117 research outputs found
Topological Entanglement Entropy of a Bose-Hubbard Spin Liquid
The Landau paradigm of classifying phases by broken symmetries was
demonstrated to be incomplete when it was realized that different quantum Hall
states could only be distinguished by more subtle, topological properties.
Today, the role of topology as an underlying description of order has branched
out to include topological band insulators, and certain featureless gapped Mott
insulators with a topological degeneracy in the groundstate wavefunction.
Despite intense focus, very few candidates for these topologically ordered
"spin liquids" exist. The main difficulty in finding systems that harbour spin
liquid states is the very fact that they violate the Landau paradigm, making
conventional order parameters non-existent. Here, we uncover a spin liquid
phase in a Bose-Hubbard model on the kagome lattice, and measure its
topological order directly via the topological entanglement entropy. This is
the first smoking-gun demonstration of a non-trivial spin liquid, identified
through its entanglement entropy as a gapped groundstate with emergent Z2 gauge
symmetry.Comment: 4+ pages, 3 figure
Absence of a Spin Liquid Phase in the Hubbard Model on the Honeycomb Lattice
A spin liquid is a novel quantum state of matter with no conventional order
parameter where a finite charge gap exists even though the band theory would
predict metallic behavior. Finding a stable spin liquid in two or higher
spatial dimensions is one of the most challenging and debated issues in
condensed matter physics. Very recently, it has been reported that a model of
graphene, i.e., the Hubbard model on the honeycomb lattice, can show a spin
liquid ground state in a wide region of the phase diagram, between a semi-metal
(SM) and an antiferromagnetic insulator (AFMI). Here, by performing numerically
exact quantum Monte Carlo simulations, we extend the previous study to much
larger clusters (containing up to 2592 sites), and find, if any, a very weak
evidence of this spin liquid region. Instead, our calculations strongly
indicate a direct and continuous quantum phase transition between SM and AFMI.Comment: 15 pages with 7 figures and 9 tables including supplementary
information, accepted for publication in Scientific Report
Characterizing quantum supremacy in near-term devices
© 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. A critical question for quantum computing in the near future is whether quantum devices without error correction can perform a well-defined computational task beyond the capabilities of supercomputers. Such a demonstration of what is referred to as quantum supremacy requires a reliable evaluation of the resources required to solve tasks with classical approaches. Here, we propose the task of sampling from the output distribution of random quantum circuits as a demonstration of quantum supremacy. We extend previous results in computational complexity to argue that this sampling task must take exponential time in a classical computer. We introduce cross-entropy benchmarking to obtain the experimental fidelity of complex multiqubit dynamics. This can be estimated and extrapolated to give a success metric for a quantum supremacy demonstration. We study the computational cost of relevant classical algorithms and conclude that quantum supremacy can be achieved with circuits in a two-dimensional lattice of 7 × 7 qubits and around 40 clock cycles. This requires an error rate of around 0.5% for two-qubit gates (0.05% for one-qubit gates), and it would demonstrate the basic building blocks for a fault-tolerant quantum computer
Low-temperature muon spin rotation studies of the monopole charges and currents in Y doped Ho2Ti2O7
In the ground state of Ho2Ti2O7 spin ice, the disorder of the magnetic moments follows the same rules as the proton disorder in water ice. Excitations take the form of magnetic monopoles that interact via a magnetic Coulomb interaction. Muon spin rotation has been used to probe the low-temperature magnetic behaviour in single crystal Ho2−xYxTi2O7 (x = 0, 0.1, 1, 1.6 and 2). At very low temperatures, a linear field dependence for the relaxation rate of the muon precession λ(B), that in some previous experiments on Dy2Ti2O7 spin ice has been associated with monopole currents, is observed in samples with x = 0, and 0.1. A signal from the magnetic fields penetrating into the silver sample plate due to the magnetization of the crystals is observed for all the samples containing Ho allowing us to study the unusual magnetic dynamics of Y doped spin ice
