360 research outputs found
Control of Antarctic phytoplankton community composition and standing stock by light availability
Quantum correlated twin atomic beams via photo-dissociation of a molecular Bose-Einstein condensate
We study the process of photo-dissociation of a molecular Bose-Einstein
condensate as a potential source of strongly correlated twin atomic beams. We
show that the two beams can possess nearly perfect quantum squeezing in their
relative numbers.Comment: Corrected LaTeX file layou
Control of Antarctic phytoplankton community composition and standing stock by light availability
Southern Ocean phytoplankton are especially subjected to pronounced seasonal and interannual changes in light availability. Although previous studies have examined the role of light in these environments, very few combined pigment-based taxonomy with flow cytometry to better discriminate the light response of various phytoplankton groups. In particular the different populations within the diverse and important taxonomic group of diatoms require further investigation. Six incubation experiments (9–10 days) were performed during the main productive period with natural seawater collected at the Western Antarctic Peninsula. Standing stock of Phaeocystis spp. cells displayed relatively fast accumulation under all levels of light (low, medium, high; 4–7, 30–50 and 150–200 µmol quanta m−2 s−1), whilst the small- and larger-sized diatom populations (4.5 and 20 µm diameter) exhibited faster accumulation in medium and high light. In contrast, intermediate-sized diatoms (11.5 µm diameter) displayed fastest net growth under low light, subsequently dominating the phytoplankton community. Low light was a key factor limiting accumulation and peak phytoplankton biomass, except one incubation displaying relatively high accumulation rates under low light. The 3-week low-light period prior to experimentation likely allowed adaptation to maximize achievable growth and seems a strong determinant of whether the different natural Antarctic phytoplankton populations sustain, thrive or decline. Our study provides improved insight into how light intensity modulates the net response of key Antarctic phytoplankton, both between and within taxonomic groups
Quantum Corrections to Dilute Bose Liquids
It was recently shown (A. Bulgac. Phys. Rev. Lett. {\bf 89}, 050402 (2002))
that an entirely new class of quantum liquids with widely tunable properties
could be manufactured from bosons (boselets), fermions (fermilets) and their
mixtures (ferbolets) by controlling their interaction properties by the means
of a Feshbach resonance. We extend the previous mean--field analysis of these
quantum liquids by computing the lowest order quantum corrections to the ground
state energy and the depletion of the Bose--Einstein condensate and by
estimating higher order corrections as well. We show that the quantum
corrections are relatively small and controlled by the diluteness parameter
, even though strictly speaking in this case there is no
low density expansion.Comment: final published version, typos corrected, updated references and
added one referenc
Entanglement of two-mode Bose-Einstein condensates
We investigate the entaglement characteristics of two general bimodal
Bose-Einstein condensates - a pair of tunnel-coupled Bose-Einstein condensates
and the atom-molecule Bose-Einstein condensate. We argue that the entanglement
is only physically meaningful if the system is viewed as a bipartite system,
where the subsystems are the two modes. The indistinguishibility of the
particles in the condensate means that the atomic constituents are physically
inaccessible and thus the degree of entanglement between individual particles,
unlike the entanglement between the modes, is not experimentally relevant so
long as the particles remain in the condensed state. We calculate the
entanglement between the modes for the exact ground state of the two bimodal
condensates and consider the dynamics of the entanglement in the tunnel-coupled
case.Comment: 11 pages, 8 figures, submitted to Physical Review A, to be presented
at the third UQ Mathematical Physics workshop, Oct. 4-6; changes made in
response to referee comment
Model study on the photoassociation of a pair of trapped atoms into an ultralong-range molecule
Using the method of quantum-defect theory, we calculate the ultralong-range
molecular vibrational states near the dissociation threshold of a diatomic
molecular potential which asymptotically varies as . The properties of
these states are of considerable interest as they can be formed by
photoassociation (PA) of two ground state atoms. The Franck-Condon overlap
integrals between the harmonically trapped atom-pair states and the
ultralong-range molecular vibrational states are estimated and compared with
their values for a pair of untrapped free atoms in the low-energy scattering
state. We find that the binding between a pair of ground-state atoms by a
harmonic trap has significant effect on the Franck-Condon integrals and thus
can be used to influence PA. Trap-induced binding between two ground-state
atoms may facilitate coherent PA dynamics between the two atoms and the
photoassociated diatomic molecule.Comment: 11 pages, 4 figures, to appear in Phys. Rev. A (September, 2003
Spectral method for the time-dependent Gross-Pitaevskii equation with a harmonic trap
We study the numerical resolution of the time-dependent Gross-Pitaevskii
equation, a non-linear Schroedinger equation used to simulate the dynamics of
Bose-Einstein condensates. Considering condensates trapped in harmonic
potentials, we present an efficient algorithm by making use of a spectral
Galerkin method, using a basis set of harmonic oscillator functions, and the
Gauss-Hermite quadrature. We apply this algorithm to the simulation of
condensate breathing and scissors modes.Comment: 23 pages, 5 figure
Quadratic-nonlinear Landau-Zener transition for association of an atomic Bose-Einstein condensate with inter-particle elastic interactions included
We study the strong coupling limit of a quadratic-nonlinear Landau-Zener
problem for coherent photo- and magneto-association of cold atoms taking into
account the atom-atom, atom-molecule, and molecule-molecule elastic scattering.
Using an exact third-order nonlinear differential equation for the molecular
state probability, we develop a variational approach which enables us to
construct a highly accurate and simple analytic approximation describing the
time dynamics of the coupled atom-molecule system. We show that the
approximation describing time evolution of the molecular state probability can
be written as a sum of two distinct terms; the first one, being a solution to a
limit first-order nonlinear equation, effectively describes the process of the
molecule formation while the second one, being a scaled solution to the linear
Landau-Zener problem (but now with negative effective Landau-Zener parameter as
long as the strong coupling regime is considered), corresponds to the remaining
oscillations which come up when the process of molecule formation is over.Comment: 19 pages, 7 figures, accepted for publication in Eur. Phys. J.
Study of Tau-pair Production in Photon-Photon Collisions at LEP and Limits on the Anomalous Electromagnetic Moments of the Tau Lepton
Tau-pair production in the process e+e- -> e+e-tau+tau- was studied using
data collected by the DELPHI experiment at LEP2 during the years 1997 - 2000.
The corresponding integrated luminosity is 650 pb^{-1}. The values of the
cross-section obtained are found to be in agreement with QED predictions.
Limits on the anomalous magnetic and electric dipole moments of the tau lepton
are deduced.Comment: 20 pages, 9 figures, Accepted by Eur. Phys. J.
- …