8,412 research outputs found
Strong and weak thermalization of infinite non-integrable quantum systems
When a non-integrable system evolves out of equilibrium for a long time,
local observables are expected to attain stationary expectation values,
independent of the details of the initial state. However, intriguing
experimental results with ultracold gases have shown no thermalization in
non-integrable settings, triggering an intense theoretical effort to decide the
question. Here we show that the phenomenology of thermalization in a quantum
system is much richer than its classical counterpart. Using a new numerical
technique, we identify two distinct thermalization regimes, strong and weak,
occurring for different initial states. Strong thermalization, intrinsically
quantum, happens when instantaneous local expectation values converge to the
thermal ones. Weak thermalization, well-known in classical systems, happens
when local expectation values converge to the thermal ones only after time
averaging. Remarkably, we find a third group of states showing no
thermalization, neither strong nor weak, to the time scales one can reliably
simulate.Comment: 12 pages, 21 figures, including additional materia
Matrix Product States: Symmetries and Two-Body Hamiltonians
We characterize the conditions under which a translationally invariant matrix
product state (MPS) is invariant under local transformations. This allows us to
relate the symmetry group of a given state to the symmetry group of a simple
tensor. We exploit this result in order to prove and extend a version of the
Lieb-Schultz-Mattis theorem, one of the basic results in many-body physics, in
the context of MPS. We illustrate the results with an exhaustive search of
SU(2)--invariant two-body Hamiltonians which have such MPS as exact ground
states or excitations.Comment: PDFLatex, 12 pages and 6 figure
Femtosecond time-resolved phase-change microscopy and ablation threshold calculations to understand ultrafast laser ablation
El trabajo recoge experimentos realizados en una configuración sonda-prueba haciendo uso de un láser de femtosegundos, con el objetivo de comprender los fenómenos de ablación de materiales sólidos ras irradiación con láseres de pulso ultracorto.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech
Gapless Hamiltonians for the toric code using the PEPS formalism
We study Hamiltonians which have Kitaev's toric code as a ground state, and
show how to construct a Hamiltonian which shares the ground space of the toric
code, but which has gapless excitations with a continuous spectrum in the
thermodynamic limit. Our construction is based on the framework of Projected
Entangled Pair States (PEPS), and can be applied to a large class of
two-dimensional systems to obtain gapless "uncle Hamiltonians".Comment: 8 pages, 2 figure
Fundamental limitations in the purifications of tensor networks
We show a fundamental limitation in the description of quantum many-body
mixed states with tensor networks in purification form. Namely, we show that
there exist mixed states which can be represented as a translationally
invariant (TI) matrix product density operator (MPDO) valid for all system
sizes, but for which there does not exist a TI purification valid for all
system sizes. The proof is based on an undecidable problem and on the
uniqueness of canonical forms of matrix product states. The result also holds
for classical states.Comment: v1: 11 pages, 1 figure. v2: very minor changes. About to appear in
Journal of Mathematical Physic
Virtues and Flaws of the Pauli Potential
Quantum simulations of complex fermionic systems suffer from a variety of
challenging problems. In an effort to circumvent these challenges, simpler
``semi-classical'' approaches have been used to mimic fermionic correlations
through a fictitious ``Pauli potential''. In this contribution we examine two
issues. First, we address some of the inherent difficulties in a widely used
version of the Pauli potential. Second, we refine such a potential in a manner
consistent with the most basic properties of a cold Fermi gas, such as its
momentum distribution and its two-body correlation function.Comment: 16 pages, 6 figure
Electroactivación de oocitos de pez cebra (Danio rerio)
[ES] El mejor tratamiento de electroactivación (B3) para oocitos de pez cebra supuso un 43% de activación frente al 18% cuando el estímulo es solo el contacto con agua[EN] In zebrafish, initial egg activation by water after being discharged from the ovarian stroma is followed by normal
cleavages only in inseminated eggs. When sperm (genetically active or not) is not used as activating agent, reproductive
techniques (as either nuclear transplant or intracytoplasmic sperm injection are ineff icient. In this work, three
experiments of egg activation by electric stimuli were performed: In the first, six activation treatments were compared
(Voltage × Pulses: 2.76 × 1; 2.76 × 2; 2.76 × 3 and 5.40 × 1; 5.40 × 2; 5.40 × 3). The group 5.40 × 3 showed the best results
(32% activated). In the second experiment, an electrical treatment of 20 min was carried out. It consisted in a sequence
of three equal electrical stimuli every 10 min (of 1 or 3 consecutive direct current square pulses for 20 ¿s each and
applied at two voltage levels, 2.76 V or 5.4 V). It was observed that the number of pulses negatively affected the rates
of damaged and lysed eggs. Moreover, only the 20 min treatment with the combination of 3 consecutive pulses at
2.76 V showed significant differences with their respective control group (43% vs 18% activated eggs, p < 0.05). In
the third experiment, negative effects of egg ageing were observed. In conclusion, the best activation treatment for
intact (non manipulated) zebrafish eggs concerns sequence B3. The electro-activation stimulus proposed here would
be the only one available once the egg has already been activated by water.Cardona Costa, J.; Perez-Camps, M.; García Ximenez, F. (2011). Electroactivation of zebrafish (danio rerio) eggs. Spanish Journal of Agricultural Research. 9(1):59-65. http://hdl.handle.net/10251/99491S59659
Influencia del ángulo de la proyección de abrasivos en la limpieza de materiales pétreos detríticos en Patrimonio Arquitectónico
In this research, the influence of the angle in abrasive blasting cleaning is studied on Montjuïc sandstone with black crust. After analyzing the properties of the soiling and the material, and their possible influence on the treatment, different cleaning tests were made at four different angles, keeping the complementary parameters constant. Taking the restorer’s perspective as a starting point, and in order to fulfill the practical requirements of an intervention —time and cost reduction—, tests were evaluated with USB digital microscope, stereomicroscope with 3D visualization and measurement, and colorimeter. From the results it is established that angles close to 75° minimize surface alteration, reducing differential erosion in the binding phases of detritic materials usually caused by this treatment.En este trabajo se estudia la influencia del ángulo de la proyección de abrasivos en la limpieza de una arenisca de Montjuïc con costra negra. Tras analizar las propiedades del material, de la suciedad y su posible influencia en el tratamiento, se realizan diferentes catas de limpieza con cuatro ángulos distintos manteniendo constantes el resto de parámetros de la proyección. Partiendo de la visión del conservador-restaurador y de un carácter práctico según las necesidades reales de una intervención —reducción de tiempos y costes—, los ensayos se evalúan con microscopio digital USB, microscopio estereoscópico con visualización y medición en 3D y colorímetro. De los resultados se puede determinar que ángulos cercanos a 75° minimizan la alteración de la superficie al reducir la erosión diferencial de las fases de unión que el tratamiento normalmente provoca en los materiales detríticos
Quantum kinetic Ising models
We introduce a quantum generalization of classical kinetic Ising models,
described by a certain class of quantum many body master equations. Similarly
to kinetic Ising models with detailed balance that are equivalent to certain
Hamiltonian systems, our models reduce to a set of Hamiltonian systems
determining the dynamics of the elements of the many body density matrix. The
ground states of these Hamiltonians are well described by matrix product, or
pair entangled projected states. We discuss critical properties of such
Hamiltonians, as well as entanglement properties of their low energy states.Comment: 20 pages, 4 figures, minor improvements, accepted in New Journal of
Physic
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