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Bell's inequality and entanglement in qubits

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arXiv 2017年

作者： Chang, Po-Yao Chu, Su-Kuan Mad, Chen-Te Center for Materials Theory Rutgers University PiscatawayNJ08854 United States Joint Quantum Institute NIST/University of Maryland College ParkMD20742 United States Joint Center for Quantum Information and Computer Science NIST/University of Maryland College ParkMD20742 United States Department of Physics and Center for Theoretical Sciences National Taiwan University Taipei10617 Taiwan

We propose an alternative evaluation of quantum entanglement by measuring the maximum violation of the Bell's inequality without performing a partial trace operation. This proposal is demonstrated by bridging the maximum violation of the Bell's inequality and the concurrence of a pure state in an n-qubit system, in which one subsystem only contains one qubit and the state is a linear combination of two product states. We apply this relation to the ground states of four qubits in the Wen-Plaquette model and show that they are maximally entangled. A topological entanglement entropy of the Wen-Plaquette model could be obtained by relating the upper bound of the maximum violation of the Bell's inequality to the concurrences of a pure state with respect to different bipartitions. Copyright © 2017, The Authors. All rights reserved.

关键词： quantum entanglement

Dimensionality-driven orthorhombic MoTe2 at room temperature

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Physical Review B 2018年第4期97卷 041410(R)-041410(R)页

作者： Rui He Shazhou Zhong Hyun Ho Kim Gaihua Ye Zhipeng Ye Logan Winford Daniel McHaffie Ivana Rilak Fangchu Chen Xuan Luo Yuping Sun Adam W. Tsen Department of Electrical and Computer Engineering Texas Tech University Lubbock Texas 79409 USA Department of Physics University of Northern Iowa Cedar Falls Iowa 50614 USA Institute for Quantum Computing University of Waterloo Waterloo Ontario Canada N2L 3G1 Department of Physics and Astronomy University of Waterloo Waterloo Ontario Canada N2L 3G1 Department of Chemistry University of Waterloo Waterloo Ontario Canada N2L 3G1 Key Laboratory of Materials Physics Institute of Solid State Physics Chinese Academy of Sciences Hefei 230031 People's Republic of China University of Science and Technology of China Hefei 230026 China High Magnetic Field Laboratory Chinese Academy of Sciences Hefei 230031 People's Republic of China Collaborative Innovation Centre of Advanced Microstructures Nanjing University Nanjing 210093 People's Republic of China

We use a combination of Raman spectroscopy and transport measurements to study thin flakes of the type-II Weyl semimetal candidate MoTe2 protected from oxidation. In contrast to bulk crystals, which undergo a phase transition from monoclinic to the inversion symmetry breaking, orthorhombic phase below ∼250K, we find that in moderately thin samples below ∼12nm, a single orthorhombic phase exists up to and beyond room temperature. This could be due to the effect of c-axis confinement, which lowers the energy of an out-of-plane hole band and stabilizes the orthorhombic structure. Our results suggest that Weyl nodes, predicated upon inversion symmetry breaking, may be observed in thin MoTe2 at room temperature.

关键词： Phase transitions Structural properties Topological materials Transition metal dichalcogenides Weyl semimetal Raman spectroscopy

Substrate surface engineering for high-quality silicon/aluminum superconducting resonators

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arXiv 2018年

作者： Earnest, C.T. Bejanin, J.H. McConkey, T.G. Peters, E.A. Korinek, A. Yuan, H. Mariantoni, M. Institute for Quantum Computing University of Waterloo 200 University Avenue West WaterlooONN2L 3G1 Canada Department of Physics and Astronomy University of Waterloo 200 University Avenue West WaterlooONN2L 3G1 Canada Department of Electrical and Computer Engineering University of Waterloo 200 University Avenue West WaterlooONN2L 3G1 Canada Canadian Centre for Electron Microscopy Department of Materials Science and Engineering McMaster University 1280 Main Street West HamiltonONL8S 4M1 Canada

