27,471 research outputs found
Explicit finite-difference and direct-simulation-MonteCarlo method for the dynamics of mixed Bose-condensate and cold-atom clouds
We present a new numerical method for studying the dynamics of quantum fluids
composed of a Bose-Einstein condensate and a cloud of bosonic or fermionic
atoms in a mean-field approximation. It combines an explicit time-marching
algorithm, previously developed for Bose-Einstein condensates in a harmonic or
optical-lattice potential, with a particle-in-cell MonteCarlo approach to the
equation of motion for the one-body Wigner distribution function in the
cold-atom cloud. The method is tested against known analytical results on the
free expansion of a fermion cloud from a cylindrical harmonic trap and is
validated by examining how the expansion of the fermionic cloud is affected by
the simultaneous expansion of a condensate. We then present wholly original
calculations on a condensate and a thermal cloud inside a harmonic well and a
superposed optical lattice, by addressing the free expansion of the two
components and their oscillations under an applied harmonic force. These
results are discussed in the light of relevant theories and experiments.Comment: 33 pages, 13 figures, 1 tabl
Numerical study of one-dimensional and interacting Bose-Einstein condensates in a random potential
We present a detailed numerical study of the effect of a disordered potential
on a confined one-dimensional Bose-Einstein condensate, in the framework of a
mean-field description. For repulsive interactions, we consider the
Thomas-Fermi and Gaussian limits and for attractive interactions the behavior
of soliton solutions. We find that the disorder average spatial extension of
the stationary density profile decreases with an increasing strength of the
disordered potential both for repulsive and attractive interactions among
bosons. In the Thomas Fermi limit, the suppression of transport is accompanied
by a strong localization of the bosons around the state k=0 in momentum space.
The time dependent density profiles differ considerably in the cases we have
considered. For attractive Bose-Einstein condensates, a bright soliton exists
with an overall unchanged shape, but a disorder dependent width. For weak
disorder, the soliton moves on and for a stronger disorder, it bounces back and
forth between high potential barriers.Comment: 13 pages, 13 figures, few typos correcte
Creating exotic condensates via quantum-phase-revival dynamics in engineered lattice potentials
In the field of ultracold atoms in optical lattices a plethora of phenomena
governed by the hopping energy and the interaction energy have been
studied in recent years. However, the trapping potential typically present in
these systems sets another energy scale and the effects of the corresponding
time scale on the quantum dynamics have rarely been considered. Here we study
the quantum collapse and revival of a lattice Bose-Einstein condensate (BEC) in
an arbitrary spatial potential, focusing on the special case of harmonic
confinement. Analyzing the time evolution of the single-particle density
matrix, we show that the physics arising at the (temporally) recurrent quantum
phase revivals is essentially captured by an effective single particle theory.
This opens the possibility to prepare exotic non-equilibrium condensate states
with a large degree of freedom by engineering the underlying spatial lattice
potential.Comment: 9 pages, 6 figure
Mesoscale asymptotic approximations to solutions of mixed boundary value problems in perforated domains
We describe a method of asymptotic approximations to solutions of mixed
boundary value problems for the Laplacian in a three-dimensional domain with
many perforations of arbitrary shape, with the Neumann boundary conditions
being prescribed on the surfaces of small voids. The only assumption made on
the geometry is that the diameter of a void is assumed to be smaller compared
to the distance to the nearest neighbour. The asymptotic approximation,
obtained here, involves a linear combination of dipole fields constructed for
individual voids, with the coefficients, which are determined by solving a
linear algebraic system. We prove the solvability of this system and derive an
estimate for its solution. The energy estimate is obtained for the remainder
term of the asymptotic approximation.Comment: 20 pages, 8 figure
Tunable transport with broken space-time symmetries
Transport properties of particles and waves in spatially periodic structures
that are driven by external time-dependent forces manifestly depend on the
space-time symmetries of the corresponding equations of motion. A systematic
analysis of these symmetries uncovers the conditions necessary for obtaining
directed transport. In this work we give a unified introduction into the
symmetry analysis and demonstrate its action on the motion in one-dimensional
periodic, both in time and space, potentials. We further generalize the
analysis to quasi-periodic drivings, higher space dimensions, and quantum
dynamics. Recent experimental results on the transport of cold and ultracold
atomic ensembles in ac-driven optical potentials are reviewed as illustrations
of theoretical considerations.Comment: Phys. Rep., in pres
Generalized HydroDynamics on an Atom Chip
The emergence of a special type of fluid-like behavior at large scales in
one-dimensional (1d) quantum integrable systems, theoretically predicted in
2016, is established experimentally, by monitoring the time evolution of the in
situ density profile of a single 1d cloud of atoms trapped on
an atom chip after a quench of the longitudinal trapping potential. The theory
can be viewed as a dynamical extension of the thermodynamics of Yang and Yang,
and applies to the whole range of repulsion strength and temperature of the
gas. The measurements, performed on weakly interacting atomic clouds that lie
at the crossover between the quasicondensate and the ideal Bose gas regimes,
are in very good agreement with the 2016 theory. This contrasts with the
previously existing 'conventional' hydrodynamic approach---that relies on the
assumption of local thermal equilibrium---, which is unable to reproduce the
experimental data.Comment: v1: 6+11 pages, 4+4 figures. v2: published version, 6+11 pages, 4+6
figure
Oscillations of rotating trapped Bose-Einstein condensates
The tensor-virial method is applied for a study of oscillation modes of
uniformly rotating Bose-Einstein condensed gases, whose rigid body rotation is
supported by an vortex array. The second order virial equations are derived in
the hydrodynamic regime for an arbitrary external harmonic trapping potential
assuming that the condensate is a superfluid at zero temperature. The
axisymmetric equilibrium shape of the condensate is determined as a function of
the deformation of the trap; its domain of stability is bounded by the
constraint on the rotation rate (measured in units of the trap
frequency .) The oscillations of the axisymmetric condensate are
stable with respect to the transverse-shear, toroidal and quasi-radial modes of
oscillations, corresponding to the , surface
deformations. In non-axisymmetric traps, the equilibrium constrains the
(dimensionless) deformation in the plane orthogonal to the rotation to the
domain with . The second harmonic oscillation modes
in non-axisymmetric traps separate into two classes which have even or odd
parity with respect to the direction of the rotation axis. Numerical solutions
show that these modes are stable in the parameter domain where equilibrium
figures exist.Comment: 16 pages, including 4 figures, uses Revtex; v2 includes a treatment
of modes in unisotropic traps; PRA in pres
Ion transport in macroscopic RF linear traps
Efficient transport of cold atoms or ions is a subject of increasing concern
in many experimental applications reaching from quantum information processing
to frequency metrology. For the scalable quantum computer architectures based
on the shuttling of individual ions, different transport schemes have been
developed, which allow to move single atoms minimizing their energy gain. In
this article we discuss the experimental implementation of the transport of a
three-dimensional ion cloud in a macroscopic linear radiofrequency (RF) trap.
The present work is based on numerical simulations done by molecular dynamics
taking into account a realistic experimental environment. The deformation of
the trapping potential and the spatial extension of the cloud during transport
appears to be the major source of the ion energy gain. The efficiency of
transport in terms of transfer probability and ion number is also discussed
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