372 research outputs found
A matrix product state based algorithm for determining dispersion relations of quantum spin chains with periodic boundary conditions
We study a matrix product state (MPS) algorithm to approximate excited states
of translationally invariant quantum spin systems with periodic boundary
conditions. By means of a momentum eigenstate ansatz generalizing the one of
\"Ostlund and Rommer [1], we separate the Hilbert space of the system into
subspaces with different momentum. This gives rise to a direct sum of effective
Hamiltonians, each one corresponding to a different momentum, and we determine
their spectrum by solving a generalized eigenvalue equation. Surprisingly, many
branches of the dispersion relation are approximated to a very good precision.
We benchmark the accuracy of the algorithm by comparison with the exact
solutions of the quantum Ising and the antiferromagnetic Heisenberg spin-1/2
model.Comment: 13 pages, 11 figures, 5 table
Entropy growth of shift-invariant states on a quantum spin chain
We study the entropy of pure shift-invariant states on a quantum spin chain.
Unlike the classical case, the local restrictions to intervals of length
are typically mixed and have therefore a non-zero entropy which is,
moreover, monotonically increasing in . We are interested in the asymptotics
of the total entropy. We investigate in detail a class of states derived from
quasi-free states on a CAR algebra. These are characterised by a measurable
subset of the unit interval. As the entropy density is known to vanishes,
is sublinear in . For states corresponding to unions of finitely many
intervals, is shown to grow slower than . Numerical
calculations suggest a behaviour. For the case with infinitely many
intervals, we present a class of states for which the entropy increases
as where can take any value in .Comment: 18 pages, 2 figure
Rigorous free fermion entanglement renormalization from wavelet theory
We construct entanglement renormalization schemes which provably approximate
the ground states of non-interacting fermion nearest-neighbor hopping
Hamiltonians on the one-dimensional discrete line and the two-dimensional
square lattice. These schemes give hierarchical quantum circuits which build up
the states from unentangled degrees of freedom. The circuits are based on pairs
of discrete wavelet transforms which are approximately related by a
"half-shift": translation by half a unit cell. The presence of the Fermi
surface in the two-dimensional model requires a special kind of circuit
architecture to properly capture the entanglement in the ground state. We show
how the error in the approximation can be controlled without ever performing a
variational optimization.Comment: 15 pages, 10 figures, one theore
Thermal States as Convex Combinations of Matrix Product States
We study thermal states of strongly interacting quantum spin chains and prove
that those can be represented in terms of convex combinations of matrix product
states. Apart from revealing new features of the entanglement structure of
Gibbs states our results provide a theoretical justification for the use of
White's algorithm of minimally entangled typical thermal states. Furthermore,
we shed new light on time dependent matrix product state algorithms which yield
hydrodynamical descriptions of the underlying dynamics.Comment: v3: 10 pages, 2 figures, final published versio
Matrix product operators for symmetry-protected topological phases: Gauging and edge theories
Projected entangled pair states (PEPS) provide a natural ansatz for the
ground states of gapped, local Hamiltonians in which global characteristics of
a quantum state are encoded in properties of local tensors. We develop a
framework to describe on-site symmetries, as occurring in systems exhibiting
symmetry-protected topological (SPT) quantum order, in terms of virtual
symmetries of the local tensors expressed as a set of matrix product operators
(MPOs) labeled by distinct group elements. These MPOs describe the possibly
anomalous symmetry of the edge theory, whose local degrees of freedom are
concretely identified in a PEPS. A classification of SPT phases is obtained by
studying the obstructions to continuously deforming one set of MPOs into
another, recovering the results derived for fixed-point models [X. Chen et al.,
Phys. Rev. B 87, 155114 (2013)]. Our formalism accommodates perturbations away
from fixed point models, opening the possibility of studying phase transitions
between different SPT phases. We also demonstrate that applying the recently
developed quantum state gauging procedure to a SPT PEPS yields a PEPS with
topological order determined by the initial symmetry MPOs. The MPO framework
thus unifies the different approaches to classifying SPT phases, via
fixed-points models, boundary anomalies, or gauging the symmetry, into the
single problem of classifying inequivalent sets of matrix product operator
symmetries that are defined purely in terms of a PEPS.Comment: 16 + 19 pages, 13 figures; v2 substantial changes to all sections,
new appendices added; v3 published versio
Extending additivity from symmetric to asymmetric channels
We prove a lemma which allows one to extend results about the additivity of
the minimal output entropy from highly symmetric channels to a much larger
class. A similar result holds for the maximal output -norm. Examples are
given showing its use in a variety of situations. In particular, we prove the
additivity and the multiplicativity for the shifted depolarising channel.Comment: 8 pages. This is the latest version of the first half of the original
paper. The other half will appear in another pape
The Spatial Limitations of Current Neutral Models of Biodiversity
The unified neutral theory of biodiversity and biogeography is increasingly accepted as an informative null model of community composition and dynamics. It has successfully produced macro-ecological patterns such as species-area relationships and species abundance distributions. However, the models employed make many unrealistic auxiliary assumptions. For example, the popular spatially implicit version assumes a local plot exchanging migrants with a large panmictic regional source pool. This simple structure allows rigorous testing of its fit to data. In contrast, spatially explicit models assume that offspring disperse only limited distances from their parents, but one cannot as yet test the significance of their fit to data. Here we compare the spatially explicit and the spatially implicit model, fitting the most-used implicit model (with two levels, local and regional) to data simulated by the most-used spatially explicit model (where offspring are distributed about their parent on a grid according to either a radially symmetric Gaussian or a ‘fat-tailed’ distribution). Based on these fits, we express spatially implicit parameters in terms of spatially explicit parameters. This suggests how we may obtain estimates of spatially explicit parameters from spatially implicit ones. The relationship between these parameters, however, makes no intuitive sense. Furthermore, the spatially implicit model usually fits observed species-abundance distributions better than those calculated from the spatially explicit model's simulated data. Current spatially explicit neutral models therefore have limited descriptive power. However, our results suggest that a fatter tail of the dispersal kernel seems to improve the fit, suggesting that dispersal kernels with even fatter tails should be studied in future. We conclude that more advanced spatially explicit models and tools to analyze them need to be developed
- …