quantum bits (qubits) with long coherence times are an important element for the implementation of medium-and large-scale quantum computers. In the case of superconducting planar qubits, understanding and improving qubits' quality can be achieved by studying superconducting planar resonators. In this Paper, we fabricate and characterize coplanar waveguide resonators made from aluminum thin films deposited on silicon substrates. We perform three different substrate treatments prior to aluminum deposition: One chemical treatment based on a hydrouoric acid clean;one physical treatment consisting of a thermal annealing at 880 C in high vacuum;one combined treatment comprising both the chemical and the physical treatments. The aim of these treatments is to remove the two-level state defects hosted by the native oxides residing at the various sample's interfaces. We first characterize the fabricated samples through cross-sectional tunneling electron microscopy acquiring electron energy loss spectroscopy maps of the samples' cross sections. These measurements show that both the chemical and the physical treatments almost entirely remove native silicon oxide from the substrate surface and that their combination results in the cleanest interface. We then study the quality of the resonators by means of microwave measurements in the \quantum regime", i.e., at a temperature T 10mK and at a mean microwave photon number hnphi 1. In this regime, we find that both surface treatments independently improve the resonator's intrinsic quality factor and that the highest quality factor is obtained for the combined treatment, Qi 0:8 million. Finally, we find that the TLS quality factor averaged over a time period of 3 h is 3 million at hnphi 10, indicating that substrate surface engineering can potentially reduce the TLS loss below other losses such as quasiparticle and vortex loss. Copyright © 2018, The Authors. All rights reserved.

关键词： High resolution transmission electron microscopy

Interaction-induced transition in the quantum chaotic dynamics of a disordered metal

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arXiv 2017年

作者： Syzranov, S.V. Gorshkov, A.V. Galitski, V.M. Physics Department University of California Santa CruzCA95064 United States Joint Quantum Institute NIST/University of Maryland College ParkMD20742 United States Joint Center for Quantum Information and Computer Science NIST/University of Maryland College ParkMD20742 United States Condensed Matter Theory Center Physics Department University of Maryland College ParkMD20742 United States

We demonstrate that a weakly disordered metal with short-range interactions exhibits a transition in the quantum chaotic dynamics when changing the temperature or the interaction strength. For weak interactions, the system displays exponential growth of the out-of-time-ordered correlator (OTOC) of the current operator. The Lyapunov exponent of this growth is temperature-independent in the limit of vanishing interaction. With increasing the temperature or the interaction strength, the system undergoes a transition to a non-chaotic behaviour, for which the exponential growth of the OTOC is absent. We conjecture that the transition manifests itself in the quasiparticle energy-level statistics and also discuss ways of its explicit observation in cold-atom setups. Copyright © 2017, The Authors. All rights reserved.

关键词： Lyapunov methods

quantum-Assisted Learning of Hardware-Embedded Probabilistic Graphical Models

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Physical Review X 2017年第4期7卷 041052-041052页

作者： Marcello Benedetti John Realpe-Gómez Rupak Biswas Alejandro Perdomo-Ortiz Quantum Artificial Intelligence Laboratory NASA Ames Research Center Moffett Field California 94035 USA Department of Computer Science University College London WC1E 6BT London United Kingdom Cambridge Quantum Computing Limited CB2 1UB Cambridge United Kingdom SGT Inc. Greenbelt Maryland 20770 USA Instituto de Matemáticas Aplicadas Universidad de Cartagena Bolívar 130001 Colombia Exploration Technology Directorate NASA Ames Research Center Moffett Field California 94035 USA USRA Research Institute for Advanced Computer Science Mountain View California 94043 USA

