150 research outputs found
Collisions and expansion of an ultracold dilute Fermi gas
We discuss the effects of collisions on the expansion of a degenerate normal
Fermi gas, following the sudden removal of the confining trap. Using a
Boltzmann equation approach, we calculate the time dependence of the aspect
ratio and the entropy increase of the expanding atomic cloud taking into
account the collisional effects due to the deformation of the distribution
function in momentum space. We find that in dilute gases the aspect ratio does
not deviate significantly from the predictions of ballistic expansion.
Conversely, if the trap is sufficiently elongated the thermal broadening of the
density distribution due to the entropy increase can be sizeable, revealing
that even at zero temperature collisions are effective in a Fermi gas.Comment: 7 pages, 3 figures, revised after comments from referees and to
include correction
Finite-time adiabatic processes: derivation and speed limit
Obtaining adiabatic processes that connect equilibrium states in a given time
represents a challenge for mesoscopic systems. In this paper, we explicitly
show how to build these finite-time adiabatic processes for an overdamped
Brownian particle in an arbitrary potential, a system that is relevant both at
the conceptual and the practical level. This is achieved by jointly engineering
the time evolutions of the binding potential and the fluid temperature.
Moreover, we prove that the second principle imposes a speed limit for such
adiabatic transformations: there appears a minimum time to connect the initial
and final states. This minimum time can be explicitly calculated for a general
compression/decompression situation.Comment: Main text: 5 pages; 18 pages with appendices and references; major
revision with results for a general non-linear potential and study of
fluctuations added; Physical Review E in pres
Fast bias inversion of a double well without residual particle excitation
We design fast bias inversions of an asymmetric double well so that the
lowest states in each well remain so and free from residual motional
excitation. This cannot be done adiabatically, and a sudden bias switch
produces in general motional excitation. The residual excitation is suppressed
by complementing a predetermined fast bias change with a linear ramp whose
time-dependent slope compensates for the displacement of the wells. The
process, combined with vibrational multiplexing and demultiplexing, can produce
vibrational state inversion without exciting internal states, just by deforming
the trap.Comment: 7 pages, 6 figure
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
Cold atom dynamics in crossed laser beam waveguides
We study the dynamics of neutral cold atoms in an -shaped crossed-beam
optical waveguide formed by two perpendicular red-detuned lasers of different
intensities and a blue-detuned laser at the corner. Complemented with a
vibrational cooling process this setting works as a one-way device or "atom
diode"
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