Order by disorder and spiral spin liquid in frustrated diamond lattice antiferromagnets
Frustration refers to competition between different interactions that cannot
be simultaneously satisfied, a familiar feature in many magnetic solids. Strong
frustration results in highly degenerate ground states, and a large suppression
of ordering by fluctuations. Key challenges in frustrated magnetism are
characterizing the fluctuating spin-liquid regime and determining the mechanism
of eventual order at lower temperature. Here, we study a model of a diamond
lattice antiferromagnet appropriate for numerous spinel materials. With
sufficiently strong frustration a massive ground state degeneracy develops
amongst spirals whose propagation wavevectors reside on a continuous
two-dimensional ``spiral surface'' in momentum space. We argue that an
important ordering mechanism is entropic splitting of the degenerate ground
states, an elusive phenomena called order-by-disorder. A broad ``spiral
spin-liquid'' regime emerges at higher temperatures, where the underlying
spiral surface can be directly revealed via spin correlations. We discuss the
agreement between these predictions and the well characterized spinel MnSc2S4
Extensive degeneracy, Coulomb phase and magnetic monopoles in an artificial realization of the square ice model
Artificial spin ice systems have been introduced as a possible mean to
investigate frustration effects in a well-controlled manner by fabricating
lithographically-patterned two-dimensional arrangements of interacting magnetic
nanostructures. This approach offers the opportunity to visualize
unconventional states of matter, directly in real space, and triggered a wealth
of studies at the frontier between nanomagnetism, statistical thermodynamics
and condensed matter physics. Despite the strong efforts made these last ten
years to provide an artificial realization of the celebrated square ice model,
no simple geometry based on arrays of nanomagnets succeeded to capture the
macroscopically degenerate ground state manifold of the corresponding model.
Instead, in all works reported so far, square lattices of nanomagnets are
characterized by a magnetically ordered ground state consisting of local
flux-closure configurations with alternating chirality. Here, we show
experimentally and theoretically, that all the characteristics of the square
ice model can be observed if the artificial square lattice is properly
designed. The spin configurations we image after demagnetizing our arrays
reveal unambiguous signatures of an algebraic spin liquid state characterized
by the presence of pinch points in the associated magnetic structure factor.
Local excitations, i.e. classical analogues of magnetic monopoles, are found to
be free to evolve in a massively degenerated, divergence-free vacuum. We thus
provide the first lab-on-chip platform allowing the investigation of collective
phenomena, including Coulomb phases and ice-like physics.Comment: 26 pages, 10 figure
Spinons and triplons in spatially anisotropic frustrated antiferromagnets
The search for elementary excitations with fractional quantum numbers is a
central challenge in modern condensed matter physics. We explore the
possibility in a realistic model for several materials, the spin-1/2 spatially
anisotropic frustrated Heisenberg antiferromagnet in two dimensions. By
restricting the Hilbert space to that expressed by exact eigenstates of the
Heisenberg chain, we derive an effective Schr\"odinger equation valid in the
weak interchain-coupling regime. The dynamical spin correlations from this
approach agree quantitatively with inelastic neutron measurements on the
triangular antiferromagnet Cs_2CuCl_4. The spectral features in such
antiferromagnets can be attributed to two types of excitations: descendents of
one-dimensional spinons of individual chains, and coherently propagating
"triplon" bound states of spinon pairs. We argue that triplons are generic
features of spatially anisotropic frustrated antiferromagnets, and arise
because the bound spinon pair lowers its kinetic energy by propagating between
chains.Comment: 16 pages, 6 figure
Identifying Topological Order by Entanglement Entropy
Topological phases are unique states of matter incorporating long-range
quantum entanglement, hosting exotic excitations with fractional quantum
statistics. We report a practical method to identify topological phases in
arbitrary realistic models by accurately calculating the Topological
Entanglement Entropy (TEE) using the Density Matrix Renormalization Group
(DMRG). We argue that the DMRG algorithm naturally produces a minimally
entangled state, from amongst the quasi-degenerate ground states in a
