell inequality can reach its maximal value under certain conditions. It is found that quantum superpositions based on |\psi_{N m}\rangle can increase the amount of entanglement, and give more ways to reach the maximal violation of the Bell inequality.     

Entanglement, Quantum Discord, and Non-locality in Bell-Diagonal States

Experimental approach to characterize the non-locality, entanglement, and quantum correlation of a multiparity quantum system is one of the important subjects in quantum information theory. Here, by investigating the violations of Bell inequality (BI), we analyze the relations among the non-locality, concurrence C, and quantum discord Q typically for a family of Bell-diagonal states. It is shown that, for the optimal measurement basis the BI is always violated, if the quantum discord is larger than 0.5031 and the concurrence is larger than 0.5605. Certainly, the BI is maximally violated for the maximal entanglement and quantum discord, i.e., C=Q=1. Our generic results are demonstrated with a thermal XY model of the two-qubit system with controllable interbit couplings.     

A Novel Scheme for Quantum Teleportation of N-Particle Generalized Bell-Type States Using Minimal Entanglement Resources

In this paper, we present an efficient scheme for the teleportation of an arbitrary N-particle generalized Bell-type state using a single Bell pair as entanglement channel. For odd and even N-particle states the scheme slightly differs and it is discussed in detail for the simplest cases, 3 and 4 particle states. Further, we have compared the quantum cost of our scheme with other teleportation schemes which use a single Bell pair as entanglement channel.

     

Quantum Dialogue Based on Entanglement Swapping and Hadamard Operation via Cavity QED

A quantum dialogue scheme based on entanglement swapping and Hadamard operation via cavity QED is proposed, which effectively uses the evolution law of atom in cavity QED. The proposed scheme uses Bell states and GHZ states as quantum resource and only needs single-atom measurements. The proposed scheme is secure against active attacks from an eavesdropper and can transmit 4 secret bits per round communication.

     

Multi-party Quantum Key Agreement Protocol with Authentication

Utilizing the advantage of quantum entanglement swapping, a multi-party quantum key agreement protocol with authentication is proposed. In this protocol, a semi-trusted third party is introduced, who prepares Bell states, and sends one particle to multiple participants respectively. After that the participants can share a Greenberger-Horne-Zeilinger state by entanglement swapping. Finally, these participants measure the particles in their hands and obtain an agreement key. Here, classical hash function and Hadamard operation are utilized to authenticate the identity of participants. The correlations of GHZ states ensure the security of the proposed protocol. To illustrated it detailly, the security of this protocol against common attacks is analyzed, which shows that the proposed protocol is secure in theory.

     

The Dynamics of Quantum Correlations Between Two Atoms in Two Coupled Cavities

The dynamics of the quantum correlation between two atoms in two single-mode cavities is studied. For the initial Bell state |Ψ ⁺〉, the quantum consonance is equal to the entanglement, and larger than quantum discord. For the initial Bell state |Φ ⁺〉, the quantum consonance is larger than entanglement, but not larger than quantum discord all the time. As the increase of the cavity-cavity coupling strength, the evolution period of quantum correlation becomes smaller. Consonance is not smooth at some points while cavities are coupled with each other.     

High-Dimensional Bell State Analysis for Photon-Atoms Hybrid System

High-dimensional Bell state analysis (HDBSA) has great application potential in the high-capacity quantum communication and quantum information processing. In this paper, we propose a scheme to completely distinguish the 2^N-dimensional Bell states of a hybrid system with the help of the nonlinear interaction between the Λ-type atoms and a photon system. We use the unit-probability quantum teleportation with non-maximum entanglement as an example to show the application of HDBSA. Finally, we discuss its possible realization with current experimental techniques. Our HDBSA protocol may pave a new way for high-capacity long-distance quantum communication.

     

The Dynamics of Quantum Entanglement and General Quantum Correlations in the Double Tavis-Cumming Model

We study the dynamics of quantum correlations involving entanglement and discord of two pairs of two-level atoms in cavity QED. In the model, two atoms A and C are coupled with a single-mode cavity field via Tavis-Cumming interaction at one location, and the same for B and D at another location. The two locations are connected by the entanglement of the atoms AB and CD while there are no any direct interactions between them. Through comparing the robustness of entanglement and discord of the atoms in various initial conditions of cavities, it is shown the discord is more robust than the entanglement and would be useful in quantum information technology.     

Intrinsic Relations of Bipartite Quantum Resources in Tripartite Systems

Quantum resources play crucial roles for displaying superiority in many quantum communication and computation tasks. To reveal the intrinsic relations hidden in these quantum resources, many efforts have been made in recent years. In this work, the correlations of the tripartite W‐type states based on bipartite quantum resources are investigated. The inter‐relations among the degree of coherence, concurrence, Bell nonlocality, and purity are presented. Considering Bell nonlocal and Bell local (satisfied the Clauser–Horne–Shimony–Holt inequality) states for the two‐qubit subsystems derived from the tripartite W‐type states, exact lower and upper boundaries of the degree of coherence versus concurrence are obtained. Interestingly, exact relation among the degree of coherence, concurrence, and purity is obtained. Moreover, coherence is also closely related to entanglement in two specific scenarios: the tripartite W‐type state under decoherence and a practical system for a renormalized spin‐1/2 Heisenberg model.     

Quantum Teleportation Under Different Collective Noise Environment

We investigate that the average fidelity of the standard quantum teleportation communication protocol when the quantum channel is affected by different local collective noise environments frequently encountered in real quantum communication protocol. We show that the quantum teleportation efficiency can be enhanced when the noise is unavoidable by choose the fit Bell state as the quantum channel, especially we can get perfect quantum teleportation efficiency under the local collective Pauli σ_y noise environment. Our work can shed some light on the application of practical standard quantum teleportation communication protocol.

