Nonlinear entanglement witnesses based on continuous-variable local orthogonal observables for bipartite systems

In this paper, a nonlinear entanglement witness criterion based on continuous-variable local orthogonal observables for bipartite states is established, which is strictly stronger than the the linear entanglement witnesses criterion introduced by Zhang et al. (Phys. Rev. Lett. 111:190501, 2013). This criterion is particularly applied to two-mode Gaussian states yielding a criterion in terms of the covariance matrix. Comparison with CCNR criterion is discussed.     

Quantum correlations in a family of bipartite separable qubit states

Quantum correlations (QCs) in some separable states have been proposed as a key resource for certain quantum communication tasks and quantum computational models without entanglement. In this paper, a family of nine-parameter separable states, obtained from arbitrary mixture of two sets of bi-qubit product pure states, is considered. QCs in these separable states are studied analytically or numerically using four QC quantifiers, i.e., measurement-induced disturbance (Luo in Phys Rev A77:022301, 2008), ameliorated MID (Girolami et al. in J Phys A Math Theor 44:352002, 2011),quantum dissonance (DN) (Modi et al. in Phys Rev Lett 104:080501, 2010), and new quantum dissonance (Rulli in Phys Rev A 84:042109, 2011), respectively. First, an inherent symmetry in the concerned separable states is revealed, that is, any nine-parameter separable states concerned in this paper can be transformed to a three-parameter kernel state via some certain local unitary operation. Then, four different QC expressions are concretely derived with the four QC quantifiers. Furthermore, some comparative studies of the QCs are presented, discussed and analyzed, and some distinct features about them are exposed. We find that, in the framework of all the four QC quantifiers, the more mixed the original two pure product states, the bigger QCs the separable states own. Our results reveal some intrinsic features of QCs in separable systems in quantum information.     

Quantum key distribution using continuous-variable non-Gaussian states

In this work, we present a quantum key distribution protocol using continuous-variable non-Gaussian states, homodyne detection and post-selection. The employed signal states are the photon added then subtracted coherent states (PASCS) in which one photon is added and subsequently one photon is subtracted from the field. We analyze the performance of our protocol, compared with a coherent state-based protocol, for two different attacks that could be carried out by the eavesdropper (Eve). We calculate the secret key rate transmission in a lossy line for a superior channel (beam-splitter) attack, and we show that we may increase the secret key generation rate by using the non-Gaussian PASCS rather than coherent states. We also consider the simultaneous quadrature measurement (intercept-resend) attack, and we show that the efficiency of Eve’s attack is substantially reduced if PASCS are used as signal states.     

Quantum network dense coding via continuous-variable graph states

We present a dense coding network based on continuous-variable graph state along with its corresponding protocol. A scheme to distill bipartite entanglement between two arbitrary modes in a graph state is provided in order to realize the dense coding network. We also analyze the capacity of network dense coding and provide a method to calculate its maximum mutual information. As an application, we analyze the performance of dense coding in a square lattice graph state network. The result showed that the mutual information of the dense coding is not largely affected by the complexity of the network. We conclude that the performance of dense coding network is very optimistic.     

Quantum anonymous voting with unweighted continuous-variable graph states

Motivated by the revealing topological structures of continuous-variable graph state (CVGS), we investigate the design of quantum voting scheme, which has serious advantages over the conventional ones in terms of efficiency and graphicness. Three phases are included, i.e., the preparing phase, the voting phase and the counting phase, together with three parties, i.e., the voters, the tallyman and the ballot agency. Two major voting operations are performed on the yielded CVGS in the voting process, namely the local rotation transformation and the displacement operation. The voting information is carried by the CVGS established before hand, whose persistent entanglement is deployed to keep the privacy of votes and the anonymity of legal voters. For practical applications, two CVGS-based quantum ballots, i.e., comparative ballot and anonymous survey, are specially designed, followed by the extended ballot schemes for the binary-valued and multi-valued ballots under some constraints for the voting design. Security is ensured by entanglement of the CVGS, the voting operations and the laws of quantum mechanics. The proposed schemes can be implemented using the standard off-the-shelf components when compared to discrete-variable quantum voting schemes attributing to the characteristics of the CV-based quantum cryptography.     

