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Elective-momentum ultrasize quantum Monte Carlo method

Physical Review B 2019年第8期99卷 085114-085114页

作者： Zi Hong Liu Xiao Yan Xu Yang Qi Kai Sun Zi Yang Meng Beijing National Laboratory for Condensed Matter Physics and Institute of Physics Chinese Academy of Sciences Beijing 100190 China School of Physical Sciences University of Chinese Academy of Sciences Beijing 100190 China Department of Physics Hong Kong University of Science and Technology Clear Water Bay Hong Kong China Center for Field Theory and Particle Physics Department of Physics Fudan University Shanghai 200433 China State Key Laboratory of Surface Physics Fudan University Shanghai 200433 China Collaborative Innovation Center of Advanced Microstructures Nanjing 210093 China Department of Physics University of Michigan Ann Arbor Michigan 48109 USA CAS Center of Excellence in Topological Quantum Computation and School of Physical Sciences University of Chinese Academy of Sciences Beijing 100190 China Songshan Lake Materials Laboratory Dongguan Guangdong 523808 China

One bottleneck of quantum Monte Carlo (QMC) simulation of strongly correlated electron systems lies at the scaling relation of computational complexity with respect to the system sizes. For generic lattice models of interacting fermions, the best methodology at hand still scales with βN3 where β is the inverse temperature and N is the system size. Such scaling behavior has greatly hampered the accessibility of the universal infrared (IR) physics of many interesting correlated electron models at (2+1)D, let alone (3+1)D. To reduce the computational complexity, we develop a new QMC method with inhomogeneous momentum-space mesh, dubbed elective-momentum ultrasize quantum Monte Carlo (EQMC) method. Instead of treating all fermionic excitations on an equal footing as in conventional QMC methods, by converting the fermion determinant into the momentum space, our method focuses on fermion modes that are directly associated with low-energy (IR) physics in the vicinity of the so-called hot spots, while other fermion modes irrelevant for universal properties are ignored. As shown in the paper, for any cutoff-independent quantities, e.g., scaling exponents, this method can achieve the same level of accuracy with orders of magnitude increase in computational efficiency. We demonstrate this method with a model of antiferromagnetic itinerant quantum critical point, realized via coupling itinerant fermions with a frustrated transverse-field Ising model on a triangle lattice. The system size of 48×48×32 (L×L×β, almost 3 times of previous investigations) are comfortably accessed with EQMC. With much larger system sizes, the scaling exponents are unveiled with unprecedentedly high accuracy, and this result sheds new light on the open debate about the nature and the universality class of itinerant quantum critical points.

关键词： Critical exponents Quantum phase transitions Quantum Monte Carlo

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Surface terminations and layer-resolved tunneling spectroscopy of the 122 iron pnictide superconductors

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Physical Review B 2019年第13期99卷 134520-134520页

作者： Ang Li Jia-Xin Yin Jihui Wang Zheng Wu Jihua Ma Athena S. Sefat Brian C. Sales David G. Mandrus Michael A. McGuire Rongying Jin Chenglin Zhang Pengcheng Dai Bing Lv Ching-Wu Chu Xuejin Liang P.-H. Hor C.-S. Ting Shuheng H. Pan Department of Physics and Texas Center for Superconductivity University of Houston Houston Texas 77204 USA Beijing National Laboratory for Condensed Matter Physics and Institute of Physics Chinese Academy of Sciences Beijing 100190 China Department of Physics Boston College Chestnut Hill Massachusetts 02467 USA Materials Science & Technology Division Oak Ridge National Laboratory Oak Ridge Tennessee 37831 USA Department of Physics & Astronomy Louisiana State University Baton Rouge Louisiana 70803 USA Department of Physics and Astronomy Rice University Houston Texas 77005 USA School of Physics University of Chinese Academy of Sciences Beijing 100190 China CAS Center for Excellence in Topological Quantum Computation University of Chinese Academy of Sciences Beijing 100190 China Songshang Lake Material Laboratory Dongguan Guangdong 523808 China

