893 research outputs found
Information propagation for interacting particle systems
We show that excitations of interacting quantum particles in lattice models
always propagate with a finite speed of sound. Our argument is simple yet
general and shows that by focusing on the physically relevant observables one
can generally expect a bounded speed of information propagation. The argument
applies equally to quantum spins, bosons such as in the Bose-Hubbard model,
fermions, anyons, and general mixtures thereof, on arbitrary lattices of any
dimension. It also pertains to dissipative dynamics on the lattice, and
generalizes to the continuum for quantum fields. Our result can be seen as a
meaningful analogue of the Lieb-Robinson bound for strongly correlated models.Comment: 4 pages, 1 figure, minor change
Constructing topological models by symmetrization: A PEPS study
Symmetrization of topologically ordered wavefunctions is a powerful method
for constructing new topological models. Here, we study wavefunctions obtained
by symmetrizing quantum double models of a group in the Projected Entangled
Pair States (PEPS) formalism. We show that symmetrization naturally gives rise
to a larger symmetry group which is always non-abelian. We prove
that by symmetrizing on sufficiently large blocks, one can always construct
wavefunctions in the same phase as the double model of . In order to
understand the effect of symmetrization on smaller patches, we carry out
numerical studies for the toric code model, where we find strong evidence that
symmetrizing on individual spins gives rise to a critical model which is at the
phase transitions of two inequivalent toric codes, obtained by anyon
condensation from the double model of .Comment: 10 pages. v2: accepted versio
Edge theories in Projected Entangled Pair State models
We study the edge physics of gapped quantum systems in the framework of
Projected Entangled Pair State (PEPS) models. We show that the effective
low-energy model for any region acts on the entanglement degrees of freedom at
the boundary, corresponding to physical excitations located at the edge. This
allows us to determine the edge Hamiltonian in the vicinity of PEPS models, and
we demonstrate that by choosing the appropriate bulk perturbation, the edge
Hamiltonian can exhibit a rich phase diagram and phase transitions. While for
models in the trivial phase any Hamiltonian can be realized at the edge, we
show that for topological models, the edge Hamiltonian is constrained by the
topological order in the bulk which can e.g. protect a ferromagnetic Ising
chain at the edge against spontaneous symmetry breaking.Comment: 5 pages, 4 figure
Coherent states of a charged particle in a uniform magnetic field
The coherent states are constructed for a charged particle in a uniform
magnetic field based on coherent states for the circular motion which have
recently been introduced by the authors.Comment: 2 eps figure
Computational Complexity of interacting electrons and fundamental limitations of Density Functional Theory
One of the central problems in quantum mechanics is to determine the ground
state properties of a system of electrons interacting via the Coulomb
potential. Since its introduction by Hohenberg, Kohn, and Sham, Density
Functional Theory (DFT) has become the most widely used and successful method
for simulating systems of interacting electrons, making their original work one
of the most cited in physics. In this letter, we show that the field of
computational complexity imposes fundamental limitations on DFT, as an
efficient description of the associated universal functional would allow to
solve any problem in the class QMA (the quantum version of NP) and thus
particularly any problem in NP in polynomial time. This follows from the fact
that finding the ground state energy of the Hubbard model in an external
magnetic field is a hard problem even for a quantum computer, while given the
universal functional it can be computed efficiently using DFT. This provides a
clear illustration how the field of quantum computing is useful even if quantum
computers would never be built.Comment: 8 pages, 3 figures. v2: Version accepted at Nature Physics; differs
significantly from v1 (including new title). Includes an extra appendix (not
contained in the journal version) on the NP-completeness of Hartree-Fock,
which is taken from v
Cosmic ray short burst observed with the Global Muon Detector Network (GMDN) on June 22, 2015
We analyze the short cosmic ray intensity increase ("cosmic ray burst": CRB)
on June 22, 2015 utilizing a global network of muon detectors and derive the
global anisotropy of cosmic ray intensity and the density (i.e. the
omnidirectional intensity) with 10-minute time resolution. We find that the CRB
was caused by a local density maximum and an enhanced anisotropy of cosmic rays
both of which appeared in association with Earth's crossing of the heliospheric
current sheet (HCS). This enhanced anisotropy was normal to the HCS and
consistent with a diamagnetic drift arising from the spatial gradient of cosmic
ray density, which indicates that cosmic rays were drifting along the HCS from
the north of Earth. We also find a significant anisotropy along the HCS,
lasting a few hours after the HCS crossing, indicating that cosmic rays
penetrated into the inner heliosphere along the HCS. Based on the latest
geomagnetic field model, we quantitatively evaluate the reduction of the
geomagnetic cut-off rigidity and the variation of the asymptotic viewing
direction of cosmic rays due to a major geomagnetic storm which occurred during
the CRB and conclude that the CRB is not caused by the geomagnetic storm, but
by a rapid change in the cosmic ray anisotropy and density outside the
magnetosphere.Comment: accepted for the publication in the Astrophysical Journa
The role of depression in the association between mobilisation timing and live discharge after hip fracture surgery: Secondary analysis of the UK National Hip Fracture Database
Purpose
The aim was to compare the probability of discharge after hip fracture surgery conditional on being alive and in hospital between patients mobilised within and beyond 36-hours of surgery across groups defined by depression.
Methods
Data were taken from the National Hip Fracture Database and included patients 60 years of age or older who underwent hip fracture surgery in England and Wales between 2014 and 2016. The conditional probability of postsurgical live discharge was estimated for patients mobilised early and for patients mobilised late across groups with and without depression. The association between mobilisation timing and the conditional probability of live discharge were also estimated separately through adjusted generalized linear models.
Results
Data were analysed for 116,274 patients. A diagnosis of depression was present in 8.31% patients. In those with depression, 7,412 (76.7%) patients mobilised early. In those without depression, 84,085 (78.9%) patients mobilised early. By day 30 after surgery, the adjusted odds ratio of discharge among those who mobilised early compared to late was 1.79 (95% CI: 1.56â2.05, p<0.001) and 1.92 (95% CI: 1.84â2.00, p<0.001) for those with and without depression, respectively.
Conclusion
A similar proportion of patients with depression mobilised early after hip fracture surgery when compared to those without a diagnosis of depression. The association between mobilisation timing and time to live discharge was observed for patients with and without depression
Lieb-Robinson Bounds for Harmonic and Anharmonic Lattice Systems
We prove Lieb-Robinson bounds for the dynamics of systems with an infinite
dimensional Hilbert space and generated by unbounded Hamiltonians. In
particular, we consider quantum harmonic and certain anharmonic lattice
systems
Single Transfer-Excitation Resonance Observed Via the Two-Photon Decay in He-like GeÂłâ°âș
We measured the 2E1 decay of the 1s2s 1S0 â1s2 1S0 transition in He-like germanium for 12- to 19-MeV/u Ge31+ +H2 collisions. The resonant population of the 2s2p 1P1 state by transfer excitation was isolated due to its cascading to the 1s2s 1S0 state. The experimental cross sections compare well with calculations using dielectronic recombination rates. The method gives the unique possibility to populate selectively the 1S0 state in heavy He-like ions
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