Generation of Entangled States of Multiple Superconducting Quantum Interference Devices in Cavity

We propose a scheme for generating the maximally entangled states of many superconducting quantum interference devices (SQUIDs) by using a quantized cavity field and classicalmicrowave pulses in cavity. In the scheme,the maximally entangled states can be generated without requiring the measurement and individual addressing of the SQUIDs.     

Cooperative Communications via Dual-Teleportation with Non-maximally Entanglement Measurements

We investigate a framework of the cooperative quantum teleportation (CQT) based on non-maximally entangled state basis (NB) measurements, instead of maximally entangled state basis (MB) measurements. It is implemented with two consecutive conventional (or direct) quantum telportations (DQT), where unknown quantum states can be transmitted in a point-to-point fashion. The security is based on the quantum-mechanical impossibility of local unitary transformations between non-maximally entangled states. It shows that the CQT can enhance the successful transmissions by self-correcting the errors introduced in the dual-teleportations.     

Efficient generation of maximally entangled states via four-wave mixing in a semiconductor quantum-dot nanostructure

We investigate the generation of the maximally entangled state of two weak-light pulses (the probe and generated pulses) via four-wave mixing (FWM) in a semiconductor quantum dot (SQD) with a biexciton-exciton cascade configuration. The results show that this maximally entangled state can propagate with an ultraslow group velocity under suitable parameter conditions. For application, our proposed scheme is probably achievable with the present technology by applying the standard GaAs/InGaAs self-assemble quantum dots (QDs). Furthermore, our calculations provide a guideline for the realization of the maximally entangled state in the SQD solid-state system, which can be much more practical than that in an atomic system because of its flexible design and the wide tunable parameters.     

Quantum Entanglement of Many Distant Bose-Einstein Condensates in an Optical Lattice by Interference

We propose a scheme to generate maximally entangled states of two distant Bose-Einstein condensates, which are trapped in different potential wells of a one-dimensional optical lattice. We show how such maximally entangled state can be used to test the Bell inequality and realize quantum teleportation of a Bose-Einstein condensate state. The scheme proposed here is based on the interference of Bose-Einstein condensates leaking out from different potential wells of optical lattice. It is briefly pointed out that this scheme can be extended to generate maximally entangled Greenberger-Horne-Zeilinger (GHZ) states of 2m (m >1) distant Bose-Einstein condensates.     

Optimal and covariant single-copy LOCC transformation between two two-qubit states

Given two two-qubit pure states characterized by their Schmidt numbers we investigate an optimal strategy to convert the states between themselves with respect to their local unitary invariance. We discuss the efficiency of this transformation and its connection to LOCC convertibility properties between two single-copy quantum states. As an illustration of the investigated transformations we present a communication protocol where in spite of all expectations a shared maximally entangled pair between two participants is the worst quantum resource.     

Projection of two biphoton qutrits onto a maximally entangled state

Bell state measurements, in which two quantum bits are projected onto a maximally entangled state, are an essential component of quantum information science. We propose and experimentally demonstrate the projection of two quantum systems with three states (qutrits) onto a generalized maximally entangled state. Each qutrit is represented by the polarization of a pair of indistinguishable photons-a biphoton. The projection is a joint measurement on both biphotons using standard linear optics elements. This demonstration enables the realization of quantum information protocols with qutrits, such as teleportation and entanglement swapping.     

Entanglement Dynamics of Two Qubits Coupled to a Noise Environment

We study the time evolution of two two-state systems （two qubits） initially in the pure entangled states or the maximally entangled mixed states interacting with the individual environmental noise. It is shown that due to environment noise, all quantum entangled states are very fragile and become a classical mixed state in a short-time limit. But the environment can affect entanglement in very different ways. The type of decoherence process for certain entangled states belongs to amplitude damping, while the others belong to dephasing decoherenee.     

纠缠薛定谔猫态的非局域性及其在热库中的演化

利用Bell-CHSH(Clauser-Horne-Shimony-Holt）不等式研究了两种纠缠的光学薛定猫态的量子非定域性及其在真空热库中的演化。计算表明，最大纠缠的薛定谔猫态具有最大非定域性。在真空热库中随着时间的演化，两种纠缠态的量子非定域性逐渐减弱直至消失。     

三粒子纠缠Ｗ态的隐形传态

提出利用三个二粒子纠缠态作为量子信道，实现三粒子纠缠Ｗ态的隐形传态的方案.首先考察量子信道是最大纠缠态的情形，然后进一步考察量子信道是非最大纠缠态的情形.发现在量子信道为非最大纠缠态时，通过引进一个辅助粒子，并构造一个幺正变换矩阵，即可以一定的概率完成三粒子纠缠Ｗ态的隐形传态. 关键词： 隐形传态 三粒子纠缠 纠缠Ｗ态 非最大纠缠量子信道     

Generation and characterization of Werner states and maximally entangled mixed states by a universal source of entanglement

We present a novel technique for generating two-photon polarization mixed states of any structure, which is based on the peculiar spatial characteristics of a high brilliance source of entangled pairs. Werner states and maximally entangled mixed states, two well-known families of mixed states important for quantum information, have been created and fully characterized by this technique. We have also investigated and tested the nonlocal properties of these states.     