The surface terminations of 122-type alkaline earth metal iron pnictides AEFe2As2(AE=Ca,Ba) are investigated with scanning tunneling microscopy/spectroscopy. Cleaving these crystals at a cryogenic temperature yields a large majority of terminations with an atomically resolved (√2 × √2)R45 or 1 × 2 lattice, as well as a very rare termination of 1 × 1 lattice symmetry. By analyzing the lattice registration and selective chemical marking, we identify these terminations as (√2 × √2)R45-reconstructed AE, 1 × 2-reconstructed As, and (√2 × √2)R45-reconstructed Fe surface layers, respectively. Layer-resolved tunneling spectroscopy on these terminating surfaces reveals a well-defined superconducting energy gap on the As terminations, while the gap features become weaker on the AE terminations and absent on the Fe terminations. The superconducting gap is hardly affected locally by the As or AE surface reconstructions. The definitive identification of the surface terminations and the associated spectroscopic signatures shed light on the essential roles of As and the pnictogen-iron-pnictogen trilayer building block in iron-based superconductivity.

关键词： Impurities in superconductors Local density of states Superconducting gap Surface reconstruction High-temperature superconductors Pnictides Scanning tunneling spectroscopy

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Simultaneous Excitation of Two Noninteracting Atoms with Time-Frequency Correlated Photon Pairs in a Superconducting Circuit

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Physical Review Letters 2020年第13期125卷 133601-133601页

作者： Wenhui Ren Wuxin Liu Chao Song Hekang Li Qiujiang Guo Zhen Wang Dongning Zheng Girish S. Agarwal Marlan O. Scully Shi-Yao Zhu H. Wang Da-Wei Wang Interdisciplinary Center for Quantum Information State Key Laboratory of Modern Optical Instrumentation and Zhejiang Province Key Laboratory of Quantum Technology and Device Department of Physics Zhejiang University Hangzhou 310027 China Institute of Physics Chinese Academy of Sciences Bejing 100190 China CAS Center for Excellence in Topological Quantum Computation University of Chinese Academy of Sciences Bejing 100190 China Institute for Quantum Science and Engineering Departments of Biological and Agricultural Engineering Physics and Astronomy Texas A&M University College Station Texas 77843 USA Baylor University Waco Texas 76706 USA

We report the first observation of simultaneous excitation of two noninteracting atoms by a pair of time-frequency correlated photons in a superconducting circuit. The strong coupling regime of this process enables the synthesis of a three-body interaction Hamiltonian, which allows the generation of the tripartite Greenberger-Horne-Zeilinger state in a single step with a fidelity as high as 0.95. We further demonstrate the inhibition of the simultaneous two-atom excitation by continuously measuring whether the first photon is emitted. This work provides a new route in synthesizing many-body interaction Hamiltonian and coherent control of entanglement.

关键词： Cavity quantum electrodynamics Quantum Zeno dynamics Quantum entanglement Quantum optics with artificial atoms Superconducting quantum optics

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Mid-Circuit Cavity Measurement in a Neutral Atom Array

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Physical Review Letters 2022年第20期129卷 203602-203602页

作者： Emma Deist Yue-Hui Lu Jacquelyn Ho Mary Kate Pasha Johannes Zeiher Zhenjie Yan Dan M. Stamper-Kurn Department of Physics University of California Berkeley California 94720 USA Challenge Institute for Quantum Computation University of California Berkeley California 94720 USA Max-Planck-Institut für Quantenoptik 85748 Garching Germany Munich Center for Quantum Science and Technology (MCQST) 80799 Munich Germany Materials Sciences Division Lawrence Berkeley National Laboratory Berkeley California 94720 USA

Subsystem readout during a quantum process, or mid-circuit measurement, is crucial for error correction in quantum computation, simulation, and metrology. Ideal mid-circuit measurement should be faster than the decoherence of the system, high-fidelity, and nondestructive to the unmeasured qubits. Here, we use a strongly coupled optical cavity to read out the state of a single tweezer-trapped Rb87 atom within a small tweezer array. Measuring either atomic fluorescence or the transmission of light through the cavity, we detect both the presence and the state of an atom in the tweezer, within only tens of microseconds, with state preparation and measurement infidelities of roughly 0.5% and atom loss probabilities of around 1%. Using a two-tweezer system, we find measurement on one atom within the cavity causes no observable hyperfine-state decoherence on a second atom located tens of microns from the cavity volume. This high-fidelity mid-circuit readout method is a substantial step toward quantum error correction in neutral atom arrays.

