4,972 research outputs found
Multimode analysis of non-classical correlations in double well Bose-Einstein condensates
The observation of non-classical correlations arising in interacting two to
size weakly coupled Bose-Einstein condensates was recently reported by Esteve
et al. [Nature 455, 1216 (2008)]. In order to observe fluctuations below the
standard quantum limit, they utilized adiabatic passage to reduce the thermal
noise to below that of thermal equilibrium at the minimum realizable
temperature. We present a theoretical analysis that takes into account the
spatial degrees of freedom of the system, allowing us to calculate the expected
correlations at finite temperature in the system, and to verify the hypothesis
of adiabatic passage by comparing the dynamics to the idealized model.Comment: 12 pages, 7 figure
Influence of quantum fluctuations on the superfluid critical velocity of a one-dimensional Bose gas
The mean-field Gross-Pitaevskii equation with repulsive interactions exhibits
frictionless flow when stirred by an obstacle below a critical velocity. Here
we go beyond the mean-field approximation to examine the influence of quantum
fluctuations on this threshold behaviour in a one-dimensional Bose gas in a
ring. Using the truncated Wigner approximation, we perform simulations of
ensembles of trajectories where the Bose gas is stirred with a repulsive
obstacle below the mean-field critical velocity. We observe the probabilistic
formation of grey solitons which subsequently decay, leading to an increase in
the momentum of the fluid. The formation of the first soliton leads to a
soliton cascade, such that the fluid rapidly accelerates to minimise the speed
difference with the obstacle. We measure the initial rate of momentum transfer,
and relate it to macroscopic tunnelling between quantised flow states in the
ring.Comment: 10 pages, 9 figure
The Projected Gross-Pitaevskii Equation for harmonically confined Bose gases
We extend the Projected Gross Pitaevskii equation formalism of Davis et al.
[Phys. Rev. Lett. \bf{87}, 160402 (2001)] to the experimentally relevant case
of harmonic potentials. We outline a robust and accurate numerical scheme that
can efficiently simulate this system. We apply this method to investigate the
equilibrium properties of a harmonically trapped three-dimensional Bose gas at
finite temperature, and consider the dependence of condensate fraction,
position and momentum distributions, and density fluctuations on temperature.
We apply the scheme to simulate an evaporative cooling process in which the
preferential removal of high energy particles leads to the growth of a
Bose-Einstein condensate. We show that a condensate fraction can be inferred
during the dynamics even in this non-equilibrium situation.Comment: 11 pages, 7 figure
Critical temperature of a trapped Bose gas: comparison of theory and experiment
We apply the Projected Gross-Pitaevskii equation (PGPE) formalism to the
experimental problem of the shift in critical temperature of a
harmonically confined Bose gas as reported in Gerbier \emph{et al.} [Phys. Rev.
Lett. \textbf{92}, 030405 (2004)]. The PGPE method includes critical
fluctuations and we find the results differ from various mean-field theories,
and are in best agreement with experimental data. To unequivocally observe
beyond mean-field effects, however, the experimental precision must either
improve by an order of magnitude, or consider more strongly interacting
systems. This is the first application of a classical field method to make
quantitative comparison with experiment.Comment: revtex4, four pages, three figures. v2: updated to published version.
Several additions to figures, and better explanations in text in response to
referee comment
Emergence of order from turbulence in an isolated planar superfluid
We study the relaxation dynamics of an isolated zero temperature
quasi-two-dimensional superfluid Bose-Einstein condensate (BEC) that is
imprinted with a spatially random distribution of quantum vortices. Following a
period of vortex annihilation, we find that the remaining vortices
self-organise into two macroscopic coherent `Onsager vortex' clusters that are
stable indefinitely. We demonstrate that this occurs due to a novel physical
mechanism --- the evaporative heating of the vortices --- that results in a
negative temperature phase transition in the vortex degrees of freedom. At the
end of our simulations the system is trapped in a non-thermal state. Our
computational results provide a pathway to observing Onsager vortex states in a
superfluid Bose gas.Comment: 10 pages, 7 figure
Guyon\u27s Sensitive Appetite
This Master’s Thesis seeks to explain the internal conflicts faced by Guyon, the titular hero of Book II of Edmund Spenser’s Faerie Queene. Starting with Thomas Aquinas’ designations of the sensitive versus the intellectual appetite, I show that Guyon struggles to maintain the dominance of his intellectual appetite as he puts his vaunted temperance to a series of tests. The hero manages to appease his sensitive appetite through the vice of curiositas, yet the power of his sensitive appetite demands dramatic and violent acts of repression to quash it in Mammon’s Cave and in the Bower of Bliss. Guyon’s intellectual appetite to maintain temperance in Gloriana’s kingdom, aided by the guidance of the Palmer, leads Guyon to succeed in his quest yet reveals the incompatibility between temperance and the desirous and glory-seeking life of a knight errant
Bistability and nonequilibrium condensation in a driven-dissipative Josephson array: a c-field model
Developing theoretical models for nonequilibrium quantum systems poses
significant challenges. Here we develop and study a multimode model of a
driven-dissipative Josephson junction chain of atomic Bose-Einstein
condensates, as realised in the experiment of Labouvie et al. [Phys. Rev. Lett.
116, 235302 (2016)]. The model is based on c-field theory, a beyond-mean-field
approach to Bose-Einstein condensates that incorporates fluctuations due to
finite temperature and dissipation. We find the c-field model is capable of
capturing all key features of the nonequilibrium phase diagram, including
bistability and a critical slowing down in the lower branch of the bistable
region. Our model is closely related to the so-called Lugiato-Lefever equation,
and thus establishes new connections between nonequilibrium dynamics of
ultracold atoms with nonlinear optics, exciton-polariton superfluids, and
driven damped sine-Gordon systems.Comment: 12 pages, 10 figure
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