Quantum network dialogue protocol based on continuous-variable GHZ states

Quantum dialogue network, as a considerable topic, promotes high efficiency and instantaneousness in quantum communication through simultaneously deducing the secret information over the quantum channel. A new quantum network dialogue protocol is proposed based on continuous-variable GHZ states. In the protocol, the quantum dialogue could be conducted simultaneously among multiple legitimate communication parties. The security of the proposed protocol is ensured by the correlation of continuous-variable GHZ entangled states and the decoy states inserted into the GHZ states in the randomly selected time slots. In addition, the proposed quantum network dialogue protocol with continuous-variable quantum states improves the communication efficiency with the perfect utilization of quantum bits greatly.     

A timed discrete-event abstraction of continuous-variable systems

The paper concerns continuous-variable systems whose state can only be observed through a quantizer, which indicates the time instant and the direction of the change of the quantized state value. These changes are considered as events. The problem is to find a concise representation of the quantized system, which allows to determine for a given initial event all timed event sequences that the quantized system can generate. Since the quantized system does not possess the Markov property, the modelling aim is to find a semi-Markov process which describes a superset of the event sequences of the quantized system. The paper shows how such a semi-Markov process can be obtained for a given quantized system. The model provides information about the occurrence time of the events and about the probability of the occurrence. The modelling problem and its solution are illustrated by considering a quantized oscillator.     

Entanglement detection and distillation for arbitrary bipartite systems

We present an inequality for detecting entanglement and distillability of arbitrary dimensional bipartite systems. This inequality provides a sufficient condition of entanglement for bipartite mixed states, and a necessary and sufficient condition of entanglement for bipartite pure states. Moreover, the inequality also gives a necessary and sufficient condition for distillability.     

Violation of Bell inequalities for arbitrary-dimensional bipartite systems

In this paper, we consider the violation of Bell inequalities for quantum system \(\mathbb {C}^K\otimes \mathbb {C}^K\) (integer \(K\ge 2\)) with group theoretical method. For general M possible measurements, and each measurement with K outcomes, the Bell inequalities based on the choice of two orbits are derived. When the observables are much enough, the quantum bounds are only dependent on M and approximate to the classical bounds. Moreover, the corresponding nonlocal games with two different scenarios are analyzed.     

Quantum correlations at negative absolute temperatures

We study behavior of quantum discord, a kind of quantum correlation, in systems of dipole–dipole interacting spins in an external magnetic field in the whole temperature range ( \(-\infty ). It was shown that negative temperatures, which are introduced to describe inversions in the population in a finite level system, provide more favorable conditions for emergence of quantum correlations including entanglement. We show that at negative temperature, the correlations become more intense and discord exists between remote spins being in separated states.     

考虑量化的多智能体系统数据驱动双向一致性控制

针对未知动力学模型非线性离散时间多智能体系统,在信息传递过程中的数据量化问题,以及智能体之间的合作与竞争关系,提出了一种数据驱动控制算法,实现了多智能体系统的双向一致性跟踪控制.首先,利用紧凑形动态线性化(CFDL)方法,将未知动力学模型的非线性智能体转化为含有时变参数的数据模型,并通过设计性能指标函数获得时变参数的估...     

Quantum protocols for zero-knowledge systems

Zero-knowledge proof system is an important protocol that can be used as a basic block for construction of other more complex cryptographic protocols. An intrinsic characteristic of a zero-knowledge systems is the assumption that is impossible for the verifier to show to a third party that he has interacted with the prover. However, it has been shown that using quantum correlations the impossibility of transferring proofs can be successfully attacked. In this work we show two new protocols for proof transference, being the first one based on teleportation and the second one without using entangled states. In this last case, we assume that the third party can communicate in advance with both verifier and prover. Following, we present a quantum zero-knowledge protocol based on quantum bit commitment that can be implemented with today technology.     

Dynamic event-triggered bipartite consensus for uncertain high-order nonlinear multi-agent systems

In this paper, the bipartite consensus problem is studied for a class of uncertain high-order nonlinear multi-agent systems. A signed digraph is presented to describe the collaborative and competitive interactions among agents. For each agent with lower triangular structure, a time-varying gain compensator is first designed by relative output information of neighboring agents. Subsequently, a distributed controller with dynamic event-triggered mechanism is proposed to drive the bipartite consensus error to zero. It is worth noting that an internal dynamic variable is introduced in triggering function, which plays an essential role in excluding the Zeno behavior and reducing energy consumption. Furthermore, the dynamic event-triggered control protocol is developed for upper triangular multi-agent systems to realize the bipartite consensus without Zeno behavior. Finally, simulation examples are provided to illustrate the effectiveness of the presented results.     

