691 research outputs found
Adsorbate Electric Fields on a Cryogenic Atom Chip
We investigate the behaviour of electric fields originating from adsorbates
deposited on a cryogenic atom chip as it is cooled from room temperature to
cryogenic temperature. Using Rydberg electromagnetically induced transparency
we measure the field strength versus distance from a 1 mm square of YBCO
patterned onto a YSZ chip substrate. We find a localized and stable dipole
field at room temperature and attribute it to a saturated layer of chemically
adsorbed rubidium atoms on the YBCO. As the chip is cooled towards 83 K we
observe a change in sign of the electric field as well as a transition from a
localized to a delocalized dipole density. We relate these changes to the onset
of physisorption on the chip surface when the van der Waals attraction
overcomes the thermal desorption mechanisms. Our findings suggest that, through
careful selection of substrate materials, it may be possible to reduce the
electric fields caused by atomic adsorption on chips, opening up experiments to
controlled Rydberg-surface coupling schemes.Comment: 5 pages, 4 figure
10 GeV dark matter candidates and cosmic-ray antiprotons
Recent measurements performed with some direct dark matter detection
experiments, e.g. CDMS-II and CoGENT (after DAMA/LIBRA), have unveiled a few
events compatible with weakly interacting massive particles. The preferred mass
range is around 10 GeV, with a quite large spin-independent cross section of
-. In this paper, we recall that a light dark
matter particle with dominant couplings to quarks should also generate
cosmic-ray antiprotons. Taking advantage of recent works constraining the
Galactic dark matter mass profile on the one hand and on cosmic-ray propagation
on the other hand, we point out that considering a thermal annihilation cross
section for such low mass candidates very likely results in an antiproton flux
in tension with the current data, which should be taken into account in
subsequent studies.Comment: 4 pages, 2 figures. V2: minor changes to match the published versio
Design of magnetic traps for neutral atoms with vortices in type-II superconducting micro-structures
We design magnetic traps for atoms based on the average magnetic field of
vortices induced in a type-II superconducting thin film. This magnetic field is
the critical ingredient of the demonstrated vortex-based atom traps, which
operate without transport current. We use Bean's critical-state method to model
the vortex field through mesoscopic supercurrents induced in the thin strip.
The resulting inhomogeneous magnetic fields are studied in detail and compared
to those generated by multiple normally-conducting wires with transport
currents. Various vortex patterns can be obtained by programming different
loading-field and transport current sequences. These variable magnetic fields
are employed to make versatile trapping potentials.Comment: 11 pages, 14 figure
Sub-Natural-Linewidth Quantum Interference Features Observed in Photoassociation of a Thermal Gas
By driving photoassociation transitions we form electronically excited
molecules (Na) from ultra-cold (50-300 K) Na atoms. Using a second
laser to drive transitions from the excited state to a level in the molecular
ground state, we are able to split the photoassociation line and observe
features with a width smaller than the natural linewidth of the excited
molecular state. The quantum interference which gives rise to this effect is
analogous to that which leads to electromagnetically induced transparency in
three level atomic systems, but here one of the ground states is a
pair of free atoms while the other is a bound molecule. The linewidth is
limited primarily by the finite temperature of the atoms.Comment: 4 pages, 5 figure
Reconfigurable self-sufficient traps for ultracold atoms based on a superconducting square
We report on the trapping of ultracold atoms in the magnetic field formed
entirely by persistent supercurrents induced in a thin film type-II
superconducting square. The supercurrents are carried by vortices induced in
the 2D structure by applying two magnetic field pulses of varying amplitude
perpendicular to its surface. This results in a self-sufficient quadrupole trap
that does not require any externally applied fields. We investigate the
trapping parameters for different supercurrent distributions. Furthermore, to
demonstrate possible applications of these types of supercurrent traps we show
how a central quadrupole trap can be split into four traps by the use of a bias
field.Comment: 5 pages, 7 figure
Density dependence of the Ionization Avalanche in ultracold Rydberg gases
We report on the behaviour of the ionization avalanche in an ensemble of
ultracold 87Rb atoms coupled to a high lying Rydberg state and investigate
extensions to the current model by including the effects of three-body
recombination and plasma expansion. To separate the two effects we study the
time dependence of the plasma formation at various densities as well as for
different nS and nD states. At medium densities and low n we observe the onset
of the avalanche as has been reported in other experiments, as well as a
subsequent turn-off of the avalanche for longer excitation times, which we
associate with plasma expansion. At higher densities and for higher lying
Rydberg states we observe a disappearance of the avalanche signature, which we
attribute to three-body recombination.Comment: 5 pages, 4 figure
All-optical generation and photoassociative probing of sodium Bose-Einstein condensates
We demonsatrate an all optical technique to evaporatively produce sodium
Bose-Einstein condensates (BEC). We use a crossed-dipole trap formed from light
near 1060 nm, and a simple ramp of the intensity to force evaporation. In
addition, we introduce photoassociation as diagnostic of the trap loading
process, and show that it can be used to detect the onset of Bose-Einstein
condensation. Finally, we demonstrate the straightforward production of
multiple traps with condensates using this technique, and that some control
over the spinor state of the BEC is achieved by positioning the trap as well.Comment: 8 pages, 10 figure
The Integrated Polarization of Spiral Galaxy Disks
We present integrated polarization properties of nearby spiral galaxies at
4.8 GHz, and models for the integrated polarization of spiral galaxy disks as a
function of inclination. Spiral galaxies in our sample have observed integrated
fractional polarization in the range < 1% to 17.6%. At inclinations less than
50 degrees, the fractional polarization depends mostly on the ratio of random
to regular magnetic field strength. At higher inclinations, Faraday
depolarization associated with the regular magnetic field becomes more
important. The observed degree of polarization is lower (<4%) for more luminous
galaxies, in particular those with L_{4.8} > 2 x 10^{21} W/Hz. The polarization
angle of the integrated emission is aligned with the apparent minor axis of the
disk for galaxies without a bar. In our axially symmetric models, the
polarization angle of the integrated emission is independent of wavelength.
Simulated distributions of fractional polarization for randomly oriented spiral
galaxies at 4.8 GHz and 1.4 GHz are presented. We conclude that polarization
measurements, e.g. with the SKA, of unresolved spiral galaxies allow
statistical studies of the magnetic field in disk galaxies using large samples
in the local universe and at high redshift. As these galaxies behave as
idealized background sources without internal Faraday rotation, they can be
used to detect large-scale magnetic fields in the intergalactic medium.Comment: 13 pages, 6 figures; Accepted for publication in The Astrophysical
Journa
Microoptical Realization of Arrays of Selectively Addressable Dipole Traps: A Scalable Configuration for Quantum Computation with Atomic Qubits
We experimentally demonstrate novel structures for the realisation of
registers of atomic qubits: We trap neutral atoms in one and two-dimensional
arrays of far-detuned dipole traps obtained by focusing a red-detuned laser
beam with a microfabricated array of microlenses. We are able to selectively
address individual trap sites due to their large lateral separation of 125 mu
m. We initialize and read out different internal states for the individual
sites. We also create two interleaved sets of trap arrays with adjustable
separation, as required for many proposed implementations of quantum gate
operations
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