Quantum thermal discord in a two-spin-1/2 XXZ model

We study the quantum discord in a two-spin-1/2 XXZ model in thermal equilibrium at temperature T in the presence of an external magnetic field B. Special attention is paid to the dependence of quantum discord on the temperature T and magnetic field B. It is found that quantum thermal discord is more robust than concurrence against temperature, in the sense that quantum thermal discord does not vanish at finite temperatures, but concurrence vanishes completely at a critical temperature.     

Quantum correlations in the dimerized spin chain at zero and finite temperatures

By using the method of exact diagonalization, we investigate the quantum correlation measured by quantum discord of the dimerized spin chain at both zero and finite temperatures. The results disclose that the quantum discord is robust at any finite parameter α and temperature T, in contrast to entanglement which shows a sudden death when the parameter α or the temperature T reaches a critical point. At finite temperature, it is interesting to find that the quantum discord QD _{2i−1, 2i} can increase with temperature T no matter if the entanglement EoF _{2i−1, 2i} exists or not. The research on the relation between the quantum discord and the quantum phase transition in the dimerized spin chain indicates that the transition can be characterized by the first derivation of the quantum discord at zero and low temperatures.     

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.     

Quantum and classical correlations for a two-qubit X structure density matrix

We derive explicit expressions for quantum discord and classical correlation for an X structure density matrix. Based on the characteristics of the expressions, the quantum discord and the classical correlation are easily obtained and compared under different initial conditions using a novel analytical method. We explain the relationships among quantum discord, classical correlation, and entanglement, and further find that the quantum discord is not always larger than the entanglement measured by concurrence in a general two-qubit X state. The new method, which is different from previous approaches, has certain guiding significance for analysing quantum discord and classical correlation of a two-qubit X state, such as a mixed state.     

Robust thermal quantum correlation and quantum phase transition of spin system on fractal lattices

We investigate the quantum correlation measured by quantum discord (QD) for thermalized ferromagnetic Heisenberg spin systems in one-dimensional chains and on fractal lattices using the decimation renormalization group approach. It is found that the QD between two non-nearest-neighbor end spins exhibits some interesting behaviors which depend on the anisotropic parameter Δ, the temperature T, and the size of system L. With increasing Δ continuously, the QD possesses a cuspate change at Δ = 0 which is a critical point of quantum phase transition (QPT). There presents the “regrowth” tendency of QD with increasing T at Δ < 0, in contrast to the “growth” of QD at Δ > 0. As the size of the system L becomes large, there still exists considerable thermal QD between long-distance end sites in spin chains and on the fractal lattices even at unentangled states, and the long-distance QD can spotlight the presence of QPT. The robustness of QD on the diamond-type hierarchical lattices is stronger than that in spin chains and Koch curves, which indicates that the fractal can affect the behaviors of quantum correlation.     

Quantum Darwinism in a Composite System: Objectivity versus Classicality

We investigate the implications of quantum Darwinism in a composite quantum system with interacting constituents exhibiting a decoherence-free subspace. We consider a two-qubit system coupled to an N-qubit environment via a dephasing interaction. For excitation preserving interactions between the system qubits, an analytical expression for the dynamics is obtained. It demonstrates that part of the system Hilbert space redundantly proliferates its information to the environment, while the remaining subspace is decoupled and preserves clear non-classical signatures. For measurements performed on the system, we establish that a non-zero quantum discord is shared between the composite system and the environment, thus violating the conditions of strong Darwinism. However, due to the asymmetry of quantum discord, the information shared with the environment is completely classical for measurements performed on the environment. Our results imply a dichotomy between objectivity and classicality that emerges when considering composite systems.     

Various Quantum Correlations in Two-Qubit Two-Axis Spin Squeezing Model in Thermal Equilibrium under an External Magnetic Field

This paper investigates the influence of the spin squeezing parameter γ, the external magnetic field B and the temperature T on the concurrence (C), the quantum discord (QD), and the geometric quantum discord (GQD) in the two-qubit two-axis spin squeezing model in thermal equilibrium under an external magnetic field. The results show that the spin squeezing parameter γ has a positive effect on all three correlations. When the system is in the ground state, the external magnetic field B has a weakening effect on the three types of quantum correlations. Particularly, the spin squeezing parameter can be used to alleviate the destructive effect of the magnetic field on the geometric quantum discord. At a relatively high temperature, the externally applied magnetic field B helps enhance the quantum discord (QD). Further, the quantum discord is more robust than concurrence, and thus is more suitable for use as a quantum resource in information processing.     