带有观测器的广义多智能体系统二分一致性

本文研究了带有观测器的广义多智能体系统的二分一致性问题.根据符号图的特性,提出了一种基于广义观测器的分布式二分一致性控制协议.以广义系统稳定性理论和代数图论为主要研究工具,分析并得到了广义多智能体系统实现二分一致性的充分条件.利用广义观测器的相对和绝对信息设计了两种新的二分一致性控制协议.数值仿真验证了理论结果的准确性和有效性.     

异质多智能体系统二分一致性的充要条件

针对由一阶智能体和二阶智能体组成的异质多智能体系统的二分一致性问题,对连续和离散系统情形分别设计了二分一致性协议。基于结构平衡的拓扑,通过规范变换实现了从具有敌对关系的系统到具有非负连接权重系统的转化,将二分一致性问题转变为一般一致性问题。进一步,运用代数图论和矩阵理论分析闭环控制系统的动态特性,得到了异质多智能体系统渐近实现二分一致性的充要条件。最后通过数值模拟验证了所得结果的有效性。     

多智能体系统的事件触发无模型迭代学习双向一致性

针对未知模型的非线性离散时间多智能体系统,研究基于事件触发迭代学习双向一致性问题.首先,利用紧凑形式动态线性化方法,建立多智能体系统的动态线性化数据模型,提出一种该数据模型的参数估计算法;其次,基于该数据模型设计输出观测器和死区控制器,并结合信号图论,构建一种事件触发分布式无模型迭代学习双向一致性控制策略;然后,通过设计李雅普诺夫函数对该控制策略的收敛性进行严格的证明;最后,通过数值仿真进一步验证该控制协议的正确性和有效性.     

随机时延下多输入多输出多智能体系统事件触发双向编队

针对动力学模型未知的多输入多输出非线性离散时间多智能体系统的随机时延问题,提出一种输入增益补偿策略,并针对其通讯受限问题,提出一种带有死区操作器的事件触发控制机制.首先,采用伪偏导技术沿时间轴方向,在智能体的每个工作点上建立一种紧格式动态线性化数据模型,并给出该数据模型的参数估计算法.在此数据模型的基础上结合符号图论,研究智能体之间的合作与竞争关系,设计一种事件触发的数据驱动双向编队控制算法.最后,通过李雅普诺夫稳定性理论、矩阵理论以及压缩映射原理论证所提出算法的收敛性,并通过仿真实验和实物实验进一步验证该算法的正确性与有效性.     

二阶混杂多智能体系统的安全二分一致性

本文在合作–竞争网络下针对一类由二阶连续时间个体和离散时间个体构成的混杂多智能体系统, 研究了系统中存在恶意个体情况下的安全二分一致性问题. 首先, 在假设智能体的恶意邻居数量上限已知的情况下, 基于删除邻居信息中极端值的思想, 设计了一种安全二分一致性容错算法. 其次, 基于鲁棒图性质进行系统转换, 运用图论、矩阵论等分析工具得到了系统实现安全二分一致性的图条件. 最后, 通过仿真实例验证了所得结果的有效性.关键词: 混杂多智能体系统; 恶意个体; 安全二分一致性; 容错算法     

Nonlinear finite-time bipartite consensus protocol for multi-agent systems associated with signed graphs

In this paper, finite-time multi-agent consensus problems are considered under networks associated with signed graphs whose edge weights can be not only positive but also negative. A nonlinear consensus protocol is proposed to guarantee the states of all agents to converge in a finite time. If the signed graph is structurally balanced, then the final consensus states of all agents are the same in modulus but not in sign. Otherwise, if the signed graph is structurally unbalanced, then the states of all agents converge to zero. Moreover, the final consensus states of agents can be provided uniformly regarding a signed-average quantity that depends on both the initial states of agents and the topology structure of the whole multi-agent network. Numerical simulations illustrate that the protocol is effective in achieving the finite-time consensus of agents under signed graphs and can particularly solve the finite-time average consensus problem of agents when their associated graph has all positive edge weights.