24,207 research outputs found
Using off-diagonal confinement as a cooling method
In a recent letter [Phys. Rev. Lett. 104, 167201 (2010)] we proposed a new
confining method for ultracold atoms on optical lattices, based on off-diagonal
confinement (ODC). This method was shown to have distinct advantages over the
conventional diagonal confinement (DC) that makes use of a trapping potential,
including the existence of pure Mott phases and highly populated condensates.
In this paper we show that the ODC method can also lead to temperatures that
are smaller than with the conventional DC method, depending on the control
parameters. We determine these parameters using exact diagonalizations for the
hard-core case, then we extend our results to the soft-core case by performing
quantum Monte Carlo (QMC) simulations for both DC and ODC systems at fixed
temperatures, and analysing the corresponding entropies. We also propose a
method for measuring the entropy in QMC simulations.Comment: 6 pages, 6 figure
Optimization of photon storage fidelity in ordered atomic arrays
A major application for atomic ensembles consists of a quantum memory for
light, in which an optical state can be reversibly converted to a collective
atomic excitation on demand. There exists a well-known fundamental bound on the
storage error, when the ensemble is describable by a continuous medium governed
by the Maxwell-Bloch equations. The validity of this model can break down,
however, in systems such as dense, ordered atomic arrays, where strong
interference in emission can give rise to phenomena such as subradiance and
"selective" radiance. Here, we develop a general formalism that finds the
maximum storage efficiency for a collection of atoms with discrete, known
positions, and a given spatial mode in which an optical field is sent. As an
example, we apply this technique to study a finite two-dimensional square array
of atoms. We show that such a system enables a storage error that scales with
atom number like ,
and that, remarkably, an array of just atoms in principle allows
for an efficiency comparable to a disordered ensemble with optical depth of
around 600.Comment: paper is now identical to published versio
Coherent State Description of the Ground State in the Tavis-Cummings Model and its Quantum Phase Transitions
Quantum phase transitions and observables of interest of the ground state in
the Tavis-Cummings model are analyzed, for any number of atoms, by using a
tensorial product of coherent states. It is found that this "trial" state
constitutes a very good approximation to the exact quantum solution, in that it
globally reproduces the expectation values of the matter and field observables.
These include the population and dipole moments of the two-level atoms and the
squeezing parameter. Agreement in the field-matter entanglement and in the
fidelity measures, of interest in quantum information theory, is also found.The
analysis is carried out in all three regions defined by the separatrix which
gives rise to the quantum phase transitions. It is argued that this agreement
is due to the gaussian structure of the probability distributions of the
constant of motion and the number of photons. The expectation values of the
ground state observables are given in analytic form, and the change of the
ground state structure of the system when the separatrix is crossed is also
studied.Comment: 38 pages, 16 figure
Energy landscape of a simple model for strong liquids
We calculate the statistical properties of the energy landscape of a minimal
model for strong network-forming liquids. Dynamics and thermodynamic properties
of this model can be computed with arbitrary precision even at low
temperatures. A degenerate disordered ground state and logarithmic statistics
for the energy distribution are the landscape signatures of strong liquid
behavior. Differences from fragile liquid properties are attributed to the
presence of a discrete energy scale, provided by the particle bonds, and to the
intrinsic degeneracy of topologically disordered networks.Comment: Revised versio
Non-Gaussian energy landscape of a simple model for strong network-forming liquids: accurate evaluation of the configurational entropy
We present a numerical study of the statistical properties of the potential
energy landscape of a simple model for strong network-forming liquids. The
model is a system of spherical particles interacting through a square well
potential, with an additional constraint that limits the maximum number of
bonds, , per particle. Extensive simulations have been carried out
as a function of temperature, packing fraction, and . The dynamics
of this model are characterized by Arrhenius temperature dependence of the
transport coefficients and by nearly exponential relaxation of dynamic
correlators, i.e. features defining strong glass-forming liquids. This model
has two important features: (i) landscape basins can be associated with bonding
patterns; (ii) the configurational volume of the basin can be evaluated in a
formally exact way, and numerically with arbitrary precision. These features
allow us to evaluate the number of different topologies the bonding pattern can
adopt. We find that the number of fully bonded configurations, i.e.
