23 research outputs found
Atomic diffraction in counter-propagating Gaussian pulses of laser light
We present an analysis of atomic diffraction due to the interaction of an
atomic beam with a pair of Gaussian light pulses. We derive a simple analytical
expression for the populations in different diffraction orders. The validity of
the obtained solution extends beyond the Raman-Nath regime, where the kinetic
energy associated with different diffraction peaks is neglected, into the
so-called channeling regime where accurate analytical expressions have not
previously been available for the diffraction. Comparison with experimental
results and exact numerical solutions demonstrate the validity of our
analytical formula.Comment: 6 pages, 5 figure
Motion of vortices in inhomogeneous Bose-Einstein condensates
We derive a general and exact equation of motion for a quantised vortex in an
inhomogeneous two-dimensional Bose-Einstein condensate. This equation expresses
the velocity of a vortex as a sum of local ambient density and phase gradients
in the vicinity of the vortex. We perform Gross-Pitaevskii simulations of
single vortex dynamics in both harmonic and hard-walled disk-shaped traps, and
find excellent agreement in both cases with our analytical prediction. The
simulations reveal that, in a harmonic trap, the main contribution to the
vortex velocity is an induced ambient phase gradient, a finding that
contradicts the commonly quoted result that the local density gradient is the
only relevant effect in this scenario. We use our analytical vortex velocity
formula to derive a point-vortex model that accounts for both density and phase
contributions to the vortex velocity, suitable for use in inhomogeneous
condensates. Although good agreement is obtained between Gross-Pitaevskii and
point-vortex simulations for specific few-vortex configurations, the effects of
nonuniform condensate density are in general highly nontrivial, and are thus
difficult to efficiently and accurately model using a simplified point-vortex
description.Comment: 13 pages, 8 figure
Coherence vortices in one spatial dimension
Coherence vortices are screw-type topological defects in the phase of
Glauber's two-point degree of quantum coherence, associated with pairs of
spatial points at which an ensemble-averaged stochastic quantum field is
uncorrelated. Coherence vortices may be present in systems whose dimensionality
is too low to support spatial vortices. We exhibit lattices of such
quantum-coherence phase defects for a one-dimensional model quantum system. We
discuss the physical meaning of coherence vortices and propose how they may be
realized experimentally.Comment: 5 pages, 3 figure
Decaying quantum turbulence in a two-dimensional Bose-Einstein condensate at finite temperature
We numerically model decaying quantum turbulence in two-dimensional
disk-shaped Bose-Einstein condensates, and investigate the effects of finite
temperature on the turbulent dynamics. We prepare initial states with a range
of condensate temperatures, and imprint equal numbers of vortices and
antivortices at randomly chosen positions throughout the fluid. The initial
states are then subjected to unitary time-evolution within the c-field
methodology. For the lowest condensate temperatures, the results of the zero
temperature Gross-Pitaevskii theory are reproduced, whereby vortex evaporative
heating leads to the formation of Onsager vortex clusters characterised by a
negative absolute vortex temperature. At higher condensate temperatures the
dissipative effects due to vortex-phonon interactions tend to drive the vortex
gas towards positive vortex temperatures dominated by the presence of vortex
dipoles. We associate these two behaviours with the system evolving toward an
anomalous non-thermal fixed point, or a Gaussian thermal fixed point,
respectively.Comment: 20 pages, 6 figures, SciPost format. Updated version includes a new
section with further analysis and an additional figur
Vibrations of a Columnar Vortex in a Trapped Bose-Einstein Condensate
We derive a governing equation for a Kelvin wave supported on a vortex line
in a Bose-Einstein condensate, in a rotating cylindrically symmetric parabolic
trap. From this solution the Kelvin wave dispersion relation is determined. In
the limit of an oblate trap and in the absence of longitudinal trapping our
results are consistent with previous work. We show that the derived Kelvin wave
dispersion in the general case is in quantitative agreement with numerical
calculations of the Bogoliubov spectrum and offer a significant improvement
upon previous analytical work.Comment: 5 pages with 1 figur