2,634 research outputs found
Quantum effects in the collective light scattering by coherent atomic recoil in a Bose-Einstein condensate
We extend the semiclassical model of the collective atomic recoil laser
(CARL) to include the quantum mechanical description of the center-of-mass
motion of the atoms in a Bose-Einstein condensate (BEC). We show that when the
average atomic momentum is less than the recoil momentum , the
CARL equations reduce to the Maxwell-Bloch equations for two momentum levels.
In the conservative regime (no radiation losses), the quantum model depends on
a single collective parameter, , that can be interpreted as the average
number of photons scattered per atom in the classical limit. When ,
the semiclassical CARL regime is recovered, with many momentum levels populated
at saturation. On the contrary, when , the average momentum
oscillates between zero and , and a periodic train of
hyperbolic secant pulses is emitted. In the dissipative regime (large radiation
losses) and in a suitable quantum limit, a sequential superfluorescence
scattering occurs, in which after each process atoms emit a hyperbolic
secant pulse and populate a lower momentum state. These results describe the
regular arrangement of the momentum pattern observed in recent experiments of
superradiant Rayleigh scattering from a BEC.Comment: submitted for publication on Phys. Rev.
The Semiclassical and Quantum Regimes of Superradiant Light Scattering from a Bose-Einstein Condensate
We show that many features of the recent experiments of Schneble et al. [D.
Schneble, Y. Torii, M. Boyd, E.W. Streed, D.E. Pritchard and W. Ketterle,
Science vol. 300, p. 475 (2003)], which demonstrate two different regimes of
light scattering by a Bose-Einstein condensate, can be described using a
one-dimensional mean-field quantum CARL model, where optical amplification
occurs simultaneously with the production of a periodic density modulation in
the atomic medium. The two regimes of light scattering observed in these
experiments, originally described as ``Kapiza-Dirac scattering'' and
``Superradiant Rayleigh scattering'', can be interpreted as the semiclassical
and quantum limits respectively of CARL lasing.Comment: 10 pages, 5 figures - to appear in Journal of Optics
The cosmic lithium problem: an observer's perspective
Using the cosmological constants derived from WMAP, the standard big bang
nucleosynthesis (SBBN) predicts the light elements primordial abundances for
4He, 3He, D, 6Li and 7Li. These predictions are in satisfactory agreement with
the observations, except for lithium which displays in old warm dwarfs an
abundance depleted by a factor of about 3. Depletions of this fragile element
may be produced by several physical processes, in different stellar
evolutionary phases, they will be briefly reviewed here, none of them seeming
yet to reproduce the observed depletion pattern in a fully convincing way.Comment: Invited review at the conference Lithium in the cosmos, Paris 27-29
Feb 2012, one reference adde
Atomic interaction effects in the superradiant light scattering from a Bose-Einstein condensate
We investigate the effects of the atomic interaction in the Superradiant
Rayleigh scattering from a Bose-Einstein condensate driven by a far-detuned
laser beam. We show that for a homogeneous atomic sample the atomic interaction
has only a dispersive effect, whereas in the inhomogeneous case it may increase
the decay of the matter-wave grating.Comment: 12 pages, 4 figures, presented to the XII International Laser Physics
Workshop, August 24-29, Hamburg, to be published in Laser Physic
Effects of atomic diffraction on the Collective Atomic Recoil Laser
We formulate a wave atom optics theory of the Collective Atomic Recoil Laser,
where the atomic center-of-mass motion is treated quantum mechanically. By
comparing the predictions of this theory with those of the ray atom optics
theory, which treats the center-of-mass motion classically, we show that for
the case of a far off-resonant pump laser the ray optics model fails to predict
the linear response of the CARL when the temperature is of the order of the
recoil temperature or less. This is due to the fact that in theis temperature
regime one can no longer ignore the effects of matter-wave diffraction on the
atomic center-of-mass motion.Comment: plain tex, 10 pages, 10 figure
The cosmological Lithium problem outside the Galaxy: the Sagittarius globular cluster M54
The cosmological Li problem is the observed discrepancy between Li abundance,
A(Li), measured in Galactic dwarf, old and metal-poor stars (traditionally
assumed to be equal to the initial value A(Li)_0), and that predicted by
standard Big Bang Nucleosynthesis calculations (A(Li)_{BBN}). Here we attack
the Li problem by considering an alternative diagnostic, namely the surface Li
abundance of red giant branch stars that in a colour magnitude diagram populate
the region between the completion of the first dredge-up and the red giant
branch bump. We obtained high-resolution spectra with the FLAMES facility at
the Very Large Telescope for a sample of red giants in the globular cluster
M54, belonging to the Sagittarius dwarf galaxy. We obtain A(Li)=+0.93+-0.11
dex, translating -- after taking into account the dilution due to the dredge
up-- to initial abundances (A(Li)_0) in the range 2.35--2.29 dex, depending on
whether or not atomic diffusion is considered. This is the first measurement of
Li in the Sagittarius galaxy and the more distant estimate of A(Li)_0 in old
stars obtained so far. The A(Li)_0 estimated in M54 is lower by ~0.35 dex than
A(Li)_{BBN}, hence incompatible at a level of ~3sigma. Our result shows that
