130 research outputs found
Exploring classically chaotic potentials with a matter wave quantum probe
We study an experimental setup in which a quantum probe, provided by a
quasi-monomode guided atom laser, interacts with a static localized attractive
potential whose characteristic parameters are tunable. In this system,
classical mechanics predicts a transition from a regular to a chaotic behavior
as a result of the coupling between the longitudinal and transverse degrees of
freedom. Our experimental results display a clear signature of this transition.
On the basis of extensive numerical simulations, we discuss the quantum versus
classical physics predictions in this context. This system opens new
possibilities for investigating quantum scattering, provides a new testing
ground for classical and quantum chaos and enables to revisit the
quantum-classical correspondence
Continuous loading of a non-dissipative atom trap
We study theoretically a scheme in which particles from an incident beam are
trapped in a potential well when colliding with particles already present in
the well. The balance between the arrival of new particles and the evaporation
of particles from the trapped cloud leads to a steady-state that we
characterize in terms of particle number and temperature. For a cigar shaped
potential, different longitudinal and transverse evaporation thresholds can be
chosen. We show that a resonance occur when the transverse evaporation
threshold coincides with the energy of the incident particles. It leads to a
dramatic increase in phase space density with respect to the incident beam.Comment: 7 pages, 2 figure
From multimode to monomode guided atom lasers: an entropic analysis
We have experimentally demonstrated a high level of control of the mode
populations of guided atom lasers (GALs) by showing that the entropy per
particle of an optically GAL, and the one of the trapped Bose Einstein
condensate (BEC) from which it has been produced are the same. The BEC is
prepared in a crossed beam optical dipole trap. We have achieved isentropic
outcoupling for both magnetic and optical schemes. We can prepare GAL in a
nearly pure monomode regime (85 % in the ground state). Furthermore, optical
outcoupling enables the production of spinor guided atom lasers and opens the
possibility to tailor their polarization
Dynamics of a classical gas including dissipative and mean field effects
By means of a scaling ansatz, we investigate an approximated solution of the
Boltzmann-Vlasov equation for a classical gas. Within this framework, we derive
the frequencies and the damping of the collective oscillations of a
harmonically trapped gas and we investigate its expansion after release of the
trap. The method is well suited to studying the collisional effects taking
place in the system and in particular to discussing the crossover between the
hydrodynamic and the collisionless regimes. An explicit link between the
relaxation times relevant for the damping of the collective oscillations and
for the expansion is established.Comment: 4 pages, 1 figur
Evaporative Cooling of a Guided Rubidium Atomic Beam
We report on our recent progress in the manipulation and cooling of a
magnetically guided, high flux beam of atoms. Typically
atoms per second propagate in a magnetic guide providing a
transverse gradient of 800 G/cm, with a temperature K, at an
initial velocity of 90 cm/s. The atoms are subsequently slowed down to cm/s using an upward slope. The relatively high collision rate (5 s)
allows us to start forced evaporative cooling of the beam, leading to a
reduction of the beam temperature by a factor of ~4, and a ten-fold increase of
the on-axis phase-space density.Comment: 10 pages, 8 figure
Collective oscillations of a classical gas confined in harmonic traps
Starting from the Boltzmann equation we calculate the frequency and the
damping of the monopole and quadrupole oscillations of a classical gas confined
in an harmonic potential. The collisional term is treated in the relaxation
time approximation and a gaussian ansatz is used for its evaluation. Our
approach provides an explicit description of the transition between the
hydrodynamic and collisionless regimes in both spherical and deformed traps.
The predictions are compared with the results of a numerical simulation.Comment: 6 pages, revtex, 2 figures include
Zeeman slowers made simple with permanent magnets in a Halbach configuration
We describe a simple Zeeman slower design using permanent magnets. Contrary
to common wire-wound setups no electric power and water cooling are required.
In addition, the whole system can be assembled and disassembled at will. The
magnetic field is however transverse to the atomic motion and an extra repumper
laser is necessary. A Halbach configuration of the magnets produces a high
quality magnetic field and no further adjustment is needed. After optimization
of the laser parameters, the apparatus produces an intense beam of slow and
cold 87Rb atoms. With a typical flux of 1 - 5 \times 10^10 atoms/s at 30 ms^-1,
our apparatus efficiently loads a large magneto-optical trap with more than
10^10 atoms in one second, which is an ideal starting point for degenerate
quantum gases experiments.Comment: 8+6 pages (article + appendices: calculation details, probe and oven
description, pictures), 18 figures, supplementary material (movie,
Mathematica programs and technical drawings
A regular Hamiltonian halting ratchet for matter wave transport
We report on the design of a Hamiltonian ratchet exploiting periodically at
rest integrable trajectories in the phase space of a modulated periodic
potential, leading to the linear non-diffusive transport of particles. Using
Bose-Einstein condensates in a modulated one-dimensional optical lattice, we
make the first observations of this new spatial ratchet transport. In the
semiclassical regime, the quantum transport strongly depends on the effective
Planck constant due to Floquet state mixing. We also demonstrate the interest
of quantum optimal control for efficient initial state preparation into the
transporting Floquet states to enhance the transport periodicity.Comment: 5 pages + supplementary materia
Bose-Einstein condensation in a stiff TOP trap with adjustable geometry
We report on the realisation of a stiff magnetic trap with independently
adjustable trap frequencies, and , in the axial and radial
directions respectively. This has been achieved by applying an axial modulation
to a Time-averaged Orbiting Potential (TOP) trap. The frequency ratio of the
trap, , can be decreased continuously from the original
TOP trap value of 2.83 down to 1.6. We have transferred a Bose-Einstein
condensate (BEC) into this trap and obtained very good agreement between its
observed anisotropic expansion and the hydrodynamic predictions. Our method can
be extended to obtain a spherical trapping potential, which has a geometry of
particular theoretical interest.Comment: 4 pages, 3 figure
Engineered swift equilibration of a Brownian particle
A fundamental and intrinsic property of any device or natural system is its
relaxation time relax, which is the time it takes to return to equilibrium
after the sudden change of a control parameter [1]. Reducing relax , is
frequently necessary, and is often obtained by a complex feedback process. To
overcome the limitations of such an approach, alternative methods based on
driving have been recently demonstrated [2, 3], for isolated quantum and
classical systems [4--9]. Their extension to open systems in contact with a
thermostat is a stumbling block for applications. Here, we design a
protocol,named Engineered Swift Equilibration (ESE), that shortcuts
time-consuming relaxations, and we apply it to a Brownian particle trapped in
an optical potential whose properties can be controlled in time. We implement
the process experimentally, showing that it allows the system to reach
equilibrium times faster than the natural equilibration rate. We also estimate
the increase of the dissipated energy needed to get such a time reduction. The
method paves the way for applications in micro and nano devices, where the
reduction of operation time represents as substantial a challenge as
miniaturization [10]. The concepts of equilibrium and of transformations from
an equilibrium state to another, are cornerstones of thermodynamics. A textbook
illustration is provided by the expansion of a gas, starting at equilibrium and
expanding to reach a new equilibrium in a larger vessel. This operation can be
performed either very slowly by a piston, without dissipating energy into the
environment, or alternatively quickly, letting the piston freely move to reach
the new volume
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