Entanglement of Formation of an Arbitrary State of Two Rebits

We consider entanglement for quantum states defined in vector spaces over the real numbers. Such real entanglement is different from entanglement in standard quantum mechanics over the complex numbers. The differences provide insight into the nature of entanglement in standard quantum theory. Wootters [Phys. Rev. Lett. 80, 2245 (1998)] has given an explicit formula for the entanglement of formation of two qubits in terms of what he calls the concurrence of the joint density operator. We give a contrasting formula for the entanglement of formation of an arbitrary state of two rebits, a rebit being a system whose Hilbert space is a 2-dimensional real vector space.     

非对称自旋-轨道耦合系统的多体量子相干含时演化

本文以具有非对称性自旋相互作用的三体自旋系统为研究对象,重点研究了三体量子相干含时演化规律.采用精确量子对角化和基于量子主方程的数值模拟方法,讨论了三体量子系统中多种量子相干组分及其退相干.研究发现,量子相干组分的含时演化与整个系统的初态量子特性紧密相关.当初态为可分离纯态时,在较短时间内,非对称相互作用有利于增加多体量子相干度.这些量子相干度因受噪声影响而逐渐衰减.当初态为类Werner态时,量子相干度的分布满足加和性,即三体量子相干度等于所有两体量子相干度之和.自旋之间非对称相互作用和环境噪声都会引起三体量子相干度大于所有两体量子相干度之和.这些结论有助于多体量子资源的制备.     

差错基、量子码与群代数

本文找到了一种研究优质差错基和量子纠错码的新方法,即群代数方法, 它为差错基和量子码提供了一种代数表示. 利用这种代数表示, 建立了一系列关于最一般量子纠错码的线性规划限. 关键词： 群代数 差错基 量子纠错码 量子信息     

负值量子条件熵与双量子系统一类混合态纠缠量度

运用负值量子条件熵研究了双量子系统一类混合态的纠缠量度.给出了负值量子条件作为条件熵纠缠度的定义，证明了条件熵纠缠满足作为2×2系统一类混合纠缠态量度的四个基本条件.当双量子系统处于纯态时，条件熵纠缠度即为部分熵纠缠度.应用条件熵纠缠度研究了真空腔场中两全同二能级原子之间纯态和一类混合态纠缠的时间演化，比较了相同条件下两全同原子系统concurrence纠缠度的时间演化.结果表明，两纠缠度演化规律完全一致，验证了负值量子条件熵可以作为双量子系统纯态和一类混合态的纠缠量度. 关键词： 双量子系统 负值量子条件熵 条件熵纠缠度 混合态纠缠度     

Quantifying quantum correlation via quantum coherence

Resource theory is applied to quantify the quantum correlation of a bipartite state and a computable measure is proposed. Since this measure is based on quantum coherence, we present another possible physical meaning for quantum correlation, i.e., the minimum quantum coherence achieved under local unitary transformations. This measure satisfies the basic requirements for quantifying quantum correlation and coincides with concurrence for pure states. Since no optimization is involved in the final definition, this measure is easy to compute irrespective of the Hilbert space dimension of the bipartite state.     

Analytically Computable Symmetric Quantum Correlations

One of the greatest challenges in developing the resource theory of a quantum feature is to establish an analytically computable quantifier, which directly limits the practicability of such quantifiers. Here, analytic quantifiers of both the symmetric quantum discord (SQD) and the symmetric measurement‐induced nonlocality (SMIN) in a bipartite system of qubits are studied on the basis of the quantum skew information. It is shown that the SMIN of any two‐qubit system and the SQD of bipartite “X”‐type states and block‐diagonal states can be analytically determined. In addition, the SQD and the SMIN are invariant with an attached quantum state. The validity of our analytical expressions is further illustrated numerically on the basis of several randomly generated density matrices.     

Discord under the influence of a quantum phase transition

This paper studies the discord of a bipartite two-level system coupling to an XY spin-chain environment in a transverse field and investigates the relationship between the discord property and the environment’s quantum phase transition.The results show that the quantum discord is also able to characterize the quantum phase transitions.We also discuss the difference between discord and entanglement,and show that quantum discord may reveal more general information than quantum entanglement for characterizing the environment’s quantum phase transition.     

Purified discord and multipartite entanglement

We study bipartite quantum discord as a manifestation of a multipartite entanglement structure in the tripartite purified system. In particular, we find that bipartite quantum discord requires the presence of both bipartite and tripartite entanglement in the purification. This allows one to understand the asymmetry of quantum discord, D(A,B)≠D(B,A)$D(A,B)\ne D(B,A)$ in terms of entanglement monogamy. As instructive special cases, we study discord for qubits and Gaussian states in detail. As a result of this we shed new light on a counterintuitive property of Gaussian states: the presence of classical correlations necessarily requires the presence of quantum correlations. Finally, our results also shed new light on a protocol for remote activation of entanglement by a third party.     

Characterizing Quantum Correlations in Arbitrary-Dimensional Bipartite Systems Using Hurwitz's Theory

Quantum correlations play vital roles in the quantum features in quantum information processing tasks. Among the measures of quantum correlations, quantum discord （QD） and entanglement of formation （EOF） axe two significant ones. Recent research has shown that there exists a relation between QD and EOF, which makes QD more significant in quantum information theory. However, until now, there exists no general method of chaxaeterizing quantum discord in high-dimensional quantum systems. In this paper, we have proposed a general method for calculating quantum discord in axbitraxy-dimensionM bipaxtite quantum systems in terms of Hurwitz＇s theory. Applications including the Werner state, the spin-1 XXZ model thermal equilibrium state, the Horodecki state, and the separable-bound-free entanglement state are investigated. We present the method of obtaining the EOF of axbitraxy-dimensional bipaxtite quantum states via purification, and the relations.hip between QD and EOF.     

