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A Fully Symmetrical Quantum Key Distribution System Capable of Preparing and Measuring Quantum States
Tianqi Dou , Jipeng Wang , Zhenhua Li , Wenxiu Qu , Shunyu Yang , Zhongqi Sun , Fen Zhou , Yanxin Han , Yuqing Huang , and Haiqiang Ma
Chin. Phys. Lett.    2020, 37 (11): 110301 .   DOI: 10.1088/0256-307X/37/11/110301
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We propose a fully symmetrical QKD system that enables quantum states to be prepared and measured simultaneously without compromising system performance. Over a 25.6 km fiber channel, we demonstrate point-to-point QKD operations with asymmetric Mach–Zehnder interferometer modules. Two interference visibilities of above 99% indicate that the proposed system has excellent stability. Consequently, the scheme not only improves the feasibility of distributing secret keys, but also enables QKD closer to more practical applications.
Mutual Restriction between Concurrence and Intrinsic Concurrence for Arbitrary Two-Qubit States
A-Long Zhou , Dong Wang, Xiao-Gang Fan , Fei Ming , and Liu Ye
Chin. Phys. Lett.    2020, 37 (11): 110302 .   DOI: 10.1088/0256-307X/37/11/110302
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Concurrence is viewed as the most commonly approach for quantifying entanglement of two-qubit states, while intrinsic concurrence contains concurrence of four pure states consisting of a special pure state ensemble concerning an arbitrary two-qubit state. Thus, a natural question arises: Whether there is a specified relation between them. We firstly examine the relation between concurrence and intrinsic concurrence for the maximally nonlocal mixed state under a special unitary operation, which is not yet rigorously proved. In order to obtain a general result, we investigate the relation between concurrence and intrinsic concurrence using randomly generated two-qubit states, and derive an inequality relation between them. Finally, we take into account the relation between concurrence and intrinsic concurrence in open systems, and reveal the ratio of the two quantum resources, which is only correlated with the experiencing channels.
Chiral State Conversion in a Levitated Micromechanical Oscillator with ${\boldsymbol In~Situ}$ Control of Parameter Loops
Peiran Yin, Xiaohui Luo, Liang Zhang, Shaochun Lin, Tian Tian, Rui Li, Zizhe Wang, Changkui Duan, Pu Huang, and Jiangfeng Du
Chin. Phys. Lett.    2020, 37 (10): 100301 .   DOI: 10.1088/0256-307X/37/10/100301
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Physical systems with gain and loss can be described by a non-Hermitian Hamiltonian, which is degenerated at the exceptional points (EPs). Many new and unexpected features have been explored in the non-Hermitian systems with a great deal of recent interest. One of the most fascinating features is that chiral state conversion appears when one EP is encircled dynamically. Here, we propose an easy-controllable levitated microparticle system that carries a pair of EPs and realize slow evolution of the Hamiltonian along loops in the parameter plane. Utilizing the controllable rotation angle, gain and loss coefficients, we can control the structure, size and location of the loops in situ. We demonstrate that, under the joint action of topological structure of energy surfaces and nonadiabatic transitions, the chiral behavior emerges both along a loop encircling an EP and even along a straight path away from the EP. This work broadens the range of parameter space for the chiral state conversion, and proposes a useful platform to explore the interesting properties of exceptional points physics.
Abundant Traveling Wave Structures of (1+1)-Dimensional Sawada–Kotera Equation: Few Cycle Solitons and Soliton Molecules
Wei Wang, Ruoxia Yao, and Senyue Lou
Chin. Phys. Lett.    2020, 37 (10): 100501 .   DOI: 10.1088/0256-307X/37/10/100501
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Traveling wave solutions have been well studied for various nonlinear systems. However, for high order nonlinear physical models, there still exist various open problems. Here, travelling wave solutions to the well-known fifth-order nonlinear physical model, the Sawada–Kotera equation, are revisited. Abundant travelling wave structures including soliton molecules, soliton lattice, kink-antikink molecules, peak-plateau soliton molecules, few-cycle-pulse solitons, double-peaked and triple-peaked solitons are unearthed.