this discrepancy is a universal problem concerning both the Milky Way and
extra-galactic systems. Either modifications of BBN calculations, or a
combination of atomic diffusion plus a suitably tuned additional mixing during
the main sequence, need to be invoked to solve the discrepancy.Comment: Accepted by MNRAS, 10 pages, 5 figures, 1 tabl
Spacetime Conformal Fluctuations and Quantum Dephasing
Any quantum system interacting with a complex environment undergoes
decoherence. Empty space is filled with vacuum energy due to matter fields in
their ground state and represents an underlying environment that any quantum
particle has to cope with. In particular quantum gravity vacuum fluctuations
should represent a universal source of decoherence. To study this problem we
employ a stochastic approach that models spacetime fluctuations close to the
Planck scale by means of a classical, randomly fluctuating metric (random
gravity framework). We enrich the classical scheme for metric perturbations
over a curved background by also including matter fields and metric conformal
fluctuations. We show in general that a conformally modulated metric induces
dephasing as a result of an effective nonlinear newtonian potential obtained in
the appropriate nonrelativistic limit of a minimally coupled Klein-Gordon
field. The special case of vacuum fluctuations is considered and a quantitative
estimate of the expected effect deduced. Secondly we address the question of
how conformal fluctuations could physically arise. By applying the random
gravity framework we first show that standard GR seems to forbid spontaneous
conformal metric modulations. Finally we argue that a different result follows
within scalar-tensor theories of gravity such as e.g. Brans-Dicke theory. In
this case a conformal modulation of the metric arises naturally as a result of
the fluctuations in the Brans-Dicke field and quantum dephasing of a test
particle is expected to occur. For large negative values of the coupling
parameter the conformal fluctuations may also contribute to alleviate the well
known problem of the large zero point energy due to quantum matter fields.Comment: PhD thesis, 164 page
6Li detection in metal-poor stars: can 3D model atmospheres solve the second lithium problem?
The presence of 6Li in the atmospheres of metal-poor halo stars is usually
inferred from the detection of a subtle extra depression in the red wing of the
7Li doublet line at 670.8 nm. However, the intrinsic line asymmetry caused by
convective flows in the photospheres of cool stars is almost indistinguishable
from the asymmetry produced by a weak 6Li blend on a (presumed) symmetric 7Li
profile. Previous determinations of the 6Li/ 7Li isotopic ratio based on 1D
model atmospheres, ignoring the convection-induced line asymmetry, must
therefore be considered as upper limits. By comparing synthetic 1D LTE and 3D
non-LTE line profiles of the Li 670.8 nm feature, we quantify the differential
effect of the convective line asymmetry on the derived 6Li abundance as a
function of effective temperature, gravity, and metallicity. As expected, we
find that the asymmetry effect systematically reduces the resulting 6Li/7Li
ratios. Depending on the stellar parameters, the 3D-1D offset in 6Li/7Li ranges
between -0.005 and -0.020. When this purely theoretical correction is taken
into account for the Asplund 2006 sample of stars, the number of significant
6Li detections decreases from 9 to 5 (2 sigma criterion), or from 5 to 2 (3
sigma criterion).
We also present preliminary results of a re-analysis of high-resolution, high
S/N spectra of individual metal-poor turn-off stars, to see whether the "second
Lithium problem" actually disappears when accounting properly for convection
and non-LTE line formation in 3D stellar atmospheres. Out of 8 stars, HD84937
seems to be the only significant (2 sigma) detection of 6Li. In view of our
results, the existence of a 6Li plateau appears questionable.Comment: To appear in the proceedings of 'Lithium in the Cosmos', Paris, Feb.
27-29, 2012, Memorie della Societa' Astronomica Italiana Supplement
Comparison of Recoil-Induced Resonances (RIR) and Collective Atomic Recoil Laser (CARL)
The theories of recoil-induced resonances (RIR) [J. Guo, P. R. Berman, B.
Dubetsky and G. Grynberg, Phys. Rev. A {\bf 46}, 1426 (1992)] and the
collective atomic recoil laser (CARL) [ R. Bonifacio and L. De Salvo, Nucl.
Instrum. Methods A {\bf 341}, 360 (1994)] are compared. Both theories can be
used to derive expressions for the gain experienced by a probe field
interacting with an ensemble of two-level atoms that are simultaneously driven
by a pump field. It is shown that the RIR and CARL formalisms are equivalent.
Differences between the RIR and CARL arise because the theories are typically
applied for different ranges of the parameters appearing in the theory. The RIR
limit considered in this paper is , while the CARL
limit is , where is the magnitude of the
difference of the wave vectors of the pump and probe fields, is the
width of the atomic momentum distribution and is a recoil
frequency. The probe gain for a probe-pump detuning equal to zero is analyzed
in some detail, in order to understand how the gain arises in a system which,
at first glance, might appear to have vanishing gain. Moreover, it is shown
that the calculations, carried out in perturbation theory have a range of
applicability beyond the recoil problem. Experimental possibilities for
observing CARL are discussed.Comment: 16 pages, 1 figure. Submitted to Physical Review
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