arXiv subjects

B. Laburthe-Tolra

Publications and source records attributed to B. Laburthe-Tolra.

27 records · Page 2Linked to original sources

arXiv2008

Radio-frequency association of molecules: the dressed state picture

We develop a theoretical model to describe the radio-frequency (rf) induced coupling of a pair of colliding atoms to a Feshbach molecule when a magnetic field arbitrarily far from the Feshbach resonance is modulated in time. We use the dressed atom picture, and show that the coupling strength in presence of rf is equal to the Feshbach coupling strength multiplied by the square of a Bessel function. The argument of this function is equal to the ratio of the atomic rf Rabi frequency to the rf frequency. We experimentally demonstrate this law by measuring the rate of rf-association of molecules using a Feshbach resonance in d-wave collisions between ultra-cold chromium atoms.

physics.atom-ph↗

arXiv2008

A Feshbach resonance in d-wave collisions

We analyse a narrow Feshbach resonance with ultra-cold chromium atoms colliding in d-wave. The resonance is made possible by dipole-dipole interactions, which couple an incoming $l=2$ collision channel with a bound molecular state with $l=0$. We find that three-body losses associated to this resonance increase with temperature, and that the loss feature width as a function of magnetic field also increases linearly with temperature. The analysis of our experimental data shows that the Feshbach coupling is small compared both to the temperature and to the density limited lifetime of the resonant bound molecular state. One consequence is that the three body losse rate is proportionnal to the square of the number of atoms, and that we can directly relate the amplitude of the losses to the Feshbach coupling parameter. We compare our measurement to a calculation of the coupling between the collisionnal channel and the molecular bound state by dipole-dipole interactions, and find a good agreement, with no adjustable parameter. An analysis of the loss lineshape is also performed, which enables to precisely measure the position of the resonance.

physics.atom-ph↗

arXiv2008

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.

physics.atom-ph↗

arXiv2008

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.

cond-mat.other↗

arXiv2008

Radio-frequency induced ground state degeneracy in a Chromium Bose-Einstein condensate

We study the effect of strong radio-frequency (rf) fields on a chromium Bose-Einstein condensate (BEC), in a regime where the rf frequency is much larger than the Larmor frequency. We use the modification of the Landé factor by the rf field to bring all Zeeman states to degeneracy, despite the presence of a static magnetic field of up to 100 mG. This is demonstrated by analyzing the trajectories of the atoms under the influence of dressed magnetic potentials in the strong field regime. We investigate the problem of adiabaticity of the rf dressing process, and relate it to how close the dressed states are to degeneracy. Finally, we measure the lifetime of the rf dressed BECs, and identify a new rf-assisted two-body loss process induced by dipole-dipole interactions.

quant-ph↗

arXiv2007

Averaging out magnetic forces with fast rf-sweeps in an optical trap for metastable chromium atoms

We introduce a novel type of time-averaged trap, in which the internal state of the atoms is rapidly modulated to modify magnetic trapping potentials. In our experiment, fast radiofrequency (rf) linear sweeps flip the spin of atoms at a fast rate, which averages out magnetic forces. We use this procedure to optimize the accumulation of metastable chomium atoms into an optical dipole trap from a magneto-optical trap. The potential experienced by the metastable atoms is identical to the bare optical dipole potential, so that this procedure allows for trapping all magnetic sublevels, hence increasing by up to 80 percent the final number of accumulated atoms.

quant-ph↗

arXiv2007

Accumulation of chromium metastable atoms into an Optical Trap

We report the fast accumulation of a large number of metastable 52Cr atoms in a mixed trap, formed by the superposition of a strongly confining optical trap and a quadrupolar magnetic trap. The steady state is reached after about 400 ms, providing a cloud of more than one million metastable atoms at a temperature of about 100 microK, with a peak density of 10^{18} atoms.m^{-3}. We have optimized the loading procedure, and measured the light shift of the 5D4 state by analyzing how the trapped atoms respond to a parametric excitation. We compare this result to a theoretical evaluation based on the available spectroscopic data for chromium atoms.

physics.atom-ph↗

arXiv2006

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.

physics.atom-ph↗

arXiv2006

Simultaneous Magneto-Optical Trapping of Bosonic and Fermionic Chromium Atoms

We report on magneto-optical trapping of fermionic 53Cr atoms. A Zeeman-slowed atomic beam provides loading rates up to 3 10^6 /s. We present systematic characterization of the magneto-optical trap (MOT). We obtain up to 5 10^5 atoms in the steady state MOT. The atoms radiatively decay from the excited P state into metastable D states, and, due to the large dipolar magnetic moment of chromium atoms in these states, they can remain magnetically trapped in the quadrupole field gradient of the MOT. We study the accumulation of metastable 53Cr atoms into this magnetic trap. We also report on the first simultaneous magneto-optical trapping of bosonic 52Cr and fermionic 53Cr atoms. Finally, we characterize the light assisted collision losses in this Bose-Fermi cold mixture.

quant-ph↗