178 research outputs found
Long-lived non-thermal states realized by atom losses in one-dimensional quasi-condensates
We investigate the cooling produced by a loss process non selective in energy
on a one-dimensional (1D) Bose gas with repulsive contact interactions in the
quasi-condensate regime. By performing nonlinear classical field calculations
for a homogeneous system, we show that the gas reaches a non-thermal state
where different modes have acquired different temperatures. After losses have
been turned off, this state is robust with respect to the nonlinear dynamics,
described by the Gross-Pitaevskii equation. We argue that the integrability of
the Gross-Pitaevskii equation is linked to the existence of such long-lived
non-thermal states, and illustrate this by showing that such states are not
supported within a non-integrable model of two coupled 1D gases of different
masses. We go beyond a classical field analysis, taking into account the
quantum noise introduced by the discreteness of losses, and show that the
non-thermal state is still produced and its non-thermal character is even
enhanced. Finally, we extend the discussion to gases trapped in a harmonic
potential and present experimental observations of a long-lived non-thermal
state within a trapped 1D quasi-condensate following an atom loss process
A cesium gas strongly confined in one dimension : sideband cooling and collisional properties
We study one-dimensional sideband cooling of Cesium atoms strongly confined
in a far-detuned optical lattice. The Lamb-Dicke regime is achieved in the
lattice direction whereas the transverse confinement is much weaker. The
employed sideband cooling method, first studied by Vuletic et al.\cite{Vule98},
uses Raman transitions between Zeeman levels and produces a spin-polarized
sample. We present a detailed study of this cooling method and investigate the
role of elastic collisions in the system. We accumulate of the atoms
in the vibrational ground state of the strongly confined motion, and elastic
collisions cool the transverse motion to a temperature of K=, where is the oscillation
frequency in the strongly confined direction. The sample then approaches the
regime of a quasi-2D cold gas. We analyze the limits of this cooling method and
propose a dynamical change of the trapping potential as a mean of cooling the
atomic sample to still lower temperatures. Measurements of the rate of
thermalization between the weakly and strongly confined degrees of freedom are
compatible with the zero energy scattering resonance observed previously in
weak 3D traps. For the explored temperature range the measurements agree with
recent calculations of quasi-2D collisions\cite{Petr01}. Transparent analytical
models reproduce the expected behavior for and also for where the 2D
features are prominent.Comment: 18 pages, 12 figure
Direct observation of quantum phonon fluctuations in a one dimensional Bose gas
We report the first direct observation of collective quantum fluctuations in
a continuous field. Shot-to-shot atom number fluctuations in small sub-volumes
of a weakly interacting ultracold atomic 1D cloud are studied using \textit{in
situ} absorption imaging and statistical analysis of the density profiles. In
the cloud centers, well in the \textit{quantum quasicondensate} regime, the
ratio of chemical potential to thermal energy is , and,
owing to high resolution, up to 20% of the microscopically observed
fluctuations are quantum phonons. Within a non-local analysis at variable
observation length, we observe a clear deviation from a classical field
prediction, which reveals the emergence of dominant quantum fluctuations at
short length scales, as the thermodynamic limit breaks down.Comment: 4 pages, 3 figures (Supplementary material 3 pages, 3 figures
Quasi 2D Bose-Einstein condensation in an optical lattice
We study the phase transition of a gas of Rb atoms to quantum degeneracy in
the combined potential of a harmonically confining magnetic trap and the
periodic potential of an optical lattice. For high optical lattice potentials
we observe a significant change in the temperature dependency of the population
of the ground state of the system. The experimental results are explained by
the subsequent formation of quasi 2D condensates in the single lattice sites.Comment: 7 pages (including 3 figures
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
Optimization of a ClââHâ inductively coupled plasma etching process adapted to nonthermalized InP wafers for the realization of deep ridge heterostructures
Inductively coupled plasmaetching using ClââHâ chemistry with no additive gas (CHâ, Ar, or Nâ) is studied to realize deep (>5ÎŒm) ridges with smooth and vertical sidewalls. The process is optimized for nonthermalized InP wafers to avoid the use of thermal grease. Cleaning of the rear side of the wafer after etching is avoided, which is suitable for an industrial process or for critical subsequent steps such as epitaxial regrowth. The influence of the ClââHâ ratio on the etching mechanism is investigated for both InP bulk layers and InGaAsâInP or InGaAlAsâInPheterostructures. The authors show that this ratio is the main parameter controlling the ridge profile, in a similar way for both bulk InP and InGa(Al)AsâInP samples. Smooth and vertical sidewalls with neither undercuts nor notches can be obtained in the 0.5â1mTpressure range for a hydrogen percentage of 35%â45% in the gas mixture. Etching rates from 900to1300nmâmin together with a selectivity over SiNx dielectric mask as high as 24:1â29:1 are measured for the InP bulk layers under these conditions. Etching does not affect the optical quality of the heterostructures as evidenced from micro-photoluminescence measurements performed on 1.6âto0.85âÎŒm-wide deep etched ridge waveguides. The process is well adapted to the realization of low loss deep ridge waveguides or buried heterostructures
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
Non-equilibrium coherence dynamics in one-dimensional Bose gases
Low-dimensional systems are beautiful examples of many-body quantum physics.
For one-dimensional systems the Luttinger liquid approach provides insight into
universal properties. Much is known of the equilibrium state, both in the
weakly and strongly interacting regime. However, it remains a challenge to
probe the dynamics by which this equilibrium state is reached. Here we present
a direct experimental study of the coherence dynamics in both isolated and
coupled degenerate 1d Bose gases. Dynamic splitting is used to create two 1d
systems in a phase coherent state. The time evolution of the coherence is
revealed in local phase shifts of the subsequently observed interference
patterns. Completely isolated 1d Bose gases are observed to exhibit a universal
sub-exponential coherence decay in excellent agreement with recent predictions
by Burkov et al. [Phys. Rev. Lett. 98, 200404 (2007)]. For two coupled 1d Bose
gases the coherence factor is observed to approach a non-zero equilibrium value
as predicted by a Bogoliubov approach. This coupled-system decay to finite
coherence is the matter wave equivalent of phase locking two lasers by
injection. The non-equilibrium dynamics of superfluids plays an important role
in a wide range of physical systems, such as superconductors, quantum-Hall
systems, superfluid Helium, and spin systems. Our experiments studying
coherence dynamics show that 1d Bose gases are ideally suited for investigating
this class of phenomena.Comment: to appear in natur
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
Influence of intrinsic decoherence on nonclassical properties of the output of a Bose-Einstein condensate
We investigate nonclassical properties of the output of a Bose-Einstein
condensate in Milburn's model of intrinsic decoherence. It is shown that the
squeezing property of the atom laser is suppressed due to decoherence.
Nevertheless, if some very special conditions were satisfied, the squeezing
properties of atom laser could be robust against the decoherence.Comment: 17 pages, 5 figures, Late
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