40,113 research outputs found
Noncommutative Einstein-Maxwell pp-waves
The field equations coupling a Seiberg-Witten electromagnetic field to
noncommutative gravity, as described by a formal power series in the
noncommutativity parameters , is investigated. A large
family of solutions, up to order one in , describing
Einstein-Maxwell null pp-waves is obtained. The order-one contributions can be
viewed as providing noncommutative corrections to pp-waves. In our solutions,
noncommutativity enters the spacetime metric through a conformal factor and is
responsible for dilating/contracting the separation between points in the same
null surface. The noncommutative corrections to the electromagnetic waves,
while preserving the wave null character, include constant polarization, higher
harmonic generation and inhomogeneous susceptibility. As compared to pure
noncommutative gravity, the novelty is that nonzero corrections to the metric
already occur at order one in .Comment: 19 revtex pages. One refrence suppressed, two references added. Minor
wording changes in the abstract, introduction and conclusio
Universality in the off-equilibrium critical dynamics of the diluted Ising model
We study the off-equilibrium critical dynamics of the three dimensional
diluted Ising model. We compute the dynamical critical exponent and we show
that it is independent of the dilution only when we take into account the
scaling-corrections to the dynamics. Finally we will compare our results with
the experimental data.Comment: Final Version, 5 Latex pages (RevTeX) plus 3 eps figure
Mapping the circumstellar SiO maser emission in R Leo
The study of the innermost circumstellar layers around AGB stars is crucial
to understand how these envelopes are formed and evolve. The SiO maser emission
occurs at a few stellar radii from the central star, providing direct
information on the stellar pulsation and on the chemical and physical
properties of these regions. Our data also shed light on several aspects of the
SiO maser pumping theory that are not well understood yet. We aim to determine}
the relative spatial distribution of the 43 GHz and 86 GHz SiO maser lines in
the oxygen-rich evolved star R Leo. We have imaged with milliarcsecond
resolution, by means of Very Long Baseline Interferometry, the 43 GHz (28SiO
v=1, 2 J=1-0 and 29SiO v=0 J=1-0) and 86 GHz (28SiO v=1 J=2-1 and 29SiO v=0
J=2-1) masing regions. We confirm previous results obtained in other
oxygen-rich envelopes. In particular, when comparing the 43 GHz emitting
regions, the 28SiO v=2 transition is produced in an inner layer, closer to the
central star. On the other hand, the 86 GHz line arises in a clearly farther
shell. We have also mapped for the first time the 29SiO v=0 J=1-0 emission in R
Leo. The already reported discrepancy between the observed distributions of the
different maser lines and the theoretical predictions is also found in R Leo.Comment: accepted for publication in A&
Energy partition and segregation for an intruder in a vibrated granular system under gravity
The difference of temperatures between an impurity and the surrounding gas in
an open vibrated granular system is studied. It is shown that, in spite of the
high inhomogeneity of the state, the temperature ratio remains constant in the
bulk of the system. The lack of energy equipartition is associated to the
change of sign of the pressure diffusion coefficient for the impurity at
certain values of the parameters of the system, leading to a segregation
criterium. The theoretical predictions are consistent with previous
experimental results, and also in agreement with molecular dynamics simulation
results reported in this paper.Comment: To appear in Phys. Rev. Let
Quantum Phase Transitions detected by a local probe using Time Correlations and Violations of Leggett-Garg Inequalities
In the present paper we introduce a way of identifying quantum phase
transitions of many-body systems by means of local time correlations and
Leggett-Garg inequalities. This procedure allows to experimentally determine
the quantum critical points not only of finite-order transitions but also those
of infinite order, as the Kosterlitz-Thouless transition that is not always
easy to detect with current methods. By means of simple analytical arguments
for a general spin- Hamiltonian, and matrix product simulations of
one-dimensional and anisotropic models, we argue that
finite-order quantum phase transitions can be determined by singularities of
the time correlations or their derivatives at criticality. The same features
are exhibited by corresponding Leggett-Garg functions, which noticeably
indicate violation of the Leggett-Garg inequalities for early times and all the
Hamiltonian parameters considered. In addition, we find that the infinite-order
transition of the model at the isotropic point can be revealed by the
maximal violation of the Leggett-Garg inequalities. We thus show that quantum
phase transitions can be identified by purely local measurements, and that
many-body systems constitute important candidates to observe experimentally the
violation of Leggett-Garg inequalities.Comment: Minor changes, 11 pages, 11 figures. Final version published in Phys.
Rev.
Quantum Hysteresis in Coupled Light-Matter Systems
We investigate the non-equilibrium quantum dynamics of a canonical
light-matter system, namely the Dicke model, when the light-matter interaction
is ramped up and down through a cycle across the quantum phase transition. Our
calculations reveal a rich set of dynamical behaviors determined by the cycle
times, ranging from the slow, near adiabatic regime through to the fast, sudden
quench regime. As the cycle time decreases, we uncover a crossover from an
oscillatory exchange of quantum information between light and matter that
approaches a reversible adiabatic process, to a dispersive regime that
generates large values of light-matter entanglement. The phenomena uncovered in
this work have implications in quantum control, quantum interferometry, as well
as in quantum information theory.Comment: 9 pages and 4 figure
Functional advantages offered by many-body coherences in biochemical systems
Quantum coherence phenomena driven by electronic-vibrational (vibronic)
interactions, are being reported in many pulse (e.g. laser) driven chemical and
biophysical systems. But what systems-level advantage(s) do such many-body
coherences offer to future technologies? We address this question for pulsed
systems of general size N, akin to the LHCII aggregates found in green plants.
We show that external pulses generate vibronic states containing particular
multipartite entanglements, and that such collective vibronic states increase
the excitonic transfer efficiency. The strength of these many-body coherences
and their robustness to decoherence, increase with aggregate size N and do not
require strong electronic-vibrational coupling. The implications for energy and
information transport are discussed.Comment: arXiv admin note: text overlap with arXiv:1706.0776
Pulsed Generation of Quantum Coherences and Non-classicality in Light-Matter Systems
We show that a pulsed stimulus can be used to generate many-body quantum
coherences in light-matter systems of general size. Specifically, we calculate
the exact real-time evolution of a driven, generic out-of-equilibrium system
comprising an arbitrary number N qubits coupled to a global boson field. A
novel form of dynamically-driven quantum coherence emerges for general N and
without having to access the empirically challenging strong-coupling regime.
Its properties depend on the speed of the changes in the stimulus.
Non-classicalities arise within each subsystem that have eluded previous
analyses. Our findings show robustness to losses and noise, and have potential
functional implications at the systems level for a variety of nanosystems,
including collections of N atoms, molecules, spins, or superconducting qubits
in cavities -- and possibly even vibration-enhanced light harvesting processes
in macromolecules.Comment: 9 pages, 4 figure
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