Geometric Global Quantum Discord of Two-qubit X States

In this paper, we find that the geometric global quantum discord proposed by Xu and the total quantum correlations proposed by Hassan and Joag are identical. Moreover, we work out the analytical formulas of the geometric global quantum discord and geometric quantum discord both for two-qubit X states, respectively. We further illustrate how to use these formulas to deal with a few particular examples. We also compare the results achieved by using three kinds of geometric quantum discords. The geometric quantum discord is verified as a tight lower bound of the geometric global quantum discord for two-qubit X states.     

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.     

Correlation Dynamics of Three-Qubit System Under a Classical Dephasing Environment

By starting from the stochastic Hamiltonian of the three correlated spins and modeling their frequency fluctuations as caused by dephasing noisy environments described by Ornstein-Uhlenbeck (OU) processes, we study the dynamics of quantum correlations, including entanglement and quantum discord. Of course, in this article, we use two definitions for the quantum discord (global quantum discord and quantum dissension). We prepared initially our open system with the Greenberger-Horne-Zeilinger (GHZ) and W states and present the exact solutions for evolution dynamics of entanglement and quantum discord between three spins under both Markovian and non-Markovian regime of this classical noise. By comparison the dynamics of entanglement with that of quantum discord we find that entanglement can be more robust than quantum discord against this noise. It is shown that by considering non-Markovian extensions the survival time of correlations prolong. Also, we compare the results of two definitions of the quantum discord and show that the quantum dissension is equal to the global quantum discord for GHZ state, but they are unequal for the W state.     

极化诱导的内建电场对Mn δ掺杂的GaN/AlGaN量子阱居里温度的调制

采用传输矩阵方法分析极化诱导的内建电场对Mn δ掺杂的GaN/Al_xGa_1-xN量子阱居里温度(T_C)的调制作用.通过解薛定谔方程计算出在不同的内建电场条件下半导体量子阱局域态内的基态空穴能级和波函数分布情况，并在此基础上确定量子阱内Mn δ掺杂情况下T_C随内建电场的变化趋势，分析了不同量子阱结构引起的内建电场分布变化及其对T_C的影响.在耦合双量子阱中通过调节左右阱的不对称性可以得到T_C近3倍的增长. 关键词： GaN 量子阱 内建电场 居里温度     

<Emphasis Type="Italic">T</Emphasis>-odd asymmetries for evaporation γ-rays in nuclear fission

T-odd asymmetries in angular distributions of evaporation γ-rays emitted by thermalized fragments resulting from the fission of axially symmetric deformed nuclei induced by cold polarized neutrons are investigated within the quantum theory of fission. The asymmetries in question are due to the anisotropy of the angular distributions of evaporation γ-rays, caused by zero wriggling vibrations of the fissioning nucleus and associated with the orientation of large fission fragment spins in the direction perpendicular to the direction n ₀ of the symmetry axis of the fissioning nucleus at the time of its separation into fragments. The angle of rotation of the vector n ₀ with respect to the asymptotic direction of the light fission fragment emission is calculated with allowance for the interference of fission amplitudes of neutron resonances excited in the fissioning nucleus as it captures the incident neutron. It is shown that the angular and energy characteristics of the T-odd asymmetry calculated for evaporation γ-rays agree with the characteristics of the experimentally investigated T-odd asymmetry in angular distributions of all γ-rays emitted by a fissioning ²³⁶U nucleus.     

The Dynamics of Quantum Discord and Entanglement of Three Atoms Coupled to Three Spatially Separate Cavities

In this paper, we study the dynamics of quantum discord and entanglement of three identical two-level atoms simultaneously resonantly interacting with three spatially separate single-mode of high-Q cavities respectively. Taking advantage of the depiction quantum discord and entanglement of formation (EoF), we conclude that the discord and entanglement of atoms and cavities can be mediated by changing some parameters and the maximum values of discord and entanglement are independent on the couplings of cavities and atoms. In particular, there also exists quantum discord sudden death as well as entanglement sudden death and the time interval of the former is shorter than that of the later in the proposed quantum system. It is shown that the discord and entanglement of any two atoms among three atoms can be transferred to the corresponding cavities, and there exists discord and entanglement exchanging between the atoms and the corresponding cavities.     