关键词： Cavity quantum electrodynamics Optical lattices & traps Quantum information processing Optical tweezers

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Quantum State Smoothing Cannot Be Assumed Classical Even When the Filtering and Retrofiltering Are Classical

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PRX Quantum 2023年第4期4卷 040340-040340页

作者： Kiarn T. Laverick Prahlad Warszawski Areeya Chantasri Howard M. Wiseman Centre for Quantum Computation and Communication Technology (Australian Research Council) Centre for Quantum Dynamics Griffith University Yuggera Country Brisbane Queensland 4111 Australia Centre of Excellence in Engineered Quantum Systems (Australian Research Council) School of Physics The University of Sydney Gadigal Country Sydney New South Wales 2006 Australia Optical and Quantum Physics Laboratory Department of Physics Faculty of Science Mahidol University Bangkok 10140 Thailand Institute for Quantum Studies Chapman University Orange California 92866 USA

State smoothing is a technique to estimate a state at a particular time, conditioned on information obtained both before (past) and after (future) that time. For a classical system, the smoothed state is a normalized product of the filtered state (a state conditioned only on the past measurement information and the initial preparation) and the retrofiltered effect (depending only on the future measurement information). For the quantum case, while there are well-established analogues of the filtered state (ρF) and the retrofiltered effect (E^R), their product does not, in general, provide a valid quantum state for smoothing. However, this procedure does seem to work when ρF and E^R are mutually diagonalizable. This fact has been used to obtain smoothed quantum states—purer than the filtered states—in a number of experiments on continuously monitored quantum systems, in cavity QED and atomic systems. In this paper we show that there is an implicit assumption underlying this technique: that if all the information were known to the observer, the true system state would be one of the diagonal basis states. This assumption does not necessarily hold, as the missing information is quantum information. It could be known to the observer only if it were turned into a classical measurement record, but then its nature would depend on the choice of measurement. We show by a simple model that, depending on that measurement choice, the smoothed quantum state can: agree with that from the classical method, disagree with it but still be co-diagonal with it, or not even be co-diagonal with it. That is, just because filtering and retrofiltering appear classical does not mean classical smoothing theory is applicable in quantum experiments.

关键词： Open quantum systems & decoherence Quantum feedback Quantum measurements Quantum stochastic processes

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Theory for the Charge-Density-Wave Mechanism of 3D Quantum Hall Effect

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Physical Review Letters 2020年第20期125卷 206601-206601页

作者： Fang Qin (覃昉) Shuai Li Z. Z. Du C. M. Wang Wenqing Zhang Dapeng Yu Hai-Zhou Lu X. C. Xie Shenzhen Institute for Quantum Science and Engineering and Department of Physics Southern University of Science and Technology (SUSTech) Shenzhen 518055 China CAS Key Laboratory of Quantum Information University of Science and Technology of China Chinese Academy of Sciences Hefei Anhui 230026 China Shenzhen Municipal Key-Lab for Advanced Quantum Materials and Devices Shenzhen 518055 China Shenzhen Key Laboratory of Quantum Science and Engineering Shenzhen 518055 China Department of Physics Shanghai Normal University Shanghai 200234 China International Center for Quantum Materials School of Physics Peking University Beijing 100871 China CAS Center for Excellence in Topological Quantum Computation University of Chinese Academy of Sciences Beijing 100190 China Beijing Academy of Quantum Information Sciences West Building 3 No. 10 Xibeiwang East Road Haidian District Beijing 100193 China

