Role of Covariance Correlation Tensor in Establishment of Criterion of Quantum Entanglement

This article discusses the role of covariance correlation tensor in the establishment of the criterion of quantum entanglement. It gives a simple example to show the powerfulness in the treatment of quantum dense coding,and illustrates the fact that this method also provides theoretical basis for establishing corresponding knotted pictures.     

Criterion of Quantum Entanglement and the Covariance Correlation Tensor in the Theory of Quantum Network

This article discusses the covariance correlation tensor (CCT) in quantum network theory for four Bell bases in detail. Furthermore, it gives the expression of the density operator in terms of CCT for a quantum network of three nodes, thus gives the criterion of entanglement for this case, i.e. the conditions of complete separability and partial separability for a given quantum state of three bodies. Finally it discusses the general case for the quantum network of m≥3 nodes.     

The Role of Entanglement in Quantum Measurement and Information Processing

The significance of the quantum feature of entanglement between physical systems is investigated in the context of quantum measurements. It is shown that, while there are measurement couplings that leave the object and probe systems nonentangled, no information transfer from object to probe can take place unless there is at least some intermittent period where the two systems are entangled.     

量子纠缠与经典关联在双光子关联成像中的作用

关联成像引起了人们广泛的兴趣,特别是,二阶关联成像是一种纯粹的量子效应还是也可能是一种经典效应还存在争议.本文分析了三种关联成像方案,其中前两种方案中用经典关联和量子纠缠的双光子光源都能成像,而第三种只能用量子纠缠的双光子光源才能成像.对比发现,纠缠双光子同时存在动量和坐标两个自由度上的关联,在三种成像方案中都可用作光源.而经典关联的双光子不具有动量和坐标的双重关联,而仅仅存在其中一种关联(经典关联),关联成像的方式也因此受到限制.     

Quantum Key Distribution Scheme Based on Dense Encoding in Entangled States

A quantum key distribution protocol, based on the quantum dense encoding in entangled states, is presented. In this protocol, we introduce an encoding process to encode two classical bits information into one of the four one-qubit unitary operations implemented by Alice and the Bell states measurement implemented by Bob in stead of direct measuring the previously shared Einstein-Podolsky-Rosen pairs by both of the distant parties, Alice and Bob. Considering the practical application we can get the conclusion that our protocol has some advantages. It not only simplifies the measurement which may induce potential errors, but also improves the effectively transmitted rate of the generated qubits by the raw key. Here we also discuss eavesdropping attacks against the scheme and the channel loss.     

General Solution for the Complete Separability of a Pure Quantum State with Real Coefficients for a Quantum Network of Any Nodes

By means of the criterion of entanglement in terms of the covariance correlation tensor in quantum network theory, this article discusses the general solution for the complete separability of the pure quantum state with real coefficients for a quantum network of any nodes.     

Separability of Mixed States of the Quantum Network of Three Nodes

This article discusses the complete separability and partial separability of the mixed states of quantumnetwork of three nodes by means of the criterion of entanglement in terms of the covariance correlation tensor in quantumnetwork theory.     

Separability of Pure States of the Quantum Network of Three Nodes

This article discusses the complete separability and partial separability of the pure states of the quantumnetwork of three nodes by means of the criterion of entanglement in terms of the covariance correlation tensor in quantumnetwork theory.     

General Solution for the Complete Separability of a Pure Quantum State with Real Coefficients for a Quantum Network of Any Nodes

By means of the criterion of entanglement in terms of the covariance correlation tensor in quantum network theory, this article discusses the generalsolution for the complete separability of the pure quantum state with real coefficients for a quantum network of any nodes.     

Separability of Pure States of the Quantum Network of Three Nodes

This article discusses the complete separability and partial separability of the pure states of the quantum network of three nodes by means of the criterion of entanglement in terms of the covariance correlation tensor in quantum network theory.     

Complete and Partial Separability Conditions of Mixed State for a Quantum Network of Any Nodes

By using the block division method in matrix calculus, this article successfully calculate the expectationvalues of the generating operators and the correlation tensors for quantum network of any nodes. Thence, by meansof the criterion of entanglement in terms of the covariance correlation tensor in quantum network theory, this articlediscusses the complete and partial separability conditions of the mixed quantum state for a quantum network of anynodes and judge the separability of a quantum state in the generalcase of any nodes for two examples.     

