226 research outputs found
General conditions for a quantum adiabatic evolution
The smallness of the variation rate of the hamiltonian matrix elements
compared to the (square of the) energy spectrum gap is usually believed to be
the key parameter for a quantum adiabatic evolution. However it is only
perturbatively valid for scaled timed hamiltonian and resonance processes as
well as off resonance possible constructive St\"{u}ckelberg interference
effects violate this usual condition for general hamiltionian. More general
adiabatic condition and exact bounds for adiabatic quantum evolution are
derived and studied in the framework of a two-level system. The usual criterion
is restored for real two level hamiltonian with small number of monotonicity
changes of the hamiltonian matrix elements and its derivative.Comment: 4 page
A Study of molecular cooling via Sisyphus processes
We present a study of Sisyphus cooling of molecules: the scattering of a
single-photon remove a substantial amount of the molecular kinetic energy and
an optical pumping step allow to repeat the process. A review of the produced
cold molecules so far indicates that the method can be implemented for most of
them, making it a promising method able to produce a large sample of molecules
at sub-mK temperature. Considerations of the required experimental parameters,
for instance the laser power and linewidth or the trap anisotropy and
dimensionality, are given. Rate equations, as well as scattering and dipolar
forces, are solved using Kinetic Monte Carlo methods for several lasers and
several levels. For NH molecules, such detailed simulation predicts a 1000-fold
temperature reduction and an increase of the phase space density by a factor of
10^7 . Even in the case of molecules with both low Franck-Condon coefficients
and a non-closed pumping scheme, 60% of trapped molecules can be cooled from
100 mK to sub-mK temperature in few seconds. Additionally, these methods can be
applied to continuously decelerate and cool a molecular bea
Laser stimulated deexcitation of Rydberg antihydrogen atoms
Antihydrogen atoms are routinely formed at CERN in a broad range of Rydberg
states. Ground-state anti-atoms, those useful for precision measurements, are
eventually produced through spontaneous decay. However given the long lifetime
of Rydberg states the number of ground-state antihydrogen atoms usable is
small, in particular for experiments relying on the production of a beam of
antihydrogen atoms. Therefore, it is of high interest to efficiently stimulate
the decay in order to retain a higher fraction of ground-state atoms for
measurements. We propose a method that optimally mixes the high angular
momentum states with low ones enabling to stimulate, using a broadband
frequency laser, the deexcitation toward low-lying states, which then
spontaneously decay to ground-state. We evaluated the method in realistic
antihydrogen experimental conditions. For instance, starting with an initial
distribution of atoms within the manifolds, as formed through charge
exchange mechanism, we show that more than 80\% of antihydrogen atoms will be
deexcited to the ground-state within 100 ns using a laser producing 2 J at 828
nm.Comment: 10 page
Phase space density limitation in laser cooling without spontaneous emission
We study the possibility to enhance the phase space density of
non-interacting particles submitted to a classical laser field without
spontaneous emission. We clearly state that, when no spontaneous emission is
present, a quantum description of the atomic motion is more reliable than
semi-classical description which can lead to large errors especially if no care
is taken to smooth structures smaller than the Heisenberg uncertainty
principle. Whatever the definition of position - momentum phase space density,
its gain is severely bounded especially when started from a thermal sample.
More precisely, the maximum phase space density, can only be improved by a
factor M for M-level atoms. This bound comes from a transfer between the
external and internal degrees of freedom. To circumvent this limit, one can use
non-coherent light fields, informational feedback cooling schemes, involve
collectives states between fields and atoms, or allow a single spontaneous
emission evenComment: 3 figures, 4 page
Laser Cooling of Molecular Anions
We propose a scheme for laser cooling of negatively charged molecules. We
briefly summarise the requirements for such laser cooling and we identify a
number of potential candidates. A detailed computation study with C, the
most studied molecular anion, is carried out. Simulations of 3D laser cooling
in a gas phase show that this molecule could be cooled down to below 1 mK in
only a few tens of milliseconds, using standard lasers. Sisyphus cooling, where
no photo-detachment process is present, as well as Doppler laser cooling of
trapped C, are also simulated. This cooling scheme has an impact on the
study of cold molecules, molecular anions, charged particle sources and
antimatter physics
Electric-field induced dipole blockade with Rydberg atoms
High resolution laser Stark excitation of np (60 < n < 85) Rydberg states of
ultra-cold cesium atoms shows an efficient blockade of the excitation
attributed to long-range dipole-dipole interaction. The dipole blockade effect
is observed as a quenching of the Rydberg excitation depending on the value of
the dipole moment induced by the external electric field. Effects of eventual
ions which could match the dipole blockade effect are discussed in detail but
are ruled out for our experimental conditions. Analytic and Monte-Carlo
simulations of the excitation of an ensemble of interacting Rydberg atoms agree
with the experiments indicates a major role of the nearest neighboring Rydberg
atom.Comment: 4 page
Dipole blockade through Rydberg Forster resonance energy transfer
High resolution laser excitation of np Rydberg states of cesium atoms shows a
dipole blockade at F\"{o}rster resonances corresponding to the resonant
dipole-dipole energy transfer of the np + np → ns + (n + 1)s reaction.
The dipole-dipole interaction can be tuned on and off by the Stark effect, and
such a process observed for relatively low n (25 − 41) is promising for
quantum gate devices. Both Penning ionization and saturation in the laser
excitation can limit the range of observation of the dipole blockadeComment: number of pages:
Triplet-singlet conversion by broadband optical pumping
We demonstrate the conversion of cold Cs_{2} molecules initially distributed
over several vibrational levels of the lowest triplet state a^{3}\Sigma_{u}^{+}
into the singlet ground state X^{1}\Sigma_{g}^{+}. This conversion is realized
by a broadband laser exciting the molecules to a well-chosen state from which
they may decay to the singlet state throug\textcolor{black}{h two sequential
single-photon emission steps: Th}e first photon populates levels with mixed
triplet-singlet character, making possible a second spontaneous emission down
to several vibrational levels of the X^{1}\Sigma_{g}^{+} states. By adding an
optical scheme for vibrational cooling, a substantial fraction of molecules are
transferred to the ground vibrational level of the singlet state. The
efficiency of the conversion process, with and without vibrational cooling, is
discussed at the end of the article. The presented conversion is general in
scope and could be extended to other molecules.Comment: 5 pages, 4 figure
Ionization of Rydberg atoms embedded in an ultracold plasma
We have studied the behavior of cold Rydberg atoms embedded in an ultracold
plasma. We demonstrate that even deeply bound Rydberg atoms are completely
ionized in such an environment, due to electron collisions. Using a fast pulse
extraction of the electrons from the plasma we found that the number of excess
positive charges, which is directly related to the electron temperature Te, is
not strongly affected by the ionization of the Rydberg atoms. Assuming a
Michie-King equilibrium distribution, in analogy with globular star cluster
dynamics, we estimate Te. Without concluding on heating or cooling of the
plasma by the Rydberg atoms, we discuss the range for changing the plasma
temperature by adding Rydberg atoms.Comment: To be published in P.R.
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