Mainstream machine-learning techniques such as deep learning and probabilistic programming rely heavily on sampling from generally intractable probability distributions. There is increasing interest in the potential advantages of using quantum computing technologies as sampling engines to speed up these tasks or to make them more effective. However, some pressing challenges in state-of-the-art quantum annealers have to be overcome before we can assess their actual performance. The sparse connectivity, resulting from the local interaction between quantum bits in physical hardware implementations, is considered the most severe limitation to the quality of constructing powerful generative unsupervised machine-learning models. Here, we use embedding techniques to add redundancy to data sets, allowing us to increase the modeling capacity of quantum annealers. We illustrate our findings by training hardware-embedded graphical models on a binarized data set of handwritten digits and two synthetic data sets in experiments with up to 940 quantum bits. Our model can be trained in quantum hardware without full knowledge of the effective parameters specifying the corresponding quantum Gibbs-like distribution; therefore, this approach avoids the need to infer the effective temperature at each iteration, speeding up learning; it also mitigates the effect of noise in the control parameters, making it robust to deviations from the reference Gibbs distribution. Our approach demonstrates the feasibility of using quantum annealers for implementing generative models, and it provides a suitable framework for benchmarking these quantum technologies on machine-learning-related tasks.

关键词： Hybrid quantum systems NP-hard problems quantum algorithms quantum information architectures & platforms Stochastic inference

quantum key distribution with distinguishable decoy states

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arXiv 2017年

作者： Huang, Anqi Sun, Shi-Hai Liu, Zhihong Makarov, Vadim Institute for Quantum Information State Key Laboratory of High Performance Computing College of Computer National University of Defense Technology Changsha410073 China Institute for Quantum Computing University of Waterloo WaterlooONN2L 3G1 Canada Department of Electrical and Computer Engineering University of Waterloo WaterlooONN2L 3G1 Canada College of Science National University of Defense Technology Changsha410073 China College of Mechatronic Engineering and Automation National University of Defense Technology Changsha410073 China Russian Quantum Center MISIS University Moscow Department of Physics and Astronomy University of Waterloo WaterlooONN2L 3G1 Canada

The decoy state protocol has been considered to be one of the most important methods to protect the security of quantum key distribution (QKD) with a weak coherent source. Here we test two experimental approaches to generating the decoy states with different intensities: modulation of the pump current of a semiconductor laser diode, and external modulation by an optical intensity modulator. The former approach shows a side-channel in the time domain that allows an attacker to distinguish s signal state from a decoy state, breaking a basic assumption in the protocol. We model a photon-number-splitting attack based on our experimental data, and show that it compromises the system's security. Then, based on the work of K. Tamaki et al. [New J. Phys. 18, 065008 (2016)], we obtain two analytical formulas to estimate the yield and error rate of single-photon pulses when the signal and decoy states are distinguishable. The distinguishability reduces the secure key rate below that of a perfect decoy-state protocol. To mitigate this reduction, we propose to calibrate the transmittance of the receiver (Bob's) unit. We apply our method to three QKD systems and estimate their secure key rates. Copyright © 2017, The Authors. All rights reserved.

关键词： Modulation

Measuring holographic entanglement entropy on a quantum simulator

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arXiv 2017年

作者： Li, Keren Han, Muxin Qu, Dongxue Huang, Zichang Long, Guilu Wan, Yidun Lu, Dawei Zeng, Bei Laflamme, Raymond Shenzhen Institute for Quantum Science and Engineering Department of Physics Southern University of Science and Technology Shenzhen518055 China Center for Quantum Computing Peng Cheng Laboratory Shenzhen518055 China Institute for Quantum Computing and Department of Physics and Astronomy University of Waterloo WaterlooONN2L 3G1 Canada State Key Laboratory of Low-Dimensional Quantum Physics and Department of Physics Tsinghua University Beijing100084 China Department of Physics Florida Atlantic University 777 Glades Road Boca RatonFL33431 United States Institut für Quantengravitation Universität Erlangen-Nürnberg Staudtstr. 7/B2 Erlangen91058 Germany State Key Laboratory of Surface Physics Fudan University Shanghai200433 China Department of Physics and Center for Field Theory and Particle Physics Fudan University Shanghai200433 China Institute for Nanoelectronic devices and Quantum computing Fudan University Shanghai200433 China Collaborative Innovation Center of Advanced Microstructures Nanjing University Nanjing210093 China Shenzhen Key Laboratory of Quantum Science and Engineering Shenzhen518055 China Department of Mathematics and Statistics University of Guelph GuelphONN1G 2W1 Canada Perimeter Institute for Theoretical Physics WaterlooONN2L 2Y5 Canada

