1,549 research outputs found
Mesoscopic mean-field theory for spin-boson chains in quantum optical systems
We present a theoretical description of a system of many spins strongly coupled to a bosonic chain. We rely on the use of a spin-wave theory describing the Gaussian fluctuations around the mean-field solution, and focus on spin-boson chains arising as a generalization of the Dicke Hamiltonian. Our model is motivated by experimental setups such as trapped ions, or atoms/qubits coupled to cavity arrays. This situation corresponds to the cooperative (E⊗β) Jahn-Teller distortion studied in solid-state physics. However, the ability to tune the parameters of the model in quantum optical setups opens up a variety of novel intriguing situations. The main focus of this paper is to review the spin-wave theoretical description of this problem as well as to test the validity of mean-field theory. Our main result is that deviations from mean-field effects are determined by the interplay between magnetic order and mesoscopic cooperativity effects, being the latter strongly size-dependent
Molecular Hydrogen Kinematics in Cepheus A
We present the radial velocity structure of the molecular hydrogen outflows
associated to the star forming region Cepheus A. This structure is derived from
doppler shift of the H_2 v=1-0 S(1) emission line obtained by Fabry-Perot
spectroscopy. The East and West regions of emission, called Cep A(E) and Cep
A(W), show radial velocities in the range -20 to 0 km/s with respect to the
molecular cloud. Cep A(W) shows an increasing velocity with position offset
from the core indicating the existence of a possible accelarating machanism.
Cep A(E) has an almost constant mean radial velocity of -18 km/s along the
region although with a large dispersion in velocity, indicating the possibility
of a turbulent outflow. A detailed analysis of the Cep A(E) region shows
evidence for the presence of a Mach disk on that outflow. Also, we argue that
the presence of a velocity gradient in Cep A(W) is indicative of a C-shock in
this region. Following Riera et al. (2003), we analyzed the data using wavelet
analysis to study the line width and the central radial velocity distributions.
We found that both outflows have complex spatial and velocity structures
characteristic of a turbulent flow.Comment: 24 pages, 15 figure
Simulating quantum-optical phenomena with cold atoms in optical lattices
We propose a scheme involving cold atoms trapped in optical lattices to
observe different phenomena traditionally linked to quantum-optical systems.
The basic idea consists of connecting the trapped atomic state to a non-trapped
state through a Raman scheme. The coupling between these two types of atoms
(trapped and free) turns out to be similar to that describing light-matter
interaction within the rotating-wave approximation, the role of matter and
photons being played by the trapped and free atoms, respectively. We explain in
particular how to observe phenomena arising from the collective spontaneous
emission of atomic and harmonic oscillator samples such as superradiance and
directional emission. We also show how the same setup can simulate Bose-Hubbard
Hamiltonians with extended hopping as well as Ising models with long-range
interactions. We believe that this system can be realized with state of the art
technology
Effective Spin Quantum Phases in Systems of Trapped Ions
A system of trapped ions under the action of off--resonant standing--waves
can be used to simulate a variety of quantum spin models. In this work, we
describe theoretically quantum phases that can be observed in the simplest
realization of this idea: quantum Ising and XY models. Our numerical
calculations with the Density Matrix Renormalization Group method show that
experiments with ion traps should allow one to access general properties of
quantum critical systems. On the other hand, ion trap quantum spin models show
a few novel features due to the peculiarities of induced effective spin--spin
interactions which lead to interesting effects like long--range quantum
correlations and the coexistence of different spin phases.Comment: 11 pages, 13 figure
Many-body theory of the quantum mirage
In recent scanning tunneling microscopy experiments, confinement in an
elliptical corral has been used to project the Kondo effect from one focus to
the other one. I solve the Anderson model at arbitrary temperatures, for an
impurity hybridized with eigenstates of an elliptical corral, each of which has
a resonant level width delta. This width is crucial. If delta < 20 meV, the
Kondo peak disappears, while if delta > 80 meV, the mirage disappears. For
particular conditions, a stronger mirage with the impurity out of the foci is
predicted.Comment: 5 pages, 5 figures. Some clarifications of the method added, and a
reference included to show that the hybridization of the impurity with bulk
states can be neglecte
Trapped Rydberg Ions: From Spin Chains to Fast Quantum Gates
We study the dynamics of Rydberg ions trapped in a linear Paul trap, and
discuss the properties of ionic Rydberg states in the presence of the static
and time-dependent electric fields constituting the trap. The interactions in a
system of many ions are investigated and coupled equations of the internal
electronic states and the external oscillator modes of a linear ion chain are
derived. We show that strong dipole-dipole interactions among the ions can be
achieved by microwave dressing fields. Using low-angular momentum states with
large quantum defect the internal dynamics can be mapped onto an effective spin
model of a pair of dressed Rydberg states that describes the dynamics of
Rydberg excitations in the ion crystal. We demonstrate that excitation transfer
through the ion chain can be achieved on a nanosecond timescale and discuss the
implementation of a fast two-qubit gate in the ion chain.Comment: 26 pages, 9 figure
Impacts of urbanization around Mediterranean cities : changes in ecosystem service supply
Unidad de excelencia MarÃa de Maeztu MdM-2015-0552Urbanization is an important driver of changes in land cover in the Mediterranean Basin and it is likely to impact the supply and demand of ecosystem services (ES). The most significant land cover changes occur in the peri-urban zone, but little is known about how these changes affect the ES supply. For eight European and four North African cities, we have quantified changes in peri-urban land cover, for periods of sixteen years (1990-2006) in the Northern African, and twenty-two years (1990-2012) in the European cities, respectively. Using an expert-based method, we derived quantitative estimates of the dynamics in the supply of twenty-seven ES. The nature of land cover changes slightly differed between European and North African Mediterranean cities, but overall it increased in urban areas and decreased in agricultural land. The capacity of the peri-urban areas of Mediterranean cities to supply ES generally reduced over the last 20-30 years. For nine ES the potential supply actually increased for all four North African cities and three out of the eight European cities. Across all cities, the ES timber, wood fuel and religious and spiritual experience increased. Given the expected increase of urban population in the Mediterranean Basin and the current knowledge of ES deficits in urban areas, the overall decrease in ES supply capacity of peri-urban areas is a risk for human well-being in the Mediterranean and poses a serious challenge for the Sustainable Development Goals in the Mediterranean basin
COBRA - Double beta decay searches using CdTe detectors
A new approach (called COBRA) for investigating double beta decay using CdTe
(CdZnTe) semiconductor detectors is proposed. It follows the idea that source
and detector are identical. This will allow simultaneous measurements of 5
- and 4 - emitters at once. Half-life limits
for neutrinoless double beta decay of Cd-116 and Te-130 can be improved by more
than one order of magnitude with respect to current limits and sensitivities on
the effective Majorana neutrino mass of less than 1 eV can be obtained.
Furthermore, for the first time a realistic chance of observing double electron
capture processes exists. Additional searches for rare processes like the
4-fold forbidden Cd-113 -decay, the electron capture of Te-123 and dark
matter detection can be performed. The achievable limits are evaluated for 10
kg of such detectors and can be scaled accordingly towards higher detector
masses because of the modular design of the proposed experiment.Comment: 13 pages, 3 eps-figures, submitte
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