Separability of Mixed States of the Quantum Network of Three Nodes

This article discusses the complete separability and partial separability of the mixed states of quantum network of three nodes by means of the crJterlon of entanglement in terms of the covarJance correlation tensor in quantum network theory.     

Correlation and Information in Quantum Channels

Quantum and classical correlations in quantum channels are investigated by means of an entangled pure state and a separable state which is closest to an entangled pure state. The coherent information and the separable information are used to estimate how much correlation is transmitted through a quantum channel. As the examples, the linear dissipative channel of qubits and the quantum erasure channel are considered.     

Complete and Partial Separability Conditions of Mixed State for a Quantum Network of Any Nodes

By using the block division method in matrix calculus, this article successfully calculate the expectation values of the generating operators and the correlation tensors for quantum network of any nodes. Thence, by means of the criterion of entanglement in terms of the covariance correlation tensor in quantum network theory, this article discusses the complete and partial separability conditions of the mixed quantum state for a quantum network of any nodes and judge the separability of a quantum state in the general case of any nodes for two examples.     

Many-Agent Controlled Teleportation of Multi-qubit Quantum Information via Quantum Entanglement Swapping

We present a method to teleport multi-qubit quantum information in an easy way from a sender to a receiver via the control of many agents in a network. Only when all the agents collaborate with the quantum information receiver can the unknown states in the sender＇s qubits be fully reconstructed in the receiver＇s qubits. In our method, agents＇s control parameters are obtained via quantum entanglement swapping. As the realization of the many-agent controlled teleportation is concerned, compared to the recent method [G.P. Yang, et al., Phys. Rev. A 70 （2004） 022329], our present method considerably reduces the preparation difficulty of initial states and the identification difficulty of entangled states, moreover, it does not need local Hadamard operations and it is more feasible in technology.     

On the Entanglement Structure in Quantum Cloning

We study the entanglement properties of the output state of a universal cloning machine. We analyse in particular bipartite and tripartite entanglement of the clones, and discuss the classical limit of infinitely many output copies.     

Spatial Entanglement of Fermions in One-Dimensional Quantum Dots

The time-dependent quantum Monte Carlo method for fermions is introduced and applied in the calculation of the entanglement of electrons in one-dimensional quantum dots with several spin-polarized and spin-compensated electron configurations. The rich statistics of wave functions provided by this method allow one to build reduced density matrices for each electron, and to quantify the spatial entanglement using measures such as quantum entropy by treating the electrons as identical or distinguishable particles. Our results indicate that the spatial entanglement in parallel-spin configurations is rather small, and is determined mostly by the spatial quantum nonlocality introduced by the ground state. By contrast, in the spin-compensated case, the outermost opposite-spin electrons interact like bosons, which prevails their entanglement, while the inner-shell electrons remain largely at their Hartree–Fock geometry. Our findings are in close correspondence with the numerically exact results, wherever such comparison is possible.     

Many-Agent Controlled Teleportation of Multi-qubit Quantum Information via Quantum Entanglement Swapping

We present a method to teleport multi-qubit quantum information in an easy way from a sender to a receiver via the control of many agents in a network. Only when all the agents collaborate with the quantum information receiver can the unknown states in the sender‘s qubits be fully reconstructed in the receiver‘s qubits. In our method,agents‘s control parameters are obtained via quantum entanglement swapping. As the realization of the many-agent controlled teleportation is concerned, compared to the recent method [C.P. Yang, et al., Phys. Rev. A 70 (2004) 022329],our present method considerably reduces the preparation difficulty of initial states and the identification difficulty of entangled states, moreover, it does not need local Hadamard operations and it is more feasible in technology.     

Separability of Pure and Mixed States of the Quantum Network of Four Nodes

This article discusses the separability of the pure and mixed states of the quantum network of four nodesby means of the criterion of entanglement in terms of the covariance correlation tensor in quantum network theory.     

Continuous-Variable Entanglement in Tunnel-Coupled Double Quantum Wells

This paper investigates the generation and evolution of continuous-variable entanglement in an asymmetric coupled-quantum well （CQW） system. Our numerical results show that this CQW system can be regarded as a source of macroscopic entangled light over a wide range of initial states of the cavity field. This investigation can be used for achieving the macroscopic entangled light in the CQW solid-state medium, which is much more praeticaJ than that in an atomic medium because of its flexible design and the controllable interference strength.