8,178 research outputs found
Mixture of ultracold lithium and cesium atoms in an optical dipole trap
We present the first simultaneous trapping of two different ultracold atomic
species in a conservative trap. Lithium and cesium atoms are stored in an
optical dipole trap formed by the focus of a CO laser. Techniques for
loading both species of atoms are discussed and observations of elastic and
inelastic collisions between the two species are presented. A model for
sympathetic cooling of two species with strongly different mass in the presence
of slow evaporation is developed. From the observed Cs-induced evaporation of
Li atoms we estimate a cross section for cold elastic Li-Cs collisions.Comment: 10 pages 9 figures, submitted to Appl. Phys. B; v2: Corrected
evaporation formulas and some postscript problem
A Pragmatic Approach to Discovery Reform: How Small Changes Can Make a Big Difference in Civil Discovery
Gravitational Instantons and Fluxes from M/F-theory on Calabi-Yau fourfolds
We compactify four-dimensional N=1 gauged supergravity theories on a circle
including fluxes for shift-symmetric scalars. Four-dimensional Taub-NUT
gravitational instantons universally correct the three-dimensional
superpotential in the absence of fluxes. In the presence of fluxes these
Taub-NUT instanton contributions are no longer gauge-invariant. Invariance can
be restored by gauge instantons on top of Taub-NUT instantons. We establish the
embedding of this scenario into M-theory. Circle fluxes and gaugings arise from
a restricted class of M-theory four-form fluxes on a resolved Calabi-Yau
fourfold. The M5-brane on the base of the elliptic fourfold dualizes into the
universal Taub-NUT instanton. In the presence of fluxes this M5-brane is
anomalous. We argue that anomaly free contributions arise from involved
M5-brane geometries dual to gauge-instantons on top of Taub-NUT instantons.
Adding a four-dimensional superpotential to the gravitational instanton
corrections leads to three-dimensional Anti-de Sitter vacua at stabilized
compactification radius. We comment on the possibility to uplift these M-theory
vacua, and to tunnel to four-dimensional F-theory vacua.Comment: 47 pages, 2 figure
On Flux Quantization in F-Theory II: Unitary and Symplectic Gauge Groups
We study the quantization of the M-theory G-flux on elliptically fibered
Calabi-Yau fourfolds with singularities giving rise to unitary and symplectic
gauge groups. We seek and find its relation to the Freed-Witten quantization of
worldvolume fluxes on 7-branes in type IIB orientifold compactifications on
Calabi-Yau threefolds. By explicitly constructing the appropriate four-cycles
on which to calculate the periods of the second Chern class of the fourfolds,
we find that there is a half-integral shift in the quantization of G-flux
whenever the corresponding dual 7-brane is wrapped on a non-spin submanifold.
This correspondence of quantizations holds for all unitary and symplectic gauge
groups, except for SU(3), which behaves mysteriously. We also perform our
analysis in the case where, in addition to the aforementioned gauge groups,
there is also a 'flavor' U(1)-gauge group.Comment: 33 pages, 4 figure
Exact Dynamics of Multicomponent Bose-Einstein Condensates in Optical Lattices in One, Two and Three Dimensions
Numerous exact solutions to the nonlinear mean-field equations of motion are
constructed for multicomponent Bose-Einstein condensates on one, two, and three
dimensional optical lattices. We find both stationary and nonstationary
solutions, which are given in closed form. Among these solutions are a
vortex-anti-vortex array on the square optical lattice and modes in which two
or more components slosh back and forth between neighboring potential wells. We
obtain a variety of solutions for multicomponent condensates on the simple
cubic lattice, including a solution in which one condensate is at rest and the
other flows in a complex three-dimensional array of intersecting vortex lines.
A number of physically important solutions are stable for a range of parameter
values, as we show by direct numerical integration of the equations of motion.Comment: 22 pages, 9 figure
Enhancing the understanding of resilience in health systems of low- and middle-income countries: a qualitative evidence synthesis
Background A country's health system faces pressure when hit by an unexpected shock, such as what we observe in the midst of the coronavirus disease 2019 (COVID-19) pandemic. The concept of resilience is highly relevant in this context and is a prerequisite for a health system capable of withstanding future shocks. By exploring how the key dimensions of the resilient health system framework are applied, the present systematic review synthesizes the vital features of resilient health systems in low- and middle-income countries. The aim of this review is to ascertain the relevance of health system resilience in the context of a major shock, through better understanding its dimensions, uses and implications. Methods The review uses the best-fit framework synthesis approach. An a priori conceptual framework was selected and a coding framework created. A systematic search identified 4284 unique citations from electronic databases and reports by non-governmental organisations, 12 of which met the inclusion criteria. Data were extracted and coded against the pre-existing themes. Themes outside of the a priori framework were collated to form a refined list of themes. Then, all twelve studies were revisited using the new list of themes in the context of each study. Results Ten themes were generated from the analysis. Five confirmed the a priori conceptual framework that capture the dynamic attributes of a resilient system. Five new themes were identified as foundational for achieving resilience: realigned relationships, foresight and motivation as drivers, and emergency preparedness and change management as organisational mechanisms. Conclusion The refined conceptual model shows how the themes inter-connect. The foundations of resilience appear to be critical especially in resource-constrained settings to unlock the dynamic attributes of resilience. This review prompts countries to consider building the foundations of resilience described here as a priority to better prepare for future shocks
Coherent transport of neutral atoms in spin-dependent optical lattice potentials
We demonstrate the controlled coherent transport and splitting of atomic wave
packets in spin-dependent optical lattice potentials. Such experiments open
intriguing possibilities for quantum state engineering of many body states.
After first preparing localized atomic wave functions in an optical lattice
through a Mott insulating phase, we place each atom in a superposition of two
internal spin states. Then state selective optical potentials are used to split
the wave function of a single atom and transport the corresponding wave packets
in two opposite directions. Coherence between the wave packets of an atom
delocalized over up to 7 lattice sites is demonstrated.Comment: 4 pages, 6 figure
Ramping fermions in optical lattices across a Feshbach resonance
We study the properties of ultracold Fermi gases in a three-dimensional
optical lattice when crossing a Feshbach resonance. By using a zero-temperature
formalism, we show that three-body processes are enhanced in a lattice system
in comparison to the continuum case. This poses one possible explanation for
the short molecule lifetimes found when decreasing the magnetic field across a
Feshbach resonance. Effects of finite temperatures on the molecule formation
rates are also discussed by computing the fraction of double-occupied sites.
Our results show that current experiments are performed at temperatures
considerably higher than expected: lower temperatures are required for
fermionic systems to be used to simulate quantum Hamiltonians. In addition, by
relating the double occupancy of the lattice to the temperature, we provide a
means for thermometry in fermionic lattice systems, previously not accessible
experimentally. The effects of ramping a filled lowest band across a Feshbach
resonance when increasing the magnetic field are also discussed: fermions are
lifted into higher bands due to entanglement of Bloch states, in good agreement
with recent experiments.Comment: 9 pages, 7 figure
Very long storage times and evaporative cooling of cesium atoms in a quasi-electrostatic dipole trap
We have trapped cesium atoms over many minutes in the focus of a CO-laser
beam employing an extremely simple laser system. Collisional properties of the
unpolarized atoms in their electronic ground state are investigated. Inelastic
binary collisions changing the hyperfine state lead to trap loss which is
quantitatively analyzed. Elastic collisions result in evaporative cooling of
the trapped gas from 25 K to 10 K over a time scale of about 150 s.Comment: 5 pages, 3 figure
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