Butterfly-Like Anisotropic Magnetoresistance and Angle-Dependent Berry Phase in a Type-II Weyl Semimetal WP$_{2}$
Kaixuan Zhang, Yongping Du, Pengdong Wang, Laiming Wei, Lin Li, Qiang Zhang, Wei Qin, Zhiyong Lin, Bin Cheng, Yifan Wang, Han Xu, Xiaodong Fan, Zhe Sun, Xiangang Wan, and Changgan Zeng
Chin. Phys. Lett.    2020, 37 (9): 090301 .   DOI: 10.1088/0256-307X/37/9/090301
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The Weyl semimetal has emerged as a new topologically nontrivial phase of matter, hosting low-energy excitations of massless Weyl fermions. Here, we present a comprehensive study of a type-II Weyl semimetal WP$_{2}$. Transport studies show a butterfly-like magnetoresistance at low temperature, reflecting the anisotropy of the electron Fermi surfaces. This four-lobed feature gradually evolves into a two-lobed variant with an increase in temperature, mainly due to the reduced relative contribution of electron Fermi surfaces compared to hole Fermi surfaces for magnetoresistance. Moreover, an angle-dependent Berry phase is also discovered, based on quantum oscillations, which is ascribed to the effective manipulation of extremal Fermi orbits by the magnetic field to feel nearby topological singularities in the momentum space. The revealed topological character and anisotropic Fermi surfaces of the WP$_{2}$ substantially enrich the physical properties of Weyl semimetals, and show great promises in terms of potential topological electronic and Fermitronic device applications.
Constructing a Maximally Entangled Seven-Qubit State via Orthogonal Arrays
Xin-Wei Zha , Min-Rui Wang, and Ruo-Xu Jiang 
Chin. Phys. Lett.    2020, 37 (9): 090302 .   DOI: 10.1088/0256-307X/37/9/090302
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Huber et al. [Phys. Rev. Lett. 118 (2017) 200502] have proved that a seven-qubit state whose three-body marginal states are all maximally mixed does not exist. Here, we propose a method to build a maximally entangled state based on orthogonal arrays to construct maximally entangled seven-qubit states. Using this method, we not only determine that a seven-qubit state whose three-body marginals are all maximally mixed does not exist, but also find the condition for maximally entangled seven-qubit states. We consider that $\pi_{\rm ME} =19/140$ is a condition for maximally entangled seven-qubit states. Furthermore, we derive three forms of maximally entangled seven-qubit states via orthogonal arrays.
The Analytic Eigenvalue Structure of the 1+1 Dirac Oscillator
Bo-Xing Cao  and Fu-Lin Zhang
Chin. Phys. Lett.    2020, 37 (9): 090303 .   DOI: 10.1088/0256-307X/37/9/090303
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We study the analytic structure for the eigenvalues of the one-dimensional Dirac oscillator, by analytically continuing its frequency on the complex plane. A twofold Riemann surface is found, connecting the two states of a pair of particle and antiparticle. One can, at least in principle, accomplish the transition from a positive energy state to its antiparticle state by moving the frequency continuously on the complex plane, without changing the Hamiltonian after transition. This result provides a visual explanation for the absence of a negative energy state with the quantum number $n=0$.
Rescaled Range Permutation Entropy: A Method for Quantifying the Dynamical Complexity of Extreme Volatility in Chaotic Time Series
Jia-Chen Zhang , Wei-Kai Ren , and Ning-De Jin
Chin. Phys. Lett.    2020, 37 (9): 090501 .   DOI: 10.1088/0256-307X/37/9/090501
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Information entropy, as a quantitative measure of complexity in nonlinear systems, has been widely researched in a variety of contexts. With the development of a nonlinear dynamic, the entropy is faced with severe challenges in dealing with those signals exhibiting extreme volatility. In order to address this problem of weighted permutation entropy, which may result in the inaccurate estimation of extreme volatility, we propose a rescaled range permutation entropy, which selects the ratio of range and standard deviation as the weight of different fragments in the time series, thereby effectively extracting the maximum volatility. By analyzing typical nonlinear systems, we investigate the sensitivities of four methods in chaotic time series where extreme volatility occurs. Compared with sample entropy, fuzzy entropy, and weighted permutation entropy, this rescaled range permutation entropy leads to a significant discernibility, which provides a new method for distinguishing the complexity of nonlinear systems with extreme volatility.