Quantum Correlations Evolution Asymmetry in Quantum Channels

It was demonstrated that the entanglement evolution of a specially designed quantum state in the bistochastic channel is asymmetric. In this work, we generalize the study of the quantum correlations, including entanglement and quantum discord, evolution asymmetry to various quantum channels. We found that the asymmetry of entanglement and quantum discord only occurs in some special quantum channels, and the behavior of the entanglement evolution may be quite different from the behavior of the quantum discord evolution. To quantum entanglement, in some channels it decreases monotonously with the increase of the quantum channel intensity. In some other channels, when we increase the intensity of the quantum channel, it decreases at first, then keeps zero for some time, and then rises up. To quantum discord, the evolution becomes more complex and you may find that it evolutes unsmoothly at some points. These results illustrate the strong dependence of the quantum correlations evolution on the property of the quantum channels.     

BCS Effect on Quantum Correlation and Tripartite Quantum Entanglement in Spinless Gapless Tomonaga-Luttinger Liquid and Cuprate Superconducting Nanowire

It has been known that quantum information offers powerful instruments to investigate the properties of many-body systems. In this framework, we touched two particular aspect of this activity, namely the quantum entanglement and discord to compare the properties of gapless Tomonaga-Luttinger Liquid (TLL) model and the effect of BCS coupling in spinless fermions of TLL as a cuprate superconducting nanowire. Using two-fermion space-spin density matrix, we investigate quantum correlation of these cases via bipartite and tripartite entanglement, as well as quantum discord. The relations of concurrence (as a measure of quantum entanglement), the lower bound of the generalized robustness of tripartite entanglement and quantum discord in terms of the relative distance between fermions and the coupling parameter were accordingly obtained. The relationship between the compressibility as a physical property of system and quantum correlations has also been studied.

     

The exponential map for representations ofU p,q (gl(2))

For the quantum groupGL _p,q (2) and the corresponding quantum algebraU _p,q (gl(2)) Fronsdal and Galindo [Lett. Math. Phys.27 (1993) 59] explicitly constructed the so-called universalT-matrix. In a previous paper [J. Phys. A28 (1995) 2819] we showed how this universalT-matrix can be used to exponentiate representations from the quantum algebra to get representations (left comodules) for the quantum group. Here, further properties of the universalT-matrix are illustrated. In particular, it is shown how to obtain comodules of the quantum algebra by exponentiating modules of the quantum group. Also the relation with the universalR-matrix is discussed.Presented at the 4th International Colloquium Quantum Groups and Integrable Systems, Prague, 22–24 June 1995.     

Nonrelativistic scale anomaly, and composite operators with complex scaling dimensions

It is demonstrated that a nonrelativistic quantum scale anomaly manifests itself in the appearance of composite operators with complex scaling dimensions. In particular, we study nonrelativistic quantum mechanics with an inverse square potential and consider a composite s-wave operator O=ψψ. We analytically compute the scaling dimension of this operator and determine the propagator 〈0|TOO^†|0〉. The operator O represents an infinite tower of bound states with a geometric energy spectrum. Operators with higher angular momenta are briefly discussed.     

Thermal tripartite quantum correlations: quantum discord and entanglement perspectives

We investigate thermal tripartite quantum correlations for a spin star network and for a new extended version of it. In a spin star network, three peripheral spins interact with the central spin identically while in extended spin star network, three peripheral spins interact with two central spatially separated spins in the same way. We exploit the method of [C.C. Rulli, M.S. Sarandy, Phys. Rev. A 84, 042109 (2011)] to evaluate the tripartite quantum discord (TQD) and the method of [M. Li, S. Fei, Z. Wang, Rep. Math. Phys 65, 289 (2010)] called as lower bound of tripartite concurrence (LBTC) to evaluate the tripartite entanglement (TE) of the the peripheral parties in both systems. It is found that thermal TQD is much more robust than thermal TE as a function of temperature T. Also, the peripheral parties of the extended spin star network, in comparison with those of the spin star one, can exhibit higher values of TQD at T > 0. This, indeed, motivates us to realise improved quantum information and quantum computation tasks at finite temperatures.     

Bath-induced correlations in an infinite-dimensional Hilbert space

Quantum correlations between two free spinless dissipative distinguishable particles (interacting with a thermal bath) are studied analytically using the quantum master equation and tools of quantum information. Bath-induced coherence and correlations in an infinite-dimensional Hilbert space are shown. We show that for temperature T> 0 the time-evolution of the reduced density matrix cannot be written as the direct product of two independent particles. We have found a time-scale that characterizes the time when the bath-induced coherence is maximum before being wiped out by dissipation (purity, relative entropy, spatial dispersion, and mirror correlations are studied). The Wigner function associated to the Wannier lattice (where the dissipative quantum walks move) is studied as an indirect measure of the induced correlations among particles. We have supported the quantum character of the correlations by analyzing the geometric quantum discord.     

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.