739 research outputs found
Optical measurement of heteronuclear cross-relaxation interactions in Tm:YAG
We investigate cross-relaxation interactions between Tm and Al in Tm:YAG
using two optical methods: spectral holeburning and stimulated echoes. These
interactions lead to a reduction in the hyperfine lifetime at magnetic fields
that bring the Tm hyperfine transition into resonance with an Al transition. We
develop models for measured echo decay curves and holeburning spectra near a
resonance, which are used to show that the Tm-Al interaction has a resonance
width of 10~kHz and reduces the hyperfine lifetime to 0.5 ms. The antihole
structure is consistent with an interaction dominated by the Al nearest
neighbors at 3.0 Angstroms, with some contribution from the next nearest
neighbors at 3.6 Angstroms.Comment: 13 pages, 9 figure
Revival of Silenced Echo and Quantum Memory for Light
We propose an original quantum memory protocol. It belongs to the class of
rephasing processes and is closely related to two-pulse photon echo. It is
known that the strong population inversion produced by the rephasing pulse
prevents the plain two-pulse photon echo from serving as a quantum memory
scheme. Indeed gain and spontaneous emission generate prohibitive noise. A
second -pulse can be used to simultaneously reverse the atomic phase and
bring the atoms back into the ground state. Then a secondary echo is radiated
from a non-inverted medium, avoiding contamination by gain and spontaneous
emission noise. However, one must kill the primary echo, in order to preserve
all the information for the secondary signal. In the present work, spatial
phase mismatching is used to silence the standard two-pulse echo. An
experimental demonstration is presented.Comment: 13 pages, 6 figure
Physical activity and smoking habit in adolescent students
El objetivo de este estudio fue detectar en qué medida afecta la actividad
física y el hábito tabáquico en estudiantes de Enseñanza Secundaria. Se
estudiaron 168 adolescentes no fumadores y fumadores recién iniciados. Se
midieron peso, talla, presión arterial, frecuencia cardíaca, actividad física, fuerza,
flexibilidad y resistencia y una prueba espirométrica. Tanto en chicas y chicos
fumadores, se evidenciaron peores resultados en la mayoría de los parámetros
espirométricos (FEV1, FEF25-75%, FVC) y un envejecimiento prematuro del
pulmón, más acentuado en chicas. La actividad físico-deportiva moderada se
asocia a adolescentes que menos fuman y tienen más facilidad para abandonar
el hábito tabáquicoThe aim of the study was to detect how physical education and smoking habits
affect secondary school students, 168 non smokers and newly initiated were
studied. Weight, height, heart rate, blood pressure, physical activity, strength,
flexibility and endurance, and spirometric tests were analyzed. In both, female
and male smokers, obtained worse results in the mayority of spirometric
parameters (FEV1, FEF25-75%, FVC) and premature lung ageing, more pointed in
females. Moderate physical-sporting activity is associated to adolescents who
smoke less and quit smoking more easil
Homogeneous Conformal String Backgrounds
We present exact solutions characterised by Bianchi-type I,II,III,V,VI
four-dimensional metric, space-independent dilaton, and vanishing torsion
background, for the low energy string effective action with zero central charge
deficit. We show that, in such a context, curvature singularities cannot be
avoided, except for the trivial case of flat spacetime and constant dilaton. We
also provide a further example of the failure of the standard prescription for
connecting conformal string backgrounds through duality transformations
associated to non-semisimple, non-Abelian isometry group.Comment: 20 pages, latex, no figures, to appear in Class. Q. Gra
Isotropization of Bianchi-Type Cosmological Solutions in Brans-Dicke Theory
The cosmic, general analitic solutions of the Brans--Dicke Theory for the
flat space of homogeneous and isotropic models containing perfect, barotropic,
fluids are seen to belong to a wider class of solutions --which includes
cosmological models with the open and the closed spaces of the
Friedmann--Robertson--Walker metric, as well as solutions for models with
homogeneous but anisotropic spaces corresponding to the Bianchi--Type metric
clasification-- when all these solutions are expressed in terms of reduced
variables. The existence of such a class lies in the fact that the scalar
field, , times a function of the mean scale factor or ``volume element'',
, which depends on time and on the barotropic index of the
equation of state used, can be written as a function of a ``cosmic time''
reduced in terms of another function of the mean scale factor depending itself
again on the barotropic index but independent of the metrics here employed.
This reduction procedure permites one to analyze if explicitly given
anisotropic cosmological solutions ``isotropize'' in the course of their time
evolution. For if so can happen, it could be claimed that there exists a
subclass of solutions that is stable under anisotropic perturbations.Comment: 15 pages, Late
Isotropization of Bianchi type models and a new FRW solution in Brans-Dicke theory
Using scaled variables we are able to integrate an equation valid for
isotropic and anisotropic Bianchi type I, V, IX models in Brans-Dicke (BD)
theory. We analyze known and new solutions for these models in relation with
the possibility that anisotropic models asymptotically isotropize, and/or
possess inflationary properties. In particular, a new solution of curve
() Friedmann-Robertson-Walker (FRW) cosmologies in Brans-Dicke theory
is analyzed.Comment: 15 pages, 4 postscript figures, to appear in Gen. Rel. Grav., special
issue dedicated in honour of Prof. H. Dehne
Spin squeezing, entanglement and quantum metrology with Bose-Einstein condensates
Squeezed states, a special kind of entangled states, are known as a useful
resource for quantum metrology. In interferometric sensors they allow to
overcome the "classical" projection noise limit stemming from the independent
nature of the individual photons or atoms within the interferometer. Motivated
by the potential impact on metrology as wells as by fundamental questions in
the context of entanglement, a lot of theoretical and experimental effort has
been made to study squeezed states. The first squeezed states useful for
quantum enhanced metrology have been proposed and generated in quantum optics,
where the squeezed variables are the coherences of the light field. In this
tutorial we focus on spin squeezing in atomic systems. We give an introduction
to its concepts and discuss its generation in Bose-Einstein condensates. We
discuss in detail the experimental requirements necessary for the generation
and direct detection of coherent spin squeezing. Two exemplary experiments
demonstrating adiabatically prepared spin squeezing based on motional degrees
of freedom and diabatically realized spin squeezing based on internal hyperfine
degrees of freedom are discussed.Comment: Phd tutorial, 23 pages, 17 figure
Analyse multifractale de signaux laser Doppler acquis au repos et après stimulation par nitroglycérine
Plane-symmetric inhomogeneous Brans-Dicke cosmology with an equation of state
We present a new exact solution in Brans-Dicke theory. The solution describes
inhomogeneous plane-symmetric perfect fluid cosmological model with an equation
of state . Some main properties of the solution are discussed.Comment: 6 pages, Late
Detection by NMR of a "local spin-gap" in quenched CsC60
We present a 13C and 133Cs NMR investigation of the CsC60 cubic quenched
phase. Previous ESR measurements suggest that this phase is metallic, but NMR
reveals contrasting electronic behavior on the local scale. The 13C
spin-lattice relaxation time (T1) exhibits a typical metallic behavior down to
50 K, but indicates that a partial spin-gap opens for T<50 K. Unexpectedly,
133Cs NMR shows that there are two inequivalent Cs sites. For one of these
sites, the NMR shift and (T1T)^{-1} follow an activated law, confirming the
existence of a spin-gap. We ascribe this spin-gap to the occurrence of
localized spin-singlets on a small fraction of the C60 molecules.Comment: 4 figure
- …