29 research outputs found
Cold atom gas at very high densities in an optical surface microtrap
An optical microtrap is realized on a dielectric surface by crossing a
tightly focused laser beam with an horizontal evanescent-wave atom mirror. The
nondissipative trap is loaded with cesium atoms through elastic
collisions from a cold reservoir provided by a large-volume optical surface
trap. With an observed 300-fold local increase of the atomic number density
approaching , unprecedented conditions of cold atoms
close to a surface are realized
Cooling atomic motion with quantum interference
We theoretically investigate the quantum dynamics of the center of mass of
trapped atoms, whose internal degrees of freedom are driven in a
-shaped configuration with the lasers tuned at two-photon resonance.
In the Lamb-Dicke regime, when the motional wave packet is well localized over
the laser wavelenght, transient coherent population trapping occurs, cancelling
transitions at the laser frequency. In this limit the motion can be efficiently
cooled to the ground state of the trapping potential. We derive an equation for
the center-of-mass motion by adiabatically eliminating the internal degrees of
freedom. This treatment provides the theoretical background of the scheme
presented in [G. Morigi {\it et al}, Phys. Rev. Lett. {\bf 85}, 4458 (2000)]
and implemented in [C.F. Roos {\it et al}, Phys. Rev. Lett. {\bf 85}, 5547
(2000)]. We discuss the physical mechanisms determining the dynamics and
identify new parameters regimes, where cooling is efficient. We discuss
implementations of the scheme to cases where the trapping potential is not
harmonic.Comment: 11 pages, 3 figure
Self-consistent model of ultracold atomic collisions and Feshbach resonances in tight harmonic traps
We consider the problem of cold atomic collisions in tight traps, where the
absolute scattering length may be larger than the trap size. As long as the
size of the trap ground state is larger than a characteristic length of the van
der Waals potential, the energy eigenvalues can be computed self-consistently
from the scattering amplitude for untrapped atoms. By comparing with the exact
numerical eigenvalues of the trapping plus interatomic potentials, we verify
that our model gives accurate eigenvalues up to milliKelvin energies for single
channel s-wave scattering of Na atoms in an isotropic harmonic trap,
even when outside the Wigner threshold regime. Our model works also for
multi-channel scattering, where the scattering length can be made large due to
a magnetically tunable Feshbach resonance.Comment: 7 pages, 4 figures (PostScript), submitted to Physical Review
Mixtures of Bosonic and Fermionic Atoms in Optical Lattices
We discuss the theory of mixtures of Bosonic and Fermionic atoms in periodic
potentials at zero temperature. We derive a general Bose--Fermi Hubbard
Hamiltonian in a one--dimensional optical lattice with a superimposed harmonic
trapping potential. We study the conditions for linear stability of the mixture
and derive a mean field criterion for the onset of a Bosonic superfluid
transition. We investigate the ground state properties of the mixture in the
Gutzwiller formulation of mean field theory, and present numerical studies of
finite systems. The Bosonic and Fermionic density distributions and the onset
of quantum phase transitions to demixing and to a Bosonic Mott--insulator are
studied as a function of the lattice potential strength. The existence is
predicted of a disordered phase for mixtures loaded in very deep lattices. Such
a disordered phase possessing many degenerate or quasi--degenerate ground
states is related to a breaking of the mirror symmetry in the lattice.Comment: 11 pages, 8 figures; added discussions; conclusions and references
expande
Large-amplitude driving of a superconducting artificial atom: Interferometry, cooling, and amplitude spectroscopy
Superconducting persistent-current qubits are quantum-coherent artificial
atoms with multiple, tunable energy levels. In the presence of large-amplitude
harmonic excitation, the qubit state can be driven through one or more of the
constituent energy-level avoided crossings. The resulting
Landau-Zener-Stueckelberg (LZS) transitions mediate a rich array of
quantum-coherent phenomena. We review here three experimental works based on
LZS transitions: Mach-Zehnder-type interferometry between repeated LZS
transitions, microwave-induced cooling, and amplitude spectroscopy. These
experiments exhibit a remarkable agreement with theory, and are extensible to
other solid-state and atomic qubit modalities. We anticipate they will find
application to qubit state-preparation and control methods for quantum
information science and technology.Comment: 13 pages, 5 figure
Expanding non homogeneous configurations of the model
A time dependent variational approach is considered to derive the equations
of movement for the model. The temporal evolution of the model
is performed numerically in the frame of the Gaussian approximation in a
lattice of 1+1 dimensions given non homogeneous initial conditions (like
bubbles) for the classical and quantum parts of the field which expands. A
schematic model for the initial conditions is presented considering the model
at finite fermionic density. The non zero fermionic density may lead either to
the restoration of the symmetry or to an even more asymmetric phase. Both kinds
of situations are considered as initial conditions and the eventual differences
in early time dynamics are discussed. In the early time evolution there is
strong energy exchange between the classical and quantum parts of the field as
the initial configuration expands. The contribution of the quantum fluctuations
is discussed especially in the strong coupling constant limit. The continuum
limit is analyzed.Comment: 23 pages (latex) plus thirteen figures in eps file
Production of a dual-species Bose-Einstein condensate of Rb and Cs atoms
We report the simultaneous production of Bose-Einstein condensates (BECs) of
Rb and Cs atoms in separate optical traps. The two samples are
mixed during laser cooling and loading but are separated by m for the
final stage of evaporative cooling. This is done to avoid considerable
interspecies three-body recombination, which causes heating and evaporative
loss. We characterize the BEC production process, discuss limitations, and
outline the use of the dual-species BEC in future experiments to produce
rovibronic ground state molecules, including a scheme facilitated by the
superfluid-to-Mott-insulator (SF-MI) phase transition
Resonance superfluidity in a quantum degenerate Fermi gas
We consider the superfluid phase transition that arises when a Feshbach resonance pairing occurs in a dilute Fermi gas. This is related to the phenomenon of superconductivity described by the seminal Bardeen-Cooper-Schrieffer (BCS) theory. In superconductivity, the phase transition is caused by a coupling between pairs of electrons within the medium. This coupling is perturbative and leads to a critical temperature Tc which is small compared to the Fermi temperature TF. Even high-Tc superconductors typically have a critical temperature which is two orders of magnitude below TF. Here we describe a resonance pairing mechanism in a quantum degenerate gas of potassium (40K) atoms which leads to superfluidity in a novel regime--a regime that promises the unique opportunity to experimentally study the crossover from the BCS phase of weakly-coupled fermions to the Bose Einstein condensate of strongly-bound composite bosons. We find that the transition to a superfluid phase is possible at the high critical temperature of about 0.5TF. It should be straightforward to verify this prediction, since these temperatures can already be achieved experimentally