The charge-density-wave (CDW) mechanism of the 3D quantum Hall effect has been observed recently in ZrTe5 [Tang et al., Nature 569, 537 (2019)]. Different from previous cases, the CDW forms on a one-dimensional (1D) band of Landau levels, which strongly depends on the magnetic field. However, its theory is still lacking. We develop a theory for the CDW mechanism of 3D quantum Hall effect. The theory can capture the main features in the experiments. We find a magnetic field induced second-order phase transition to the CDW phase. We find that electron-phonon interactions, rather than electron-electron interactions, dominate the order parameter. We extract the electron-phonon coupling constant from the non-Ohmic I−V relation. We point out a commensurate-incommensurate CDW crossover in the experiment. More importantly, our theory explores a rare case, in which a magnetic field can induce an order-parameter phase transition in one direction but a topological phase transition in other two directions, both depend on one magnetic field.

关键词： Charge density waves Hall effect Quantum Hall effect Quantum transport 3-dimensional systems

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On the spectrum and string tension of U(1) lattice gauge theory in 2 + 1 dimensions

arXiv

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arXiv 2018年

作者： Athenodorou, Andreas Teper, Michael Computation-based Science and Technology Research Center Cyprus Institute 20 Kavafi Str. Nicosia2121 Rudolf Peierls Centre for Theoretical Physics Clarendon Laboratory University of Oxford Parks Road OxfordOX1 3PU United Kingdom

We calculate the low-lying spectra of glueballs and confining flux tubes in the U(1) lattice gauge theory in 2 + 1 dimensions. We see that up to modest lattice spacing corrections, the glueball states are consistent with being multiparticle states composed of non-interacting massive JPC= 0--particles. We observe that the ag2→ 0 limit is, as expected, unconventional, and follows the well-known saddle-point analysis of Polyakov to a good approximation. The spectrum of closed (winding) flux tubes exhibits the presence of a massive world-sheet excitation whose mass is consistent with that of the bulk screening mass. These U(1) calculations are intended to complement existing lattice calculations of the properties of SU(N > 2) and SO(N > 3) gauge theories in D = 2 + 1. Copyright © 2018, The Authors. All rights reserved.

关键词： Magnetic flux

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Antiferromagnetism in the kagome-lattice compound α−Cu3Mg(OH)6Br2

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Physical Review B 2019年第15期100卷 155129-155129页

作者： Yuan Wei Zili Feng Clarina dela Cruz Wei Yi Zi Yang Meng Jia-Wei Mei Youguo Shi Shiliang Li Beijing National Laboratory for Condensed Matter Physics Institute of Physics Chinese Academy of Sciences Beijing 100190 China School of Physical Sciences University of Chinese Academy of Sciences Beijing 100190 China Neutron Scattering Division Neutron Sciences Directorate Oak Ridge National Laboratory Oak Ridge Tennessee 37831 USA International Center for Materials Nanoarchitectonics National Institute for Materials Science 1-1 Namiki Tsukuba Ibaraki 305-0044 Japan CAS Center of Excellence in Topological Quantum Computation and School of Physical Sciences University of Chinese Academy of Sciences Beijing 100190 China Songshan Lake Materials Laboratory Dongguan Guangdong 523808 China Department of Physics The University of Hong Kong China Shenzhen Institute for Quantum Science and Engineering and Department of Physics Southern University of Science and Technology Shenzhen 518055 China Center of Materials Science and Optoelectronics Engineering University of Chinese Academy of Sciences Beijing 100049 China

The antiferromagnetism in α−Cu3Mg(OH)6Br2 was studied by magnetic-susceptibility, specific-heat, and neutron-diffraction measurements. The crystal structure consists of Cu2+ kagome layers with Mg2+ ions occupying the centers of the hexagons, separated by Br1− ions. The magnetic system orders antiferromagnetically at 5.4 K with the magnetic moments aligned ferromagnetically within the kagome planes. The ordered moment is 0.94μB, suggesting little quantum and geometrical fluctuations. By comparing the magnetic and specific-heat properties with those of the haydeeite, we suggest that α−Cu3Mg(OH)6Br2 may be described by the two-dimensional spin-1/2 Heisenberg kagome model and is in the region of the ferromagnetic-order side of the phase diagram.

