关键词：
Global quantum discord (GQD);Lipkin–Meshkov–Glick (LMG) model;Multipartite nonlocality;Quantum phase transitions
摘要：
We have investigated the multipartite nonlocality and global quantum discord (GQD) in the antiferromagnetic Lipkin-Meshkov-Glick (LMG) model. We find that nonlocality and GQD can indicate quantum phase transitions by singular behaviors. The results also show that GQD always behaves a linear growth with log(10) N in the LMG model, which is different from linear scaling in one-dimensional chains. And this scaling behavior is not affected by anisotropic parameter gamma. Moreover, the nonlocality (denoted by.) is discovered to scale as log(10) S vertical bar(h) similar to aN+b for the transverse-field LMG model (i.e., gamma = 0). However, scaling behaviors can be well fitted by formula S vertical bar(h) = a 1/N + b for the LMG model with gamma > 0. By comparing the results of GQD and nonlocality, we have observed that nonlocality decreases but GQD increases as the size of the system grows. These results indicate that gamma has a significant influence on the nonlocality and GQD in the LMG model.
关键词：
Global quantum discord;Quantum phase transitions;Thermal tensor network
摘要：
We characterize multi-site quantum correlations in 1D finite-size XXZ chains at finite temperatures with global quantum discord. With the help of an exact diagonalization method and a thermal-tensor-network algorithm, the thermal-state discord G((rho) over cap (T)) is evaluated efficiently by well-developed optimization algorithms. Firstly, we find that in a finite temperature region, G((rho) over cap (T)) shows some footprint of the quantum phase transition of the model. The underlying mechanism is that (rho) over cap (T) captures the level crossing in the low-lying excited states. Secondly, we study the temperature dependence of G((rho) over cap (T)). When the anisotropy is strong, G((rho) over cap (T)) will show a broad thermal plateau. We offer a quantitative explanation of the plateau by truncating the thermal-state operator (rho) over cap (T) with respect to several low-lying states. Thirdly, we investigate the scaling behavior of G((rho) over cap (T)). We find that when N is large enough, G((rho) over cap (T)) would show a linear scaling. Finally, combined with the thermal tensor networks and the linear scaling behavior, we successfully figure out reliable results for G((rho) over cap (T)) in the full-temperature regions with N up to 16. We believe that the thermal tensor networks will play a role in studying general multi-site quantum correlations in 1D chains at finite temperatures.
摘要：
In quantum lattice models, in the large- N limit, boundary conditions have little effect upon local observables for sites in the centers of the lattices. In this paper, we will study the boundary effects upon multipartite nonlocality (a kind of multipartite quantum correlation associated with Bell-type inequalities) in one-dimensional finite-size spin chains, both for zero temperature and for finite temperatures. We define a quantity δSS to characterize the boundary effects, where S is a measure of global multipartite nonlocality of the entire lattice, and δS is the difference of the measure induced by changing the boundary conditions. We find δSS does not vanish in the large- N limit. Instead, at zero temperature, with the increase of N, δSS would increase steadily in the vicinity of the quantum phase transition point of the models, and converge to a nonzero constant in noncritical regions. It shows clearly that boundary effects generally exist, in the form of multipartite correlations, in long chains. The boundary effects are explained by the competition between the two orders of the models. In addition, based on these numerical results, we construct a Bell inequality, which is violated by chains with periodic (closed) boundary conditions and not violated by chains with open boundary conditions. Furthermore, we study δSTST of finite-size chains at finite temperatures, and show that boundary effects survive in finite temperature regions.
