362 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
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
Spin textures in condensates with large dipole moments
We have solved numerically the ground states of a Bose-Einstein condensate in
the presence of dipolar interparticle forces using a semiclassical approach.
Our motivation is to model, in particular, the spontaneous spin textures
emerging in quantum gases with large dipole moments, such as 52Cr or Dy
condensates, or ultracold gases consisting of polar molecules. For a
pancake-shaped harmonic (optical) potential, we present the ground state phase
diagram spanned by the strength of the nonlinear coupling and dipolar
interactions. In an elongated harmonic potential, we observe a novel helical
spin texture. The textures calculated according to the semiclassical model in
the absence of external polarizing fields are predominantly analogous to
previously reported results for a ferromagnetic F = 1 spinor Bose-Einstein
condensate, suggesting that the spin textures arising from the dipolar forces
are largely independent of the value of the quantum number F or the origin of
the dipolar interactions.Comment: 9 pages, 6 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
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