6,654 research outputs found
Breakdown of integrability in a quasi-one-dimensional ultracold bosonic gas
We demonstrate that virtual excitations of higher radial modes in an atomic
Bose gas in a tightly confining waveguide result in effective three-body
collisions that violate integrability in this quasi-one-dimensional quantum
system and give rise to thermalization. The estimated thermalization rates are
consistent with recent experimental results in quasi-1D dynamics of ultracold
atoms.Comment: 4 pages, 3 figures, revtex
Ultracold atoms in radio-frequency-dressed potentials beyond the rotating wave approximation
We study dressed Bose-Einstein condensates in an atom chip radio-frequency
trap. We show that in this system sufficiently strong dressing can be achieved
to cause the widely used rotating wave approximation (RWA) to break down. We
present a full calculation of the atom - field coupling which shows that the
non-RWA contributions quantitatively alter the shape of the emerging dressed
adiabatic potentials. The non-RWA contributions furthermore lead to additional
allowed transitions between dressed levels. We use RF spectroscopy of
Bose-Einstein condensates trapped in the dressed state potentials to directly
observe the transition from the RWA to the beyond-RWA regime.Comment: 6 pages, 4 figure
Controlled quantum stirring of Bose-Einstein condensates
By cyclic adiabatic change of two control parameters of an optical trap one
can induce a circulating current of condensed bosons. The amount of particles
that are transported per period depends on the "radius" of the cycle, and this
dependence can be utilized in order to probe the interatomic interactions. For
strong repulsive interaction the current can be regarded as arising from a
sequence of Landau-Zener crossings. For weaker interaction one observes either
gradual or coherent mega crossings, while for attractive interaction the
particles are glued together and behave like a classical ball. For the analysis
we use the Kubo approach to quantum pumping with the associated Dirac monopoles
picture of parameter space.Comment: 12 pages, 8 figure
Adiabatic radio frequency potentials for the coherent manipulation of matter waves
Adiabatic dressed state potentials are created when magnetic sub-states of
trapped atoms are coupled by a radio frequency field. We discuss their
theoretical foundations and point out fundamental advantages over potentials
purely based on static fields. The enhanced flexibility enables one to
implement numerous novel configurations, including double wells, Mach-Zehnder
and Sagnac interferometers which even allows for internal state-dependent atom
manipulation. These can be realized using simple and highly integrated wire
geometries on atom chips.Comment: 13 pages, 2 figure
Splitting and merging an elongated Bose-Einstein condensate at finite temperature
We analyze coherence effects during the splitting of a quasi one-dimensional
condensate into two spatially separated ones and their subsequent merging into
a single condensate. Our analysis takes into account finite-temperature
effects, where phase fluctuations play an important role. We show that, at
zero-temperature, the two split condensates can be merged into a single one
with a negligible phase difference. By increasing temperature to a finite value
below the critical point for condensation (), i.e., , a
considerable enhancement of phase and density fluctuations appears during the
process of splitting and merging. Our results show that if the process of
splitting and merging is sufficiently adiabatic, the whole process is quite
insensitive to phase fluctuations and even at high temperatures, a single
condensate can be produced.Comment: 8 pages, 6 figure
An optical lattice on an atom chip
Optical dipole traps and atom chips are two very powerful tools for the
quantum manipulation of neutral atoms. We demonstrate that both methods can be
combined by creating an optical lattice potential on an atom chip. A
red-detuned laser beam is retro-reflected using the atom chip surface as a
high-quality mirror, generating a vertical array of purely optical oblate
traps. We load thermal atoms from the chip into the lattice and observe cooling
into the two-dimensional regime where the thermal energy is smaller than a
quantum of transverse excitation. Using a chip-generated Bose-Einstein
condensate, we demonstrate coherent Bloch oscillations in the lattice.Comment: 3 pages, 2 figure
Numerical simulation of the turbulent convective buoyant flow of sodium over a backward- facing step
A forced convective and a buoyancy-aided turbulent liquid sodium flow over a
backward-facing step with a constant heat flux applied on the indented wall is simulated.
Linear eddy viscosity models are used for the Reynolds stresses. Turbulent heat fluxes are
modelled with a single gradient diffusion hypotheses with two different approaches to evaluate
the turbulent Prandtl number. Moreover, the influence of turbulence on heat transfer to sodium
is also assessed through simulations with zero turbulent thermal diffusivity. The results are
compared with DNS data from literature. The velocity and turbulent kinetic energy profiles
predicted by all models are in good agreement with the DNS data. The local Nusselt number
trend is qualitatively well captured, however, its magnitude is underestimated by all models
for the mixed convection case. For forced convection, the heat transfer is overestimated by all
heat flux models. The simulation with neglected turbulent heat transfer shows the best overall
agreement for the forced convection case. For the mixed convection best agreement is obtained
using a correlation to locally evaluate the turbulent thermal diffusivity
Numerical simulation of the turbulent convective buoyant flow of sodium over a backward- facing step
A forced convective and a buoyancy-aided turbulent liquid sodium flow over a backward-facing step with a constant heat flux applied on the indented wall is simulated. Linear eddy viscosity models are used for the Reynolds stresses. Turbulent heat fluxes are modelled with a single gradient diffusion hypotheses with two different approaches to evaluate the turbulent Prandtl number. Moreover, the inuence of turbulence on heat transfer to sodium is also assessed through simulations with zero turbulent thermal diffusivity. The results are compared with DNS data from literature. The velocity and turbulent kinetic energy profiles predicted by all models are in good agreement with the DNS data. The local Nusselt number trend is qualitatively well captured, however, its magnitude is underestimated by all models for the mixed convection case. For forced convection, the heat transfer is overestimated by all heat flux models. The simulation with neglected turbulent heat transfer shows the best overall agreement for the forced convection case. For the mixed convection best agreement is obtained using a correlation to locally evaluate the turbulent thermal diffusivity
Multi-layer atom chips for versatile atom micro manipulation
We employ a combination of optical UV- and electron-beam-lithography to
create an atom chip combining sub-micron wire structures with larger
conventional wires on a single substrate. The new multi-layer fabrication
enables crossed wire configurations, greatly enhancing the flexibility in
designing potentials for ultra cold quantum gases and Bose-Einstein
condensates. Large current densities of >6 x 10^7 A/cm^2 and high voltages of
up to 65 V across 0.3 micron gaps are supported by even the smallest wire
structures. We experimentally demonstrate the flexibility of the next
generation atom chip by producing Bose-Einstein condensates in magnetic traps
created by a combination of wires involving all different fabrication methods
and structure sizes.Comment: 4 pages, 5 figure
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