78 research outputs found
A magnetic lens for cold atoms controlled by a rf field
We report on a new type of magnetic lens that focuses atomic clouds using a
static inhomogeneous magnetic field in combination with a radio-frequency
field. The experimental study is performed with a cloud of cold cesium atoms.
The rf field adiabatically deforms the magnetic potential of a coil and
therefore changes its focusing properties. The focal length can be tuned
precisely by changing the rf frequency value. Depending on the rf antenna
position relative to the DC magnetic profile, the focal length of the atomic
lens can be either decreased or increased by the rf field
Optimized production of large Bose Einstein Condensates
We suggest different simple schemes to efficiently load and evaporate a
''dimple'' crossed dipolar trap. The collisional processes between atoms which
are trapped in a reservoir load in a non adiabatic way the dimple. The
reservoir trap can be provided either by a dark SPOT Magneto Optical Trap, the
(aberrated) laser beam itself or by a quadrupolar or quadratic magnetic trap.
Optimal parameters for the dimple are derived from thermodynamical equations
and from loading time, including possible inelastic and Majorana losses. We
suggest to load at relatively high temperature a tight optical trap. Simple
evaporative cooling equations, taking into account gravity, the possible
occurrence of hydrodynamical regime, Feshbach resonance processes and three
body recombination events are given. To have an efficient evaporation the
elastic collisional rate (in s) is found to be on the order of the
trapping frequency and lower than one hundred times the temperature in
micro-Kelvin. Bose Einstein condensates with more than atoms should be
obtained in much less than one second starting from an usual MOT setup.Comment: 14 page
Thermodynamics of a Bose Einstein condensate with free magnetization
We study thermodynamic properties of a gas of spin 3 52Cr atoms across Bose
Einstein condensation. Magnetization is free, due to dipole-dipole interactions
(DDIs). We show that the critical temperature for condensation is lowered at
extremely low magnetic fields, when the spin degree of freedom is thermally
activated. The depolarized gas condenses in only one spin component, unless the
magnetic field is set below a critical value, below which a non ferromagnetic
phase is favored. Finally we present a spin thermometry efficient even below
the degeneracy temperature.Comment: 4 pages, 4 figure
Anisotropic excitation spectrum of a dipolar quantum Bose gas
We measure the excitation spectrum of a dipolar Chromium Bose Einstein
Condensate with Raman-Bragg spectroscopy. The energy spectrum depends on the
orientation of the dipoles with respect to the excitation momentum,
demonstrating an anisotropy which originates from the dipole-dipole
interactions between the atoms. We compare our results with the Bogoliubov
theory based on the local density approximation, and, at large excitation
wavelengths, with numerical simulations of the time dependent Gross-Pitaevskii
equation. Our results show an anisotropy of the speed of soundComment: 3 figure
Dipolar atomic spin ensembles in a double-well potential
We experimentally study the spin dynamics of mesoscopic ensembles of
ultracold magnetic spin-3 atoms located in two separated wells of an optical
dipole trap. We use a radio-frequency sweep to selectively flip the spin of the
atoms in one of the wells, which produces two separated spin domains of
opposite polarization. We observe that these engineered spin domains are
metastable with respect to the long-range magnetic dipolar interactions between
the two ensembles. The absence of inter-cloud dipolar spin-exchange processes
reveals a classical behavior, in contrast to previous results with atoms loaded
in an optical lattice. When we merge the two subsystems, we observe
spin-exchange dynamics due to contact interactions which enable the first
determination of the s-wave scattering length of 52Cr atoms in the S=0
molecular channel a_0=13.5^{+11}_{-10.5}a_B (where a_B is the Bohr radius).Comment: 9 pages, 7 figure
Resonant demagnetization of a dipolar BEC in a 3D optical lattice
We study dipolar relaxation of a chromium BEC loaded into a 3D optical
lattice. We observe dipolar relaxation resonances when the magnetic energy
released during the inelastic collision matches an excitation towards higher
energy bands. A spectroscopy of these resonances for two orientations of the
magnetic field provides a 3D band spectroscopy of the lattice. The narrowest
resonance is registered for the lowest excitation energy. Its line-shape is
sensitive to the on-site interaction energy. We use such sensitivity to probe
number squeezing in a Mott insulator, and we reveal the production of
three-body states with entangled spin and orbital degrees of freedom.Comment: 5 pages, 3 Figures, Supplemental Materia
All-Optical Production of Chromium Bose-Einstein Condensates
We report on the production of ^52Cr Bose Einstein Condensates (BEC) with an
all-optical method. We first load 5.10^6 metastable chromium atoms in a 1D
far-off-resonance optical trap (FORT) from a Magneto Optical Trap (MOT), by
combining the use of Radio Frequency (RF) frequency sweeps and depumping
towards the ^5S_2 state. The atoms are then pumped to the absolute ground
state, and transferred into a crossed FORT in which they are evaporated. The
fast loading of the 1D FORT (35 ms 1/e time), and the use of relatively fast
evaporative ramps allow us to obtain in 20 s about 15000 atoms in an almost
pure condensate.Comment: 4 pages, 4 figure
Spontaneous Evolution of Rydberg Atoms into an Ultracold Plasma
We have observed the spontaneous evolution of a dense sample of Rydberg atoms into an ultracold plasma, in spite of the fact that each of the atoms may initially be bound by up to 100 cm21. When the atoms are initially bound by 70 cm21, this evolution occurs when most of the atoms are translationally cold, ,1 mK, but a small fraction, 1%, is at room temperature. Ionizing collisions between hot and cold Rydberg atoms and blackbody photoionization produce an essentially stationary cloud of cold ions, which traps electrons produced later. The trapped electrons rapidly collisionally ionize the remaining cold Rydberg atoms to form a cold plasma
Accumulation and thermalization of cold atoms in a finite-depth magnetic trap
We experimentally and theoretically study the continuous accumulation of cold
atoms from a magneto-optical trap (MOT) into a finite depth trap, consisting in
a magnetic quadrupole trap dressed by a radiofrequency (RF) field. Chromium
atoms (52 isotope) in a MOT are continuously optically pumped by the MOT lasers
to metastable dark states. In presence of a RF field, the temperature of the
metastable atoms that remain magnetically trapped can be as low as 25 microK,
with a density of 10^17 atoms.m-3, resulting in an increase of the phase-space
density, still limited to 7.10^-6 by inelastic collisions. To investigate the
thermalization issues in the truncated trap, we measure the free evaporation
rate in the RF-truncated magnetic trap, and deduce the average elastic cross
section for atoms in the 5D4 metastable states, equal to 7.0 10^-16m2.Comment: 9 pages, 10 Figure
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