     

Controllable Quantum Network Coding Scheme Based on Quantum Walk

Quantum entanglement is one of the key methods in quantum information processing, but it is difficult to prepare quantum entanglement. Quantum walk is widely used in quantum computation and quantum simulation, it can be applied to the preparation of quantum entangled states. In this paper, a controllable quantum network coding scheme based on quantum walk is proposed. With the help of quantum walk, the scheme preliminary realized the entanglement distribution of butterfly network, reduced entanglement resources and enhanced scalability. According to the existing technology, it is feasible to implement the quantum network coding scheme proposed in this paper.

     

Quantum correlations in non-Markovian environments

We have studied the analytical Markovian and non-Markovian dynamics of quantum correlations, such as entanglement, quantum discord and Bell nonlocalities for three noisy qubits. Quantum correlation as measured by quantum discord is found to be immune to death contrary to entanglement and Bell nonlocality for initial GHZ- or W-type mixed states.     

Tighter Weighted Polygamy Inequalities of Multipartite Entanglement in Arbitrary-Dimensional Quantum Systems

We investigate polygamy relations of multipartite entanglement in arbitrary-dimensional quantum systems. By improving an inequality and using the βth (0 ≤ β ≤ 1) power of entanglement of assistance, we provide a new class of weighted polygamy inequalities of multipartite entanglement in arbitrary-dimensional quantum systems. We show that these new polygamy relations are tighter than the ones given in Kim (Phys. Rev. A 97, 042332 2018).

     

Analyzing and Revising Quantum Dialogue Without Information Leakage Based on the Entanglement Swapping Between any Two Bell States and the Shared Secret Bell State

Information leakage in quantum dialogue (QD) or bidirectional quantum secure direct communication (BQSDC) was found ten years ago, but enough attention was not paid and even wrong conclusions were drawn. It is indeed necessary to emphasize the importance. Here, we find information leakage exists in the QD protocol based on entanglement swapping between any two Bell states and the shared secret Bell state. To be specific, half of the interchanged information is leaked out unconsciously. This is not allowed in a truly secure QD protocol. Afterward, this QD protocol is improved to the one without information leakage. Compared with the original one, there are some obvious advantages in the improved version.

     

Quantum Communication Scheme Using Non-symmetric Quantum Channel

A theoretical quantum communication scheme based on entanglement swapping and superdense coding is proposed with a 3-dimensional Bell state and 2-dimensional Bell state function as quantum channel. quantum key distribution and quantum secure direct communication can be simultaneously accomplished in the scheme. The scheme is secure and has high source capacity. At last, we generalize the quantum communication scheme to d-dimensional quantum channel.     

Quantum Dialogue by Using Non-Symmetric Quantum Channel

A protocol for quantum dialogue is proposed to exchange directly the communicator＇s secret messages by using a three-dimensional Bell state and a two-dimensional Bell state as quantum channel with quantum superdence coding, local collective unitary operations, and entanglement swapping. In this protocol, during the process of trans- mission of particles, the transmitted particles do not carry any secret messages and are transmitted only one time. The protocol has higher source capacity than protocols using symmetric two-dimensional states. The security is ensured by the unitary operations randomly performed on all checking groups before the particle sequence is transmitted and the application of entanglement swapping.     

Quantum Private Comparison Based on χ-Type Entangled States

A two-party quantum private comparison (QPC) protocol is constructed with χ-type entangled states in this paper. The proposed protocol employs a semi-honest third party (TP) that is allowed to misbehave on his own but cannot conspire with the adversary. The proposed protocol need perform Bell basis measurements and single-particle measurements but neither unitary operations nor quantum entanglement swapping technology. The proposed protocol possesses good security toward both the outside attack and the participant attack. TP only knows the comparison result of the private information from two parties in the proposed protocol.

     

Some Measurement-Based Characterizations of Separability of Bipartite States

Importance of quantum entanglement has been demonstrated in various applications. Usually, separability of a bipartite state is defined by its algebraic structure, i.e. a convex combination of product states. But it seems to be hard to check separability (equivalently, entanglement) of a state from its algebraic structure. In this note, we give some characterizations of separability of bipartite states based on POVM measurements. For bipartite pure states, we prove the separability, Bell locality, unsteerability and classical correlation are the same. As a consequence, every entangled pure bipartite state is always Bell nonlocal, steerable and quantum correlated.

     

Quantum Entanglement in Ground State of Extended Hubbard Model

Metal-insulator and CDW-SDW transitions are studied in the one-dimensional Extended Hubbard Model at half-filling by analysing the behaviour of local entanglement in fermionic systems. 1D traditional Hubbard model exhibits metal-insulator transition at critical point U_c = 0, where local entanglement reaches its maximum value. Moreover, a transition between charge- and spin-density- wave (CDW-SDW) occurs in 1D Extended Hubbard Model tUV with long-range interaction at straight line U = 2 V. The analysis of our obtained results shows that CDW-SDW transition has curious properties whose can be used in quantum information processing.

     

Quantum Entanglement and Teleportation in a Vertical Quantum Dot

We study the thermal entanglement and teleportation using quantum dot as the quantum channel. We firstly investigate the evolution of entanglement in the vertical quantum dot, then focus on the effects of the important parameters of the system on the teleported fidelity under different conditions. We obtain the critical temperature of suddenly dead entanglement. Based on Bell measurements in two subspaces, the isotropy and anisotropy subspaces, we can find that the anisotropy measurements always overmatch the isotropy ones. Moreover, we obtain the high-fidelity teleportation for quantum dot as quantum channel when the parameters are adjusted. The possible applications of quantum dot are expected in quantum teleportation