Witness for Non-Quasi Maximally Entangled States

Maximally entangled states, defined as those states that have the maximal entanglement of formation under some entanglement measure, are the ideal resource for many quantum missions. In this paper, we call a convex roof of maximally entangled pure states a quasi maximally entangled state. First, we present the concept of a witness for non-quasi maximally entangled states, which is an observable that can distinguish some non-quasi maximally entangled states from quasi maximally entangled ones. Then we prove that every non-quasi maximally entangled state can be witnessed by a witness and obtain some necessary and sufficient conditions for an observable to be a witness for non-quasi maximally entangled states. Lastly, we give some classes of Hermitian operators, which can become witnesses. Especially, we compute non-quasi maximally entangled states that can be detected by a specific product operator.     

New form Simple Structure of Maximally Eight-qubit Entanglement State

In a recent paper (Zha et al. Laser Phys. Lett. 10, 045201, 2013), presented a criterion of maximally mutiqubit entangled states (MMES). Though there are several known examples of maximally entangled quantum states of two, three, five and six qubits, the mathematical structure for multi-qubit entanglement of more than seven qubits is less clear. With an emphasis on eight qubits, by this criterion, two new forms of maximally eight-qubit MMES is obtained, where the subsystems of 1, 2, and 3-qubits are all completely mixed. We believe that the new form eight-qubit maximally entangled state can play an important role in quantum information.     

纠缠相干态的纠缠浓缩

利用线性光学元器件对光场量子态进行操纵,可以实现远程的量子纠缠调控和量子通讯.通过分析光学分束器对相干态光场的作用,发现当初始光场态是两个两部分纠缠态的直乘时,让其中的两模通过光学分束器作用后再对其进行光子计数,另外两模将会塌缩到新的纠缠态.基于这个特点,提出了一个实现部分纠缠相干态纠缠浓缩的方案.在这个方案中,两个部分纠缠相干态被用来作为量子信道,通过光学分束器作用后对光场进行光子数探测时,如果测量到光场的两模分别处于奇光子数态和零光子数态,则光场另外的两模将塌缩到最大纠缠态,从而完成纠缠浓缩的过程.计算结果表明,对于纠缠相干态,无论其初始的纠缠是多么微弱,利用这种方法总有一定的几率可以从中提纯出最大纠缠态.     

Quantum Measurement of Two-Qubit System in Damping Noise Environment

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-ShimonyHolt(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.     

Remote State Preparation of a Two-Atom Entangled State in Cavity QED

A physical scheme for remotely preparing a diatomic entangled state based on the cavity QED technique is presented in this paper. The quantum channel is composed of a two-atom entangled state and a three-atom entangled W state. The non-resonant interaction between two atoms and cavity is utilized at sender’s side to distribute the information among the quantum channel, and the original state can be transmitted to either one of the two receivers. It shows that an extra cavity and an atom are needed at the final receiver’s side as an auxiliary system if the non-maximally entangled states are worked as the quantum channel. The total success probabilities for the two receivers are not equal to each other except that the states of the quantum channel are maximally entangled.     

Teleportation of Mixture of Diagonal Bell States via Bound Entangled States

We propose a protocol for teleportation of arbitrary mixture of diagonal Bell states, it is shown that the channel can be constructed with either pure maximally entangled states or mixed bound entangled states. We also present protocols to realize the controlled teleportation of mixture of diagonal Bell states via multi-particle mixed states. Our results show that bound entangled states are also important and useful resources in quantum communication tasks.     

Multiparty quantum secret conference based on quantum encryption with pure entangled states

We present a scheme for multiparty quantum remote secret conference (MQRSC) with pure entangled states, not maximally entangled multipartite quantum systems. The conferees first share a private quantum key, a sequence of pure entangled states and then use them to encode and decode the secret messages. The conferees exploit the decoy-photon technique to ensure the security of the transmission of qubits. This MQRSC scheme is more feasible and efficient than others.     

Average Fidelity of Teleportation in Quantum Noise Channel

The effects of amplitude damping in quantum noise channels on average fidelity of quantum teleportation are analyzed in Bloch sphere representation for every stage of teleportation. When the quantum channels are varied from maximally entangled states to non-maximally entangled states, it is found that the effects of noise channels on the fidelity are nearly equivalent to each other for strong quantum noise. The degree of damage on the fidelity of non-maximally entangled channels is smaller than that of maximally entangled channels. The average fidelity of values larger than 2/3 may be one representation indirectly showing how much the unavoidable quantum noise is.     

From Quantum State Targeting to Bell Inequalities

Quantum state targeting is a quantum game which results from combining traditional quantum state estimation with additional classical information. We consider a particular version of the game and show how it can be played with maximally entangled states. The optimal solution of the game is used to derive a Bell inequality for two entangled qutrits. We argue that the nice properties of the inequality are direct consequences of the method of construction.     

Non-local implementation of single-qubit rotation operation

We present two optimal schemes for non-local inplementing a single-qubit rotation operation via a maximally entangled quantum channel. We report on the quantitative relations between the quantum action,entangled and classical communication resources required in the implementation. We also put forward two schemes for conclusive implementing the non-local quantum single-qubit rotation via a partially entangled quantum channel. Both these methods can appropriately be referred to as qubit-assisted processes.