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Annales de Physique

Trapping metastable chromium atoms in a crossed optical dipole trap

Quentin Beaufils, Radu Chicireanu, A. Pouderous, B. Laburthe-Tolra, É. Maréchal, L. Vernac, J. Keller, O. Gorceix

Published Jan 1, 2007• 0 citations
Publisher

Abstract

We report the fast accumulation of up to 1 million 52Cr metastable atoms in a mixed trap formed by the superposition of a quadrupolar magnetic trap and a strongly confining optical trap. The cloud is at a temperature of 100 μK with a peak density of 1018 atoms/m3, which is a promising starting point to reach quantum degeneracy by forced evaporation in an optical trap.

DOI
10.1051/anphys:2008035

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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.

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We report on the generation of a Bose-Einstein condensate in a gas of chromium atoms, which have an exceptionally large magnetic dipole moment and therefore underlie anisotropic long-range interactions. The preparation of the chromium condensate requires novel cooling strategies that are adapted to its special electronic and magnetic properties. The final step to reach quantum degeneracy is forced evaporative cooling of 52Cr atoms within a crossed optical dipole trap. At a critical temperature of T(c) approximately 700 nK, we observe Bose-Einstein condensation by the appearance of a two-component velocity distribution. We are able to produce almost pure condensates with more than 50,000 condensed 52Cr atoms.

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Atomes froids de chrome piegés magnétiquement et optiquement@ : premières étapes vers la condensation

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

Quentin Beaufils, Radu Chicireanu, A. Pouderous +6 more

We introduce a time-averaged trap in which the internal state of the atoms is rapidly modulated to modify the magnetic trapping potential. In our experiment, fast radio-frequency linear sweeps flip the spins of atoms at a fast rate, which averages out the magnetic forces. We use this procedure to optimize the accumulation of metastable chromium atoms in 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% the final number of accumulated atoms.

arXiv:0711.0663•Physical Review A•May 29, 2008•7 citations
We report the fast accumulation of up to 1 million 52Cr metastable atoms in a mixed trap formed by the superposition of a quadrupolar magnetic trap and a strongly confining optical trap. The cloud is at a temperature of 100 μK with a peak density of 1018 atoms/m3, which is a promising starting point to reach quantum degeneracy by forced evaporation in 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.
We report on the generation of a Bose-Einstein condensate in a gas of chromium atoms, which have an exceptionally large magnetic dipole moment and therefore underlie anisotropic long-range interactions. The preparation of the chromium condensate requires novel cooling strategies that are adapted to its special electronic and magnetic properties. The final step to reach quantum degeneracy is forced evaporative cooling of 52Cr atoms within a crossed optical dipole trap. At a critical temperature of T(c) approximately 700 nK, we observe Bose-Einstein condensation by the appearance of a two-component velocity distribution. We are able to produce almost pure condensates with more than 50,000 condensed 52Cr atoms.
In the perspective of studying dipole-dipole interactions in quantum degenerate Bose and Fermi gases, we have built an experimental setup for cooling and trapping Chromium (Cr) atoms. In this respect, one needs to use appropriate technological solution, such as a high-temperature oven (∼ 1500◦C) and a laser system providing high laser power at 425nm. Our experiments allow us to obtain magneto-optical traps for the two main isotopes of Cr (i.e. the bosonic 52Cr and the fermionic 53Cr). These traps are characterized by large light-assisted collisional loss rates, which have been studied both experimentally and theoretically. The existence of radiative decay channels towards two long-lived metastable states allows the magnetic trapping and accumulation of fairly large atom numbers of cold Cr atoms. We have studied the possibility of modifying the shape of these traps with the use of RF magnetic fields, which allowed the study of collisional properties of Cr atoms in the metastable states. Finally, we have demonstrated a new continuous loading technique of an optical dipole trap with more than one million metastable 52Cr atoms, at 100μK. The transfer into the ground state and the polarization in the lowest-energy Zeeman sublevel opens the perspective for reaching the Bose-Einstein condensation of Cr through evaporative cooling.
We discuss our experimental results on forced evaporative cooling of cold rubidium 87 Rb atoms to quantum degeneracy in an Optical Dipole Trap. The atoms are first trapped and cooled in a magneto-optical trap (MOT) loaded from a continuous beam of cold atoms [1]. More than 10 10 atoms are trapped in the MOT and then about 10 8 atoms are transferred to a Quasi-Electrostatic Trap (QUEST) formed by tightly focused CO2 laser (λ = 10.6μ m ) beams intersecting at their foci in an orthogonal configuration in the horizontal plane. Before loading the atoms into the dipole trap, the phase-space density of the atomic ensemble was increased making use of sub-doppler cooling at large detuning and the temporal dark MOT technique. In a MOT the phase-space density of the atomic ensemble is six orders of magnitude less than what is required to achieve quantum degeneracy. After transferring atoms into the dipole trap efficiently, phase-space density increases by a factor of 10 3 . Further increase in phase-space density to quantum degeneracy is achieved by forced evaporative cooling of atoms in the dipole trap. The evaporative cooling process involves a gradual reduction of the trap depth by ramping down the trapping laser intensity over a second. The temperature of the cold atomic cloud was measured by time-of-flight (TOF) technique. The spatial distribution of the atoms is measured using absorption imaging. We report results of evaporative cooling in a single beam and in a crossed double-beam dipole traps. Due to the large initial phase space density, and large initial number of atoms trapped, the quantum phase transition occurs after about 600 ms of evaporative cooling in our optimized crossed dipole trap.
We 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 (52Cr)(^{52}\mathrm{Cr})(52Cr) in a MOT are continuously optically pumped by the MOT lasers to metastable dark states. In the presence of a rf field, the temperature of the metastable atoms that remain magnetically trapped can be as low as $25\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{K}$, with a density of ${10}^{17}\phantom{\rule{0.3em}{0ex}}\mathrm{atoms}\phantom{\rule{0.3em}{0ex}}{\mathrm{m}}^{\ensuremath{-}3}$, resulting in an increase of the phase-space density, still limited to $7.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ 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^{5}D_{4}5D4​ metastable states, ${\ensuremath{\sigma}}_{el}=7.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}\phantom{\rule{0.3em}{0ex}}{\mathrm{m}}^{2}$.
We introduce a time-averaged trap in which the internal state of the atoms is rapidly modulated to modify the magnetic trapping potential. In our experiment, fast radio-frequency linear sweeps flip the spins of atoms at a fast rate, which averages out the magnetic forces. We use this procedure to optimize the accumulation of metastable chromium atoms in 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% the final number of accumulated atoms.