关键词：
global quantum discord;quantum discord;quantum phase transitions;two-dimensional lattices
摘要：
Global quantum discord (GQD) in two‐dimensional (2D) transverse‐field Ising models on N × N square lattices at zero temperature and finite temperatures are studied. First, the GQD G(ρ2×2) defined on the reduced density matrix ρ2×2 of a 2 × 2 sub‐square in the center of the N × N lattices, with N=4, 5, 6, 7, ∞ is studied. It is found that G(ρ2×2) shows little size dependence in the strong‐field and weak‐field limit, and presents dramatic size dependence in the middle‐field region. The results are explained by short‐range correlations in non‐critical regions and long‐range correlations in critical regions of the models, respectively. Then the scaling behavior of the GQD G(|ψg〉) of the ground states for the entire N × N squares is studied. It is found that G(|ψg〉) contains a 1D contribution (corresponding to the side length of the squares) and a 2D contribution (corresponding to the area of the squares). Furthermore, the effect of thermodynamic fluctuations on GQD by investigating the thermal‐state GQD G(e−HkBT) is considered. In high‐temperature regions, GQD is always destroyed by thermodynamic fluctuations for any strength of the magnetic field. However, in low‐temperature regions, some thermal enhancement of GQD under weak fields and some thermal robustness of GQD under strong fields is observed. The results are explained by the low‐lying energy states and the energy gap of the models. Various progresses have been made in understanding quantum correlations in 1D quantum lattices. However, our understanding for 2D quantum lattices is still at a low level. How would quantum correlations scale with the size of the 2D lattices? How would external physical parameters affect the strength and the robustness of quantum correlation in the 2D lattices? Here, these questions will be answered in the 2D transverse‐field Ising squares.
摘要：
A new scheme for three-dimensional (3D) atom localization is proposed based on measuring the probe absorption spectra in a four-level diamond-configuration atomic system, in which the atom interacts with three orthogonal standing-wave laser fields. Due to spatial-dependent interaction between atom and fields, position information of the atom can be obtained by measuring the absorption spectra of the weak probe field. The results show that atom localization properties can be significantly improved and some interesting spatial localization structures such as double-layer lantern-like, single-layer lantern-like, gourd-like, cylinder-like, and ellipsoid-like patterns can be achieved when we adjust system parameters properly. Most importantly, we can find the atom at a particular position in 3D space with 100% probability under appropriate conditions.
摘要：
In a previous study, we have proposed a procedure to study global quantum discord in 1D chains whose ground states are described by matrix product states [Z.-Y. Sun et al., Ann. Phys. 359, 115 (2015)]. In this paper, we show that with a very simple generalization, the procedure can be used to investigate quantum mixed states described by matrix product density operators, such as quantum chains at finite temperatures and 1D subchains in high-dimensional lattices. As an example, we study the global discord in the ground state of a 2D transverse-field Ising lattice, and pay our attention to the scaling behavior of global discord in 1D sub-chains of the lattice. We find that, for any strength of the magnetic field, global discord always shows a linear scaling behavior as the increase of the length of the sub-chains. In addition, global discord and the so-called “discord density” can be used to indicate the quantum phase transition in the model. Furthermore, based upon our numerical results, we make some reliable predictions about the scaling of global discord defined on the n × n sub-squares in the lattice.
摘要：
Due to its high sensitivity to weak forces, the classical torsion pendulum was widely employed in the fields of precision gravitational measurement, which can be evaluated by testing of Newtonian inverse square law (ISL). In order to get much higher precision for the tests of ISL at short ranges, such as at millimeter and/or sub-millimeter level, the torsion pendulum cannot work precisely in free oscillation motion due to the electrostatic disturbance. This paper presents a method using proportional-integral-differential (PID) technology to make pendulum to keep quasi-static. A closed-loop system with PID controller is realized and calibration results show the system has the potential to get the precision of test of the ISL violation at the order of 10
<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−16</sup>
N·m at millimeter range.
关键词：
Global quantum discord;Matrix product state;Quantum phase transition;Spin chain
摘要：
Global quantum discord (GQD) is a measure of quantum correlation for multi-site quantum states. In this paper, we propose an efficient procedure to calculate global quantum discord (G(n)) for consecutive n-site subsystems in infinite-size matrix product states (MPSs). We apply the method to study the scaling behavior of G(n) in several one-dimensional infinite quantum spin chains, i.e., (1) a three-site interaction model whose ground state can be exactly expressed as MPSs, and (2) a spin-1/2 XXZ chain whose ground state is approximately expressed as MPSs with the help of infinite time-evolving block decimation (iTEBD) algorithm. In both models, as the increase of n, Gn shows an approximately linear growth. Unambiguous clue for the convergence of the incremental Delta G(n) = G(n) - G(n-1) is observed when n is large enough. Moreover, in non-critical (gapped) regions Delta G(n) converges very fast, while in critical (gapless) regions it converges relatively slow. The behaviors are explained in a model-independent physical picture with finite-range correlations. Based on these results, we propose to use "discord per site" (G(n)/n) to describe the global correlations in infinite-size spin chains. Moreover, we find that the global discord shows a size-independent maximum at the infinite-order quantum phase transition point of the XXZ model. (C) 2015 Elsevier Inc. All rights reserved.
