837 research outputs found

### Ultracold atomic Fermi-Bose mixtures in bichromatic optical dipole traps: a novel route to study fermion superfluidity

The study of low density, ultracold atomic Fermi gases is a promising avenue
to understand fermion superfluidity from first principles. One technique
currently used to bring Fermi gases in the degenerate regime is sympathetic
cooling through a reservoir made of an ultracold Bose gas. We discuss a
proposal for trapping and cooling of two-species Fermi-Bose mixtures into
optical dipole traps made from combinations of laser beams having two different
wavelengths. In these bichromatic traps it is possible, by a proper choice of
the relative laser powers, to selectively trap the two species in such a way
that fermions experience a stronger confinement than bosons. As a consequence,
a deep Fermi degeneracy can be reached having at the same time a softer
degenerate regime for the Bose gas. This leads to an increase in the
sympathetic cooling efficiency and allows for higher precision thermometry of
the Fermi-Bose mixture

### On the treatment of $\ell$-changing proton-hydrogen Rydberg atom collisions

Energy-conserving, angular momentum-changing collisions between protons and
highly excited Rydberg hydrogen atoms are important for precise understanding
of atomic recombination at the photon decoupling era, and the elemental
abundance after primordial nucleosynthesis. Early approaches to $\ell$-changing
collisions used perturbation theory for only dipole-allowed ($\Delta \ell=\pm
1$) transitions. An exact non-perturbative quantum mechanical treatment is
possible, but it comes at computational cost for highly excited Rydberg states.
In this note we show how to obtain a semi-classical limit that is accurate and
simple, and develop further physical insights afforded by the non-perturbative
quantum mechanical treatment

### On the use of the proximity force approximation for deriving limits to short-range gravitational-like interactions from sphere-plane Casimir force experiments

We discuss the role of the proximity force approximation in deriving limits
to the existence of Yukawian forces - predicted in the submillimeter range by
many unification models - from Casimir force experiments using the sphere-plane
geometry. Two forms of this approximation are discussed, the first used in most
analyses of the residuals from the Casimir force experiments performed so far,
and the second recently discussed in this context in R. Decca et al. [Phys.
Rev. D 79, 124021 (2009)]. We show that the former form of the proximity force
approximation overestimates the expected Yukawa force and that the relative
deviation from the exact Yukawa force is of the same order of magnitude, in the
realistic experimental settings, as the relative deviation expected between the
exact Casimir force and the Casimir force evaluated in the proximity force
approximation. This implies both a systematic shift making the actual limits to
the Yukawa force weaker than claimed so far, and a degree of uncertainty in the
alpha-lambda plane related to the handling of the various approximations used
in the theory for both the Casimir and the Yukawa forces. We further argue that
the recently discussed form for the proximity force approximation is
equivalent, for a geometry made of a generic object interacting with an
infinite planar slab, to the usual exact integration of any additive two-body
interaction, without any need to invoke approximation schemes. If the planar
slab is of finite size, an additional source of systematic error arises due to
the breaking of the planar translational invariance of the system, and we
finally discuss to what extent this may affect limits obtained on power-law and
Yukawa forces.Comment: 11 page, 5 figure

### Ehrenfest Dynamics and Frictionless Cooling Methods

Recently introduced methods which result in shortcuts to adiabaticity,
particularly in the context of frictionless cooling, are rederived and
discussed in the framework of an approach based on Ehrenfest dynamics. This
construction provides physical insights into the emergence of the Ermakov
equation, the choice of its boundary conditions, and the use of minimum
uncertainty states as indicators of the efficiency of the procedure.
Additionally, it facilitates the extension of frictionless cooling to more
general situations of physical relevance, such as optical dipole trapping
schemes. In this context, we discuss frictionless cooling in the short-time
limit, a complementary case to the one considered in the literature, making
explicit the limitations intrinsic to the technique when the full
three-dimensional case is analyzed.Comment: 9 pages, 4 figures, v2: To appear in Physical Review A. (some minor
typos corrected and some references added

### Squeezing and robustness of frictionless cooling strategies

Quantum control strategies that provide shortcuts to adiabaticity are
increasingly considered in various contexts including atomic cooling. Recent
studies have emphasized practical issues in order to reduce the gap between the
idealized models and actual ongoing implementations. We rephrase here the
cooling features in terms of a peculiar squeezing effect, and use it to
parametrize the robustness of frictionless cooling techniques with respect to
noise-induced deviations from the ideal time-dependent trajectory for the
trapping frequency. We finally discuss qualitative issues for the experimental
implementation of this scheme using bichromatic optical traps and lattices,
which seem especially suitable for cooling Fermi-Bose mixtures and for
investigating equilibration of negative temperature states, respectively.Comment: 9 pages, 7 figures; To appear in Physical Review