configurations in which all particles are bonded to neighbors, is
extensive, suggesting that the configurational entropy of the low temperature
fluid is finite. We also evaluate the energy dependence of the configurational
entropy close to the fully bonded state, and show that it follows a logarithmic
functional form, differently from the quadratic dependence characterizing
fragile liquids. We suggest that the presence of a discrete energy scale,
provided by the particle bonds, and the intrinsic degeneracy of fully bonded
disordered networks differentiates strong from fragile behavior.Comment: Final version. Journal of Chemical Physics 124, 204509 (2006
Characterization of Carbon-Contaminated B4C-Coated Optics after Chemically Selective Cleaning with Low-Pressure RF Plasma
Boron carbide (B4C) is one of the few materials that is expected to be mostly
resilient with respect to the extremely high brilliance of the photon beam
generated by free electron lasers (FELs) and is thus of considerable interest
for optical applications in this field. However, as in the case of many other
optics operated at modern light source facilities, B4C-coated optics are
subject to ubiquitous carbon contaminations. These contaminations represent a
serious issue for the operation of high performance FEL beamlines due to severe
reduction of photon flux, beam coherence, creation of destructive interference,
and scattering losses. A variety of B4C cleaning technologies were developed at
different laboratories with varying success. We present a study regarding the
low-pressure RF plasma cleaning of carbon contaminated B4C test samples via
inductively coupled O2/Ar, H2/Ar, and pure O2 RF plasma produced following
previous studies using the same IBSS GV10x downstream plasma source. Results
regarding the chemistry, morphology as well as other aspects of the B4C optical
coating before and after the plasma cleaning are reported. We conclude from
these comparative plasma processes that pure O2 feedstock plasma only exhibits
the required chemical selectivity for maintaining the integrity of the B4C
optical coating.Comment: 27 pages, 15 figure
Quantitative determination of five hydroxy acids, precursors of relevant wine aroma compounds in wine and other alcoholic beverages
A method for the quantitative determination of 2-hydroxy-2-methylbutanoic (2OH2MB), 2-hydroxy-3-methylbutanoic (2OH3MB), 3-hydroxy-3-methylbutanoic (3OH3MB), 2-hydroxy-4-methylpentanoic (2OH4MP) and 3-hydroxybutanoic (3OHB) acids has been optimized, validated and applied to a set of wines and other alcoholic beverages. The analytes were preconcentrated in a solid phase extraction cartridge and derivatized with 2, 3, 4, 5, 6-pentafluorobenzyl bromide at room temperature for 30 min, followed by GC-MS analysis. Detection limits were between 0.5 and 29 µg L-1, and linearity was maintained up to 3 or 12 mg L-1, depending on the analyte. Recovery values were between 85 and 106 %, and reproducibility was better than 12 % RSD in most cases. The first specific study of these analytes in wine and other alcoholic beverages is herein reported. Concentrations ranged from the method detection limits to 7820, 519, 8510, 3470 and 2500 µg L-1 for 2OH2MB, 2OH3MB, 3OH3MB, 2OH4MP and 3OHB, respectively, which may have relevant sensory effects. These products were not found in distillates (except 3OHB) but were all present in beer. 2OH2MB, 3OH3MB and 3OHB were found in unfermented grape derivatives. Sherry wines had the highest levels of all except for 3OHB
Determination of 2-, 3-, 4-methylpentanoic and cyclohexanecarboxylic acids in wine: development of a selective method based on solid phase extraction and gas chromatography-negative chemical ionization mass spectrometry and its application to different wines and alcoholic beverages
A method to analyse 2-methylpentanoic, 3-methylpentanoic and 4-methylpentanoic acids as well as cyclohexanecarboxylic acid has been developed and applied to wine and other alcoholic beverages. Selective isolation with solid phase extraction, derivatization with 2,3,4,5,6-pentafluorobenzyl bromide at room temperature for 30 minutes, and further analysis by gas chromatography-mass spectrometry in negative chemical ionization mode provides detection limits between 0.4 and 2.4 ng/L. Good linearity up to 3.6 µg/L, satisfactory reproducibility (RSD < 10%) and signal recovery of around 100% represent a robust method of analysis. Concentration data of these analytes in wine and other alcoholic beverages are reported for the first time. The levels found ranged from the method detection limits to 2630 ng/L, 2040 ng/L and 3810 ng/L for 2-, 3- and 4- methylpentanoic acids, respectively, and to 1780 ng/L for cyclohexanecarboxylic acid. There are significant differences depending on the type of wine or beverage. Distilled beverages, beer and aged wines have higher contents in methylpentanoic and cyclohexanecarboxylic acids
The XDSPRES CL-based package for reducing OSIRIS cross-dispersed spectra
We present a description of the CL-based package XDSPRES, which aims at being
a complete reducing facility for cross-dispersed spectra taken with the Ohio
State Infrared Imager/Spectrometer, as installed at the SOAR telescope. This
instrument provides spectra in the range between 1.2um and 2.35um in a single
exposure, with resolving power of R ~ 1200. XDSPRES consists of two tasks,
namely xdflat and doosiris. The former is a completely automated code for
preparing normalized flat field images from raw flat field exposures. Doosiris
was designed to be a complete reduction pipeline, requiring a minimum of user
interaction. General steps towards a fully reduced spectrum are explained, as
well as the approach adopted by our code. The software is available to the
community through the web site http://www.if.ufrgs.br/~ruschel/software.Comment: 14 pages, 10 figure
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