A Generalized Geometric Measurement of Quantum Discord:Exact Treatment

A generalization of the geometric measure of quantum discord is introduced in this article, based on Hellinger distance. Our definition has virtues of computability and independence of local measurement. In addition it also does not suffer from the recently raised critiques about quantum discord. The exact result can be obtained for bipartite pure states with arbitrary levels, which is completely determined by the Schmidt decomposition. For bipartite mixed states the exact result can also be found for a special case. Furthermore the generalization into multipartite case is direct. It is shown that it can be evaluated exactly when the measured state is invariant under permutation or translation. In addition the detection of quantum phase transition is also discussed for Lipkin–Meshkov–Glick and Dicke model.     

Separability Criteria for Quantum Mixed States

We investigate separability of mixed states in bipartite and multipartite quantum systems. If a quantum state in a bipartite system of arbitrary dimension (or in $2\times2\times N$ quantum systems) is separable, we show that some quantity in relation to Hermitian matrix is positive.     

Using a quantum dot system to realize perfect state transfer

There are some disadvantages to Nikolopoulos et al.'s protocol [Nikolopoulos G M, Petrosyan D and Lambropoulos P 2004 Europhys. Lett. 65 297] where a quantum dot system is used to realize quantum communication. To overcome these disadvantages, we propose a protocol that uses a quantum dot array to construct a four-qubit spin chain to realize perfect quantum state transfer (PQST). First, we calculate the interaction relation for PQST in the spin chain. Second, we review the interaction between the quantum dots in the Heitler-London approach. Third, we present a detailed program for designing the proper parameters of a quantum dot array to realize PQST.     

Some Aspects of Quantum Entanglement for CAR Systems

We show some distinct features of quantum entanglement for bipartite CAR systems such as the failure of triangle inequality of von Neumann entropy and the possible change of our entanglement degree under local operations. Those are due to the nonindependence of CAR systems and never occur in any algebraic independent systems. We introduce a new notion half-sided entanglement.     

Polarization-Dependent Mode Analyzer and Controlled-Z Gate in Ti:LiNbO3 Quantum Photonic Circuits

Abstract

On-chip integrated photonic circuits are crucial for further progress toward quantum technologies and in the science of quantum optics. The quantum controlled-Z gate is an example of the maximally entangling gate, which is universal for quantum computing when coupled with single-qubit gates. This article demonstrates a deterministic controlled-Z photonic quantum gate based on titanium in-diffused channel waveguides in which polarization and modal degrees of freedom of a single photon are used for encoding the control and target qubits, respectively.     

An Holistic Extension for Classical Logic via Quantum Fredkin Gate

An holistic extension for classical propositional logic is introduced in the framework of quantum computation with mixed states. The mentioned extension is obtained by applying the quantum Fredkin gate to non-factorizable bipartite states. In particular, an extended notion of classical contradiction is studied in this holistic framework.     

任意正多边形量子环自旋输运的普遍解

在有关偶数正多边形量子环对称连接特殊情形的自旋输运特性的研究基础上,进一步探讨了任意正多边形量子环的自旋输运性质.不仅解析地求解了相关电子散射问题,而且得到了 Landauer-Buttiker 电导的普遍公式,并讨论了它的圆环极限和 Aharonov-Casher 相位问题.结合数值计算,研究了正多边形量子环的Landauer-Buttiker 电导随多边形边数、引线连接方式、自旋轨道耦合强度以及电子波矢的周期变化特性和零点分布规律. 关键词： Rashba 自旋-轨道耦合 Aharonov-Casher 相位 量子网络 量子输运     

任意正多边形量子环自旋输运的普遍解

在有关偶数正多边形量子环对称连接特殊情形的自旋输运特性的研究基础上,进一步探讨了任意正多边形量子环的自旋输运性质.不仅解析地求解了相关电子散射问题,而且得到了 Landauer-Buttiker 电导的普遍公式,并讨论了它的圆环极限和 Aharonov-Casher 相位问题.结合数值计算,研究了正多边形量子环的Landauer-Buttiker 电导随多边形边数、引线连接方式、自旋轨道耦合强度以及电子波矢的周期变化特性和零点分布规律.     

Quantum Secure Direct Communication by Using General Entangled States

We revisited the quantum secure direct communication (QSDC) by using a notation (Li et al. in Phys. Lett. A 297:121, 2002; Li et al. in Int. J. Theor. Phys. 46(7):1815, 2007) and generalized QSDC to arbitrary finite dimensions, we also figured out the decoding formula for QSDC of arbitrary finite dimensions. In order to overcome the channel noisy, we present a scheme through introduce n auxilians by using quantum swapping.     

Security proof of counterfactual quantum cryptography against general intercept-resend attacks and its vulnerability

Counterfactual quantum cryptography,recently proposed by Noh,is featured with no transmission of signal particles.This exhibits evident security advantages,such as its immunity to the well-known photon-number-splitting attack.In this paper,the theoretical security of counterfactual quantum cryptography protocol against the general interceptresend attacks is proved by bounding the information of an eavesdropper Eve more tightly than in Yin’s proposal [Phys.Rev.A 82 042335(2010)].It is also shown that practical counterfactual quantum cryptography implementations may be vulnerable when equipped with imperfect apparatuses,by proving that a negative key rate can be achieved when Eve launches a time-shift attack based on imperfect detector efficiency.