181 research outputs found
Experimental evidence for the breakdown of a Hartree-Fock approach in a weakly interacting Bose gas
We study the formation of a quasi-condensate in a nearly one dimensional,
weakly interacting trapped atomic Bose gas. We show that a Hartree Fock
(mean-field) approach fails to explain the presence of the quasi-condensate in
the center of the cloud: the quasi-condensate appears through an
interaction-driven cross-over and not a saturation of the excited states.
Numerical calculations based on Bogoliubov theory give an estimate of the
cross-over density in agreement with experimental results.Comment: submitted to Phys. Rev. Letter
Limitation of the modulation method to smooth wire guide roughness
It was recently demonstrated that wire guide roughness can be suppressed by
modulating the wire currents so that the atoms experience a time-averaged
potential without roughness. We theoretically study the limitations of this
technique. At low modulation frequency, we show that the longitudinal potential
modulation produces a heating of the cloud and we compute the heating rate. We
also give a quantum derivation of the rough conservative potential associated
with the micro-motion of the atoms. At large modulation frequency, we compute
the loss rate due to non adiabatic spin flip and show it presents resonnances
at multiple modulation frequencies. These studies show that the modulation
technique works for a wide range of experimental parameters. We also give
conditions to realise radio-frequency evaporative cooling in such a modulated
trap.Comment: 11 page
Mapping out the quasicondensate transition through the dimensional crossover from one to three dimensions
By measuring the density fluctuations in a highly elongated weakly interacting Bose gas, we observe and quantify the transition from the ideal gas to a quasicondensate regime throughout the dimensional crossover from a purely one-dimensional (1D) to an almost three-dimensional (3D) gas. We show that that the entire transition region and the dimensional crossover are described surprisingly well by the modified Yang-Yang model. Furthermore, we find that at low temperatures the linear density at the quasicondensate transition scales according to an interaction-driven scenario of a longitudinally uniform 1D Bose gas, whereas at high temperatures it scales according to the degeneracy-driven critical scenario of transverse condensation of a 3D ideal gas
Fabrication and Optical Properties of a Fully Hybrid Epitaxial ZnO-Based Microcavity in the Strong Coupling Regime
In order to achieve polariton lasing at room temperature, a new fabrication
methodology for planar microcavities is proposed: a ZnO-based microcavity in
which the active region is epitaxially grown on an AlGaN/AlN/Si substrate and
in which two dielectric mirrors are used. This approach allows as to
simultaneously obtain a high-quality active layer together with a high photonic
confinement as demonstrated through macro-, and micro-photoluminescence
({\mu}-PL) and reflectivity experiments. A quality factor of 675 and a maximum
PL emission at k=0 are evidenced thanks to {\mu}-PL, revealing an efficient
polaritonic relaxation even at low excitation power.Comment: 12 pages, 3 figure
Realizing a stable magnetic double-well potential on an atom chip
We discuss design considerations and the realization of a magnetic
double-well potential on an atom chip using current-carrying wires. Stability
requirements for the trapping potential lead to a typical size of order microns
for such a device. We also present experiments using the device to manipulate
cold, trapped atoms
Patterned silicon substrates: a common platform for room temperature GaN and ZnO polariton lasers
A new platform for fabricating polariton lasers operating at room temperature
is introduced: nitride-based distributed Bragg reflectors epitaxially grown on
patterned silicon substrates. The patterning allows for an enhanced strain
relaxation thereby enabling to stack a large number of crack-free AlN/AlGaN
pairs and achieve cavity quality factors of several thousands with a large
spatial homogeneity. GaN and ZnO active regions are epitaxially grown thereon
and the cavities are completed with top dielectric Bragg reflectors. The two
structures display strong-coupling and polariton lasing at room temperature and
constitute an intermediate step in the way towards integrated polariton
devices
Box traps on an atom chip for one-dimensional quantum gases
We present the implementation of tailored trapping potentials for ultracold
gases on an atom chip. We realize highly elongated traps with box-like
confinement along the long, axial direction combined with conventional harmonic
confinement along the two radial directions. The design, fabrication and
characterization of the atom chip and the box traps is described. We load
ultracold (K) clouds of Rb in a box trap, and demonstrate
Bose-gas focusing as a means to characterize these atomic clouds in arbitrarily
shaped potentials. Our results show that box-like axial potentials on atom
chips are very promising for studies of one-dimensional quantum gases.Comment: 9 pages 4 figure
Momentum distribution of 1D Bose gases at the quasi-condensation crossover: theoretical and experimental investigation
We investigate the momentum distribution of weakly interacting 1D Bose gases
at thermal equilibrium both experimentally and theoretically. Momentum
distribution of single 1D Bose gases is measured using a focusing technique,
whose resolution we improve via a guiding scheme. The momentum distribution
compares very well with quantum Monte Carlo calculations for the Lieb-Liniger
model at finite temperature, allowing for an accurate thermometry of the gas
that agrees with (and improves upon) the thermometry based on in situ density
fluctuation measurements. The quasi-condensation crossover is investigated via
two different experimental parameter sets, corresponding to the two different
sides of the crossover. Classical field theory is expected to correctly
describe the quasi-condensation crossover of weakly interacting gases. We
derive the condition of validity of the classical field theory, and find that,
in typical experiments, interactions are too strong for this theory to be
accurate. This is confirmed by a comparison between the classical field
predictions and the numerically exact quantum Monte Carlo calculations.Comment: 8 page
LO-phonon assisted polariton lasing in a ZnO based microcavity
Polariton relaxation mechanisms are analysed experimentally and theoretically
in a ZnO-based polariton laser. A minimum lasing threshold is obtained when the
energy difference between the exciton reservoir and the bottom of the lower
polariton branch is resonant with the LO phonon energy. Tuning off this
resonance increases the threshold, and exciton-exciton scattering processes
become involved in the polariton relaxation. These observations are
qualitatively reproduced by simulations based on the numerical solution of the
semi-classical Boltzmann equations
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