### Comprehensive rate coefficients for electron collision induced transitions in hydrogen

Energy-changing electron-hydrogen atom collisions are crucial to regulating
the energy balance in astrophysical and laboratory plasmas and relevant to the
formation of stellar atmospheres, recombination in H-II clouds, primordial
recombination, three-body recombination and heating in ultracold and fusion
plasmas. Computational modeling of electron-hydrogen collision has been
attempted through quantum mechanical scattering state-to-state calculations of
transitions involving low-lying energy levels in hydrogen (with principal
quantum number n < 7) and at large principal quantum numbers using classical
trajectory techniques. Analytical expressions are proposed which interpolates
the current quantum mechanical and classical trajectory results for
electron-hydrogen scattering in the entire range of energy levels, for nearly
all temperature range of interest in astrophysical environments. An asymptotic
expression for the Born cross-section is interpolated with a modified
expression derived previously for electron-hydrogen scattering in the Rydberg
regime using classical trajectory Monte Carlo simulations. The derived formula
is compared to existing numerical data for transitions involving low principal
quantum numbers, and the dependence of the deviations upon temperature is
discussed.Comment: To appear in The Astrophysical Journa

### Continuous quantum measurement of a Bose-Einstein condensate: a stochastic Gross-Pitaevskii equation

We analyze the dynamics of a Bose-Einstein condensate undergoing a continuous
dispersive imaging by using a Lindblad operator formalism. Continuous strong
measurements drive the condensate out of the coherent state description assumed
within the Gross-Pitaevskii mean-field approach. Continuous weak measurements
allow instead to replace, for timescales short enough, the exact problem with
its mean-field approximation through a stochastic analogue of the
Gross-Pitaevskii equation. The latter is used to show the unwinding of a dark
soliton undergoing a continuous imaging.Comment: 13 pages, 10 figure

### Development of a high sensitivity torsional balance for the study of the Casimir force in the 1-10 micrometer range

We discuss a proposal to measure the Casimir force in the parallel plate
configuration in the $1-10\mu$m range via a high-sensitivity torsional balance.
This will allow to measure the thermal contribution to the Casimir force
therefore discriminating between the various approaches discussed so far. The
accurate control of the Casimir force in this range of distances is also
required to improve the limits to the existence of non-Newtonian forces in the
micrometer range predicted by unification models of fundamental interactions.Comment: 10 pages, 2 figure

### Modelling thermionic emission by using a two-level mechanical system (A pedagogical approach to the Boltzmann factor)

The Boltzmann factor is at the basis of a great amount of thermodynamic and statistical physics, both classical and quantum. It describes the behaviour of natural systems that exchange energy with the environment. However, why does the expression have that specific form? The Feynman Lectures on Physics justifies it heuristically by referencing to the âexponential atmosphereâ example. Thermodynamics textbooks usually give a more or less complete explanation that mainly involves a mathematical analysis, where it is hard to see the logic flow. Moreover, the necessary mathematics is not at the level of high school or college studentsâ preparation. Here we present an experiment and a simulation aimed at deriving the Boltzmann factor expression and illustrating the fundamental concepts and principles of statistical mechanics. Experiments and simulations are used in order to visualise the mechanisms involved; the experiments use easily available laboratory
equipment, and simulations are developed in NetLogo, a software environment that, besides having a really friendly interface, allows the user to easily interact with the
algorithm, as well as to modify it

### Black hole production at lepton colliders

Production of black holes has been discussed in a variety of extensions of
the Standard Model, and related bounds have been established from data taken at
the Large Hadron Collider. We show that, if the Higgs particle has a fully
gravitational content via the equivalence principle, enhanced cross-sections of
black holes at colliders should be expected within the Standard Model itself.
The case of black hole production by precision measurements at electron
colliders is discussed. The Coulomb repulsion strongly suppresses the related
cross-section with respect to the one based on the hoop conjecture, making the
possible production of black holes still unfeasible with current beam
technology. At the same time, this suggests the reanalysis of the bounds, based
on the hoop conjecture, already determined in hadronic collisions for
extra-dimensional models.Comment: 10 pages, 6 figure

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