Search for 2D Ferromagnets: Molecular Beam Epitaxy is a Critical Tool
Matthias Batzill
Chin. Phys. Lett.    2020, 37 (8): 080101 .   DOI: 10.1088/0256-307X/37/8/080101
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Meter-Level Optical Delay Line on a Low-Loss Lithium Niobate Nanophotonics Chip
Shining Zhu
Chin. Phys. Lett.    2020, 37 (8): 080102 .   DOI: 10.1088/0256-307X/37/8/080102
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Strong Anisotropy of 3D Diffusion in Living Cells
Xiaosong Chen
Chin. Phys. Lett.    2020, 37 (8): 080103 .   DOI: 10.1088/0256-307X/37/8/080103
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Direct Strong Measurement of a High-Dimensional Quantum State
Chen-Rui Zhang, Meng-Jun Hu, Guo-Yong Xiang, Yong-Sheng Zhang, Chuan-Feng Li, and Guang-Can Guo
Chin. Phys. Lett.    2020, 37 (8): 080301 .   DOI: 10.1088/0256-307X/37/8/080301
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It is of great importance to determine an unknown quantum state for fundamental studies of quantum mechanics, yet it is still difficult to characterize systems of large dimensions in practice. Although the scan-free direct measurement approach based on a weak measurement scheme was proposed to measure a high-dimensional photonic state, how weak the interaction should be to give a correct estimation remains unclear. Here we propose and experimentally demonstrate a technique that measures a high-dimensional quantum state with the combination of scan-free measurement and direct strong measurement. The procedure involves sequential strong measurement, in which case no approximation is made similarly to the conventional direct weak measurement. We use this method to measure a transverse state of a photon with effective dimensionality of $65000$ without the time-consumed scanning process. Furthermore, the high fidelity of the result and the simplicity of the experimental apparatus show that our approach can be readily used to measure the complex field of a beam in diverse applications such as wavefront sensing and quantitative phase imaging.
Machine Learning for Many-Body Localization Transition
Wen-Jia Rao
Chin. Phys. Lett.    2020, 37 (8): 080501 .   DOI: 10.1088/0256-307X/37/8/080501
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We employ the methods of machine learning to study the many-body localization (MBL) transition in a 1D random spin system. By using the raw energy spectrum without pre-processing as training data, it is shown that the MBL transition point is correctly predicted by the machine. The structure of the neural network reveals the nature of this dynamical phase transition that involves all energy levels, while the bandwidth of the spectrum and nearest level spacing are the two dominant patterns and the latter stands out to classify phases. We further use a comparative unsupervised learning method, i.e., principal component analysis, to confirm these results.
Negative Thermal Transport in Conduction and Advection
Liujun Xu and Jiping Huang
Chin. Phys. Lett.    2020, 37 (8): 080502 .   DOI: 10.1088/0256-307X/37/8/080502
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Negative refractive index has drawn a great deal of attention due to its unique properties and practical applications in wave systems. To promote the related physics in thermotics, here we manage to coin a complex thermal conductivity whose imaginary part corresponds to the real part of complex refractive index. Therefore, the thermal counterpart of negative refractive index is just negative imaginary thermal conductivity, which is featured by the opposite directions of energy flow and wave vector in thermal conduction and advection, thus called negative thermal transport herein. To avoid violating causality, we design an open system with energy exchange and explore three different cases to reveal negative thermal transport. We further provide experimental suggestions with a solid ring structure. All finite-element simulations agree with theoretical analyses, indicating that negative thermal transport is physically feasible. These results have potential applications such as designing the inverse Doppler effect in thermal conduction and advection.
Pressure Generation above 35 GPa in a Walker-Type Large-Volume Press
Yu-Chen Shang, Fang-Ren Shen, Xu-Yuan Hou, Lu-Yao Chen, Kuo Hu, Xin Li, Ran Liu, Qiang Tao, Pin-Wen Zhu, Zhao-Dong Liu, Ming-Guang Yao, Qiang Zhou, Tian Cui, and Bing-Bing Liu
Chin. Phys. Lett.    2020, 37 (8): 080701 .   DOI: 10.1088/0256-307X/37/8/080701
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Pressure generation to a higher pressure range in a large-volume press (LVP) denotes our ability to explore more functional materials and deeper Earth's interior. Pressure generated by normal tungsten carbide (WC) anvils in a commercial way is mostly limited to 25 GPa in LVPs due to the limitation of their hardness and design of cell assemblies. We adopt three newly developed WC anvils for ultrahigh pressure generation in a Walker-type LVP with a maximum press load of 1000 ton. The hardest ZK01F WC anvils exhibit the highest efficiency of pressure generation than ZK10F and ZK20F WC anvils, which is related to their performances of plastic deformations. Pressure up to 35 GPa at room temperature is achieved at a relatively low press load of 4.5 MN by adopting the hardest ZK01F WC anvils with three tapering surfaces in conjunction with an optimized cell assembly, while pressure above 35 GPa at 1700 K is achieved at a higher press load of 7.5 MN. Temperature above 2000 K can be generated by our cell assemblies at pressure below 30 GPa. We adopt such high-pressure and high-temperature techniques to fabricate several high-quality and well-sintered polycrystalline minerals for practical use. The present development of high-pressure techniques expands the pressure and temperature ranges in Walker-type LVPs and has wide applications in physics, materials, chemistry, and Earth science.