关键词： Antiferromagnetism Frustrated magnetism 2-dimensional systems Kagome lattice Heisenberg model Neutron scattering Specific heat measurements Susceptibility measurements

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Gaussian time-dependent variational principle for the Bose-Hubbard model

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Physical Review B 2019年第9期100卷 094529-094529页

作者： Tommaso Guaita Lucas Hackl Tao Shi Claudius Hubig Eugene Demler J. Ignacio Cirac Max Planck Institute of Quantum Optics Hans-Kopfermann-Straße 1 D-85748 Garching bei München Germany Munich Center for Quantum Science and Technology Schellingstraße 4 D-80799 München Germany CAS Key Laboratory of Theoretical Physics Institute of Theoretical Physics Chinese Academy of Sciences Beijing 100190 China CAS Center for Excellence in Topological Quantum Computation University of Chinese Academy of Sciences Beijing 100049 China Lyman Laboratory Department of Physics Harvard University 17 Oxford St. Cambridge Massachusetts 02138 USA

We systematically extend Bogoliubov theory beyond the mean-field approximation of the Bose-Hubbard model in the superfluid phase. Our approach is based on the time-dependent variational principle applied to the family of all Gaussian states (i.e., Gaussian TDVP). First, we find the best ground-state approximation within our variational class using imaginary time evolution in 1D, 2D, and 3D. We benchmark our results by comparing to Bogoliubov theory and DMRG in 1D. Second, we compute the approximate one- and two-particle excitation spectrum as eigenvalues of the linearized projected equations of motion (linearized TDVP). We find the gapless Goldstone mode, a continuum of two-particle excitations and a doublon mode. We discuss the relation of the gap between Goldstone mode and two-particle continuum to the excitation energy of the Higgs mode. Third, we compute linear response functions for perturbations describing density variation and lattice modulation and discuss their relations to experiment. Our methods can be applied to any perturbations that are linear or quadratic in creation/annihilation operators. Finally, we provide a comprehensive overview how our results are related to well-known methods, such as traditional Bogoliubov theory and random phase approximation.

关键词： Superfluids Bose-Hubbard model Variational approach

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Surface Chern-Simons theory for third-order topological insulators and superconductors

arXiv

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arXiv 2024年

作者： Huang, Zhi-Hao Tan, Yi Jia, Wei Zhang, Long Liu, Xiong-Jun International Center for Quantum Materials School of Physics Peking University Beijing100871 China Hefei National Laboratory Hefei230088 China Key Laboratory of Quantum Theory and Applications MoE Lanzhou Center for Theoretical Physics Key Laboratory of Theoretical Physics of Gansu Province Lanzhou University Lanzhou730000 China School of Physics Institute for Quantum Science and Engineering Huazhong University of Science and Technology Wuhan430074 China International Quantum Academy Shenzhen518048 China CAS Center for Excellence in Topological Quantum Computation University of Chinese Academy of Sciences Beijing100190 China

Three-dimensional 3rd-order topological insulators (TOTIs) and superconductors (TOTSCs), as the highestorder topological phases hosting zero corner modes in physical dimension, has sparked extensive research interest. However, such topological states have not been discovered in reality due to the lack of experimental schemes of realization. Here, we propose a novel surface Chern-Simons (CS) theory for 3rd-order topological phases, and show that the theory enables a feasible and systematic design of TOTIs and TOTSCs. We show that the emergence of zero Dirac (Majorana) corner modes is entirely captured by an emergent 2 CS term that can be further characterized by a novel two-particle Wess-Zumino (WZ) term uncovered here in the surfaces of three-dimensional topological materials. Importantly, our proposed CS term characterization and two-particle WZ term mechanism provide a unique perspective to design TOTIs (TOTSCs) in terms of minimal ingredients, feasibly guiding the search for underlying materials, with promising candidates being discussed. This work shall advance both the theoretical and experimental research for highest-order topological matters. Copyright © 2024, The Authors. All rights reserved.

关键词： Superconducting materials

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