quantum simulation promises to have wide applications in many fields where problems are hard to model with classical computers. Various quantum devices of different platforms have been built to tackle the problems in, say, quantum chemistry, condensed matter physics, and high-energy physics. Here, we report an experiment towards the simulation of quantum gravity by simulating the holographic entanglement entropy. On a six-qubit nuclear magnetic resonance quantum simulator, we demonstrate a key result of Anti-de Sitter/conformal field theory(AdS/CFT ) correspondence-the Ryu-Takayanagi formula is demonstrated by measuring the relevant entanglement entropies on the perfect tensor state. The fidelity of our experimentally prepared the six-qubit state is 85.0% via full state tomography and reaches 93.7%if the signal-decay due to decoherence is taken into account. Our experiment serves as the basic module of simulating more complex tensor network states that exploring AdS/CFT correspondence. As the initial experimental attempt to study AdS/CFT via quantum information processing, our work opens up new avenues exploring quantum gravity phenomena on quantum simulators. Copyright © 2017, The Authors. All rights reserved.

关键词： quantum chemistry

Exactly soluble model of boundary degeneracy

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Physical Review B 2017年第4期95卷 045309-045309页

作者： Sriram Ganeshan Alexey V. Gorshkov Victor Gurarie Victor M. Galitski Simons Center of Geometry and Physics Stony Brook New York 11794 Condensed Matter Theory Center Department of Physics University of Maryland College Park Maryland 20742 USA Joint Quantum Institute NIST/University of Maryland College Park Maryland 20742 USA Joint Center for Quantum Information and Computer Science NIST/University of Maryland College Park Maryland 20742 USA Physics Department University of Colorado Boulder Colorado 80309 USA School of Physics Monash University Melbourne Victoria 3800 Australia

We investigate the topological degeneracy that can be realized in Abelian fractional quantum spin Hall states with multiply connected gapped boundaries. Such a topological degeneracy (also dubbed as “boundary degeneracy”) does not require superconducting proximity effect and can be created by simply applying a depletion gate to the quantum spin Hall material and using a generic spin-mixing term (e.g., due to backscattering) to gap out the edge modes. We construct an exactly soluble microscopic model manifesting this topological degeneracy and solve it using the recently developed technique [S. Ganeshan and M. Levin, Phys. Rev. B 93, 075118 (2016)]. The corresponding string operators spanning this degeneracy are explicitly calculated. It is argued that the proposed scheme is experimentally reasonable.

关键词： Anyons Fractional quantum Hall effect Proximity effect quantum field theory Synthetic gauge fields Luttinger liquid model

Achieving Exact Cluster Recovery Threshold via Semidefinite Programming: Extensions

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Achieving Exact Cluster Recovery Threshold via Semidefinite ...

作者： Hajek, Bruce Wu, Yihong Xu, Jiaming Coordinated Science Laboratory Department of Electrical and Computer Engineering University of Illinois at Urbana-Champaign UrbanaIL61801-3887 United States Simons Institute for the Theory of Computing University of California at Berkeley BerkeleyCA94720 United States

Resolving a conjecture of Abbe, Bandeira, and Hall, the authors have recently shown that the semidefinite programming (SDP) relaxation of the maximum likelihood estimator achieves the sharp threshold for exactly recovering the community structure under the binary stochastic block model (SBM) of two equal-sized clusters. The same was shown for the case of a single cluster and outliers. Extending the proof techniques, in this paper, it is shown that SDP relaxations also achieve the sharp recovery threshold in the following cases: 1) binary SBM with two clusters of sizes proportional to network size but not necessarily equal;2) SBM with a fixed number of equal-sized clusters;and 3) binary censored block model with the background graph being Erd's-Rényi. Furthermore, a sufficient condition is given for an SDP procedure to achieve exact recovery for the general case of a fixed number of clusters plus outliers. These results demonstrate the versatility of SDP relaxation as a simple, general purpose, computationally feasible methodology for community detection. © 2016 IEEE.

关键词： Stochastic systems