摘要：
In this paper, we characterize the global multipartite nonlocality in one-dimensional infinite-size spin-1/2 chains. For this purpose, we calculate the multipartite nonlocality of n-site subchains, and take the large- n limit. As n increases from 2 to 100, we find that the average revenue and the marginal revenue of nonlocality converge gradually, thus they can be used to characterize the global multipartite nonlocality in infinite-size chains. Furthermore, in the vicinity of the quantum phase transition (QPT) points of the transverse-field Ising model and the XXZ model, in the large- n limit, multipartite nonlocality becomes extremely sensitive to external perturbation, that is, arbitrarily small perturbation away from the QPT points would lead to a drastic change in the global nonlocality. This signal of QPTs is easily distinguishable, and can be used to detect QPTs. We also draw a vivid physical picture of multipartite correlations and QPTs by analogy with magnetic domains in ferromagnets.
作者机构：
[Sun Z.Y.; Huang H.L.] School of Electrical and Electronic Engineering, Wuhan Polytechnic University, Wuhan 430023, China;[Ding L.J.] College of Science, China Three Gorges University, Yichang 443002, China
会议名称：
3rd International Conference on Advanced Structure, Materials and Engineering, ASME 2014
摘要：
<jats:p>The finite-temperature entanglement entropy for an alternating spin-1/2 chain with multi-spin interactions is investigated by means of Green’s function theory combined with Jordan-Wigner transformation, to identify the spin-Peierls (<jats:italic>SP</jats:italic>) transition. It is found that the two-site thermal entanglement entropy is a useful tool to characterize the<jats:italic>SP</jats:italic>transition. In addition, the competition between multi-spin interaction and Peierls-dimerization plays a central role in the critical phenomenon of the system.</jats:p>
摘要：
<jats:p>The effect of arbitrary orientation in the magnetic field on the entanglement and dense coding of a two-qubit <jats:italic>XX</jats:italic> model is investigated. The concurrence and optimal dense coding capacity are calculated for different orientations of the magnetic field. It is found that the entanglement can be maximized by rotating the magnetic field to an optimal direction at given temperature. Furthermore, there exists critical concurrence <jats:italic>C</jats:italic><jats:sub>c</jats:sub>, beyond which the thermal state is unfeasible for optimal dense coding.</jats:p>
期刊：
Solid State Communications,2014年185:30-34 ISSN：0038-1098
通讯作者：
Huang, H. L.
作者机构：
[Huang, H. L.; Wu, Y. Y.; Sun, Z. Y.] Wuhan Polytech Univ, Sch Elect & Elect Engn, Wuhan 430000, Peoples R China.;[Wang, B.] ENN Grp Co Ltd, Langfang 065001, Hebei, Peoples R China.
通讯机构：
[Huang, H. L.] W;Wuhan Polytech Univ, Sch Elect & Elect Engn, Wuhan 430000, Peoples R China.
关键词：
A. Two-dimensional spin model;D. Bell inequality;D. Nonlocality;D. Phase transition
摘要：
In this paper, we use the Bell inequality to study the bipartite correlation in an exactly soluble two-dimensional Ising-Heisenberg spin system. The Bell inequality can detect not only the quantum phase transition, but also the thermal phase transitions, or the system. The property or bipartite correlation in the system is also analyzed. In the quantum anti-ferromagnetic phase, the Bell inequality is violated thus nonlocality is present. It is interesting that the nonlocality is enhanced by thermal fluctuation, and similar results have not been observed in anti-ferromagnetic phases. In the ferromagnetic phase, the quantum correlation turns out to be very novel, which cannot be captured by entanglement or nonlocality. (C) 2014 Elsevier Ltd. All rights reserved.