Characterization of Scanning SQUID Probes Based on 3D Nano-Bridge Junctions in Magnetic Field
Yin-Ping Pan, Yue Wang, Ruo-Ting Yang, Yan Tang, Xiao-Yu Liu, Hua Jin, Lin-Xian Ma, Yi-Shi Lin, Zhen Wang, Jie Ren, Yi-Hua Wang, and Lei Chen
Chin. Phys. Lett.    2020, 37 (8): 080702 .   DOI: 10.1088/0256-307X/37/8/080702
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We develop superconducting quantum interference device (SQUID) probes based on 3D nano-bridge junctions for the scanning SQUID microscopy. The use of these nano-bridge junctions enables imaging in the presence of a high magnetic field. Conventionally, a superconducting ground layer has been employed for better magnetic shielding. In our study, we prepare a number of scanning SQUID probes with and without a ground layer to evaluate their performance in external magnetic fields. The devices show the improved magnetic modulation up to 1.4 T. It is found that the ground layer reduces the inductance, and increases the modulation depth and symmetricity of the gradiometer design in the absence of the field. However, the layer is not compatible with the use of the scanning SQUID probe in the field because it decreases its working field range. Moreover, by adding the layer, the mutual inductance between the feedback coil and the SQUID also decreases linearly as a function of the field.
Enhancing Phase Sensitivity in Mach–Zehnder Interferometers for Arbitrary Input States
Hongbin Liang, Jiancheng Pei, and Xiaoguang Wang
Chin. Phys. Lett.    2020, 37 (7): 070301 .   DOI: 10.1088/0256-307X/37/7/070301
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To enhance the phase sensitivity of Mach–Zehnder interferometers, we use a tunable phase shift before the light beams are injected into the interferometer. The analytical result of the optimal phase shift is obtained, which only depends on the initial input states. For a non-zero optimal phase shift, the phase sensitivity of the interferometers in the output ports is always enhanced. We can achieve this enhancement for most states, including entangled and mixed states. The optimal phase shift is exhibited in three examples. Compared to previous methods, this scheme provides a general way to improve phase sensitivity and could find wide applications in optical phase estimations.
A Two-Dimensional Architecture for Fast Large-Scale Trapped-Ion Quantum Computing
Y.-K. Wu  and L.-M. Duan
Chin. Phys. Lett.    2020, 37 (7): 070302 .   DOI: 10.1088/0256-307X/37/7/070302
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Building blocks of quantum computers have been demonstrated in small to intermediate-scale systems. As one of the leading platforms, the trapped ion system has attracted wide attention. A significant challenge in this system is to combine fast high-fidelity gates with scalability and convenience in ion trap fabrication. Here we propose an architecture for large-scale quantum computing with a two-dimensional array of atomic ions trapped at such large distance which is convenient for ion-trap fabrication but usually believed to be unsuitable for quantum computing as the conventional gates would be too slow. Using gate operations far outside of the Lamb–Dicke region, we show that fast and robust entangling gates can be realized in any large ion arrays. The gate operations are intrinsically parallel and robust to thermal noise, which, together with their high speed and scalability of the proposed architecture, makes this approach an attractive one for large-scale quantum computing.
A New Approach for Residual Stress Analysis of GH3535 Alloy by Using Two-Dimensional Synchrotron X-Ray Diffraction
Sheng Jiang, Ji-Chao Zhang, Shuai Yan, and Xiao-Li Li
Chin. Phys. Lett.    2020, 37 (7): 070701 .   DOI: 10.1088/0256-307X/37/7/070701
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We propose a new method to evaluate residual stress based on the analysis of a portion of a Debye ring with two-dimensional synchrotron x-ray diffraction. The residual stress of a nickel-based alloy GH3535 evaluated by the proposed method is determined to be $-1149\pm34$ MPa based on the quantitative analysis of the deformation of the (200) reflection, and the residual stress obtained by analyzing THE (111) plane is $-933\pm 68$ MPa. The results demonstrate that the GH3535 alloy surface is highly compressive, as expected for a polishing surface treatment. The proposed method provides insight into the field of residual stress measurement and quantitative understanding of the residual stress states in GH3535.