topological phase. This proposal both explains the success of this method, and
the absence of ground state degeneracy found in prior DMRG sightings of
topological phases. We demonstrate the effectiveness of the calculational
procedure by obtaining the TEE for several microscopic models, with an accuracy
of order when the circumference of the cylinder is around ten times
the correlation length. As an example, we definitively show the ground state of
the quantum antiferromagnet on the kagom\'e lattice is a topological
spin liquid, and strongly constrain the full identification of this phase of
matter.Comment: 20 pages, 6 figure
Протокол функционального обследования аноректальной зоны и классификация нарушений: международный консенсус и Российские рекомендации
This manuscript summarizes consensus reached by the International Anorectal Physiology Working Group (IAPWG) for the performance, terminology used, and interpretation of anorectal function testing including anorectal manometry (focused on high-resolution manometry), the rectal sensory test, and the balloon expulsion test. Based on these measurements, a classification system for disorders of anorectal function is proposed. Aim to provide information about methods of diagnosis and new classification of functional anorectal disorders to a wide range of specialists general practitioners, therapists, gastroenterologists, coloproctologists all who face the manifestations of these diseases in everyday practice and determine the diagnostic and therapeutic algorithm. Current paper provides agreed statements of IAPWG Consensus and comments (in italics) of Russian experts on real-world practice, mainly on methodology of examination. These comments in no way intended to detract from the provisions agreed by the international group of experts. We hope that these comments will help to improve the quality of examination based on the systematization of local experience with the use of the methods discussed and the results obtained. Key recommendations: the International Anorectal Physiology Working Group protocol for the performance of anorectal function testing recommends a standardized sequence of maneuvers to test rectoanal reflexes, anal tone and contractility, rectoanal coordination, and rectal sensation. Major findings not seen in healthy controls defined by the classification are as follows: rectoanal areflexia, anal hypotension and hypocontractility, rectal hyposensitivity, and hypersensitivity. Minor and inconclusive findings that can be present in health and require additional information prior to diagnosis include anal hypertension and dyssynergia
Experimental signatures of emergent quantum electrodynamics in PrHfO
In a quantum spin liquid, the magnetic moments of the constituent electron
spins evade classical long-range order to form an exotic state that is quantum
entangled and coherent over macroscopic length scales [1-2]. Such phases offer
promising perspectives for device applications in quantum information
technologies, and their study can reveal fundamentally novel physics in quantum
matter. Quantum spin ice is an appealing proposal of one such state, in which
the fundamental ground state properties and excitations are described by an
emergent U(1) lattice gauge theory [3-7]. This quantum-coherent regime has
quasiparticles that are predicted to behave like magnetic and electric
monopoles, along with a gauge boson playing the role of an artificial photon.
However, this emergent lattice quantum electrodynamics has proved elusive in
experiments. Here we report neutron scattering measurements of the rare-earth
pyrochlore magnet PrHfO that provide evidence for a quantum spin
ice ground state. We find a quasi-elastic structure factor with pinch points -
a signature of a classical spin ice - that are partially suppressed, as
expected in the quantum-coherent regime of the lattice field theory at finite
temperature. Our result allows an estimate for the speed of light associated
with magnetic photon excitations. We also reveal a continuum of inelastic spin
excitations, which resemble predictions for the fractionalized, topological
excitations of a quantum spin ice. Taken together, these two signatures suggest
that the low-energy physics of PrHfO can be described by emergent
quantum electrodynamics. If confirmed, the observation of a quantum spin ice
ground state would constitute a concrete example of a three-dimensional quantum
spin liquid - a topical state of matter which has so far mostly been explored
in lower dimensionalities.Comment: 15 pages, 3 figure
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