176 research outputs found
Ultracold gases far from equilibrium
Ultracold atomic quantum gases belong to the most exciting challenges of
modern physics. Their theoretical description has drawn much from classical
field equations. These mean-field approximations are in general reliable for
dilute gases in which the atoms collide only rarely with each other, and for
situations where the gas is not too far from thermal equilibrium. With
present-day technology it is, however, possible to drive and observe a system
far away from equilibrium. Functional quantum field theory provides powerful
tools to achieve both, analytical understanding and numerical computability,
also in higher dimensions, of far-from-equilibrium quantum many-body dynamics.
In the article, an outline of these approaches is given, including methods
based on the two-particle irreducible effective action as well as on
renormalisation-group theory. Their relation to near-equilibrium kinetic theory
is discussed, and the distinction between quantum and classical statistical
fluctuations is shown to naturally emerge from the functional-integral
description. Example applications to the evolution of an ultracold atomic Bose
gas in one spatial dimension underline the power of the methods. The article is
compiled from the notes for lectures held at 46. Internationale
Universitaetswochen fuer Theoretische Physik 2008 in Schladming, Austria.Comment: 59 pages, 26 figures; Compiled from notes for lectures held at 46.
Internationale Universitaetswochen fuer Theoretische Physik 2008 in
Schladming, Austria. To be published in Eur. Phys. J. Special Topic
High light intensity photoassociation in a Bose-Einstein condensate
We investigate theoretically the molecular yield in photoassociation of
Bose-Einstein condensed sodium atoms for light intensities of the order of and
above those applied in a recent experiment. Our results show that the rate at
which ground state molecules may be formed saturates at high light intensities
whereas the loss rate of condensate atoms does not. This is caused by the
opposing roles of the short and long range pair correlations present near
resonance under the influence of the laser and is crucial for the development
of efficient photoassociation procedures in a condensate.Comment: 4 pages RevTeX, 4 Figures, numerical errors corrected in revised
versio
Large parity violating effects in atomic dysprosium with nearly degenerate Floquet eigenvalues
In this article we study effects of parity nonconservation in atomic
dysprosium, where one has a pair of nearly degenerate levels of opposite
parity. We consider the time evolution of this two-level system within
oscillatory electric and magnetic fields. These are chosen to have a periodical
structure with the same period, such that a Floquet matrix describes the time
evolution of the quantum states. We show that, if the states are unstable, the
eigenvalues of the Floquet matrix may have contributions proportional to the
square root of the parity violating interaction matrix element while they
are almost degenerate in their parity even part. This leads to beat frequencies
proportional to which are expected to be larger by several orders
of magnitude compared to ordinary P-violating contributions which are of order
. However, for the simple field configurations we considered, it still
seems to be difficult to observe these P-violating beat effects, since the
states decay too fast. On the other hand, we found that, within only a few
Floquet cycles, very large parity violating asymmetries with respect to
experimental setups of opposite chirality may be obtained. The electric and
magnetic fields as well as the time intervals necessary for this are in an
experimentally accessible range. For statistically significant effects beyond
one standard deviation a number of about atoms is required. Our ideas
may be applied directly to other 2-level atomic systems and different field
configurations. We hope that these ideas will stimulate experimental work in
this direction.Comment: Sep 1999, 29pp, 10 Fig
Universal scaling at non-thermal fixed points of a two-component Bose gas
Quasi-stationary far-from-equilibrium critical states of a two-component Bose
gas are studied in two spatial dimensions. After the system has undergone an
initial dynamical instability it approaches a non-thermal fixed point. At this
critical point the structure of the gas is characterised by ensembles of
(quasi-)topological defects such as vortices, skyrmions and solitons which give
rise to universal power-law behaviour of momentum correlation functions. The
resulting power-law spectra can be interpreted in terms of
strong-wave-turbulence cascades driven by particle transport into
long-wave-length excitations. Scaling exponents are determined on both sides of
the miscible-immiscible transition controlled by the ratio of the intra-species
to inter-species couplings. Making use of quantum turbulence methods, we
explain the specific values of the exponents from the presence of transient
(quasi-)topological defects.Comment: 13 pages, 12 figure
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