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arXiv:physics/0612133 · Physical Review A

Accumulation and thermalization of cold atoms in a finite-depth magnetic trap

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

Published Aug 10, 2007• 10 citations
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Abstract

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 $(^{52}\mathrm{Cr})$ 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 $^{5}D_{4}$ metastable states, ${\ensuremath{\sigma}}_{el}=7.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}\phantom{\rule{0.3em}{0ex}}{\mathrm{m}}^{2}$.

DOI
10.1103/physreva.76.023406

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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\ensuremathμK25\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{K}25\ensuremathμK, with a density of 1017atomsm\ensuremath−3{10}^{17}\phantom{\rule{0.3em}{0ex}}\mathrm{atoms}\phantom{\rule{0.3em}{0ex}}{\mathrm{m}}^{\ensuremath{-}3}1017atomsm\ensuremath−3, resulting in an increase of the phase-space density, still limited to 7.0\ifmmode×\else\texttimes\fi10\ensuremath−67.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}7.0\ifmmode×\else\texttimes\fi10\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, \ensuremathσel=7.0\ifmmode×\else\texttimes\fi10\ensuremath−16m2{\ensuremath{\sigma}}_{el}=7.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}\phantom{\rule{0.3em}{0ex}}{\mathrm{m}}^{2}\ensuremathσel​=7.0\ifmmode×\else\texttimes\fi10\ensuremath−16m2.
We have realized a scheme for continuous loading of a magnetic trap (MT). 52Cr{}^{52}\mathrm{Cr}52Cr atoms are continuously captured and cooled in a magneto-optical trap (MOT). Optical pumping to a metastable state decouples atoms from the cooling light. Due to their high magnetic moment $(6 {\ensuremath{\mu}}_{B}),$ low-field seeking metastable atoms are trapped in the magnetic quadrupole field provided by the MOT. Limited by inelastic collisions between atoms in the MOT and in the MT, we load 108{10}^{8}108 metastable atoms at a rate of 108{10}^{8}108 atoms/s below $100 \ensuremath{\mu}\mathrm{K}$ into the MT. Optical repumping after the loading allows us to realize a MT of ground-state chromium atoms.
We report on magneto-optical trapping of fermionic 53Cr^{53}\mathrm{Cr}53Cr atoms. A Zeeman-slowed atomic beam provides loading rates up to $3\ifmmode\times\else\texttimes\fi{}{10}^{6}\phantom{\rule{0.3em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. We present systematic characterization of the magneto-optical trap (MOT). We obtain up to $5\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms in the steady-state MOT. The atoms radiatively decay from the excited PPP state into metastable DDD 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^{53}\mathrm{Cr}53Cr atoms in this magnetic trap. We also report on the simultaneous magneto-optical trapping of bosonic 52Cr^{52}\mathrm{Cr}52Cr and fermionic 53Cr^{53}\mathrm{Cr}53Cr atoms. Finally, we characterize the light-assisted collision losses in this Bose-Fermi cold mixture.
We report on the production of a cold mixture of fermionic 53Cr^{53}\mathrm{Cr}53Cr and 6Li^{6}\mathrm{Li}6Li atoms delivered by two Zeeman-slowed atomic beams and collected within a magneto-optical trap (MOT). For lithium, we obtain clouds of up to $4\ifmmode\times\else\texttimes\fi{}{10}^{8}$ atoms at temperatures of about $500\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{K}$. A gray optical molasses stage allows us to decrease the gas temperature down to $45(5)\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{K}$. For chromium, we obtain MOTs of more than $1.1\ifmmode\times\else\texttimes\fi{}{10}^{6}$ atoms. The availability of magnetically trappable metastable DDD states, from which PPP-state atoms can radiatively decay onto, enables to accumulate into the MOT quadrupole samples of almost 107{10}^{7}107 53Cr^{53}\mathrm{Cr}53Cr atoms. After repumping DDD-state atoms back into the cooling cycle, a final cooling stage decreases the chromium temperature down to $145(5)\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{K}$. While the presence of a lithium MOT decreases the lifetime of magnetically trapped 53Cr^{53}\mathrm{Cr}53Cr atoms, we obtain, within a 5-s duty cycle, samples of about $4\ifmmode\times\else\texttimes\fi{}{10}^{6}$ chromium and $3\ifmmode\times\else\texttimes\fi{}{10}^{8}$ lithium atoms. Our work provides a crucial step toward the production of degenerate Cr-Li Fermi mixtures.
We report the magnetic trapping of metastable 3P2{}^{3}{P}_{2}3P2​ atomic strontium. Atoms are cooled in a magneto-optical trap (MOT) operating on the dipole-allowed ${}^{1}{S}_{0}{\ensuremath{-}}^{1}{P}_{1}$ transition at 461 nm. Decay via ${}^{1}{\stackrel{\ensuremath{\rightarrow}}{{P}_{1}}}^{1}{\stackrel{\ensuremath{\rightarrow}}{{D}_{2}}}^{3}{P}_{2}$ continuously loads a magnetic trap formed by the quadrupole magnetic field of the MOT. Over 108{10}^{8}108 atoms at a density of $8\ifmmode\times\else\texttimes\fi{}{10}^{9}{\mathrm{cm}}^{\ensuremath{-}3}$ and temperature of 1 mK are trapped. The atom temperature is significantly lower than what would be expected from the kinetic and potential energies of atoms as they are transferred from the MOT. This suggests the occurrence of thermalization and evaporative cooling in the magnetic trap.
We have investigated the atom trapping geometry for trapping of 87Rb^{87}{Rb}87Rb atoms in a radio-frequency (rf) dressed potential generated after superposing a strong linearly polarized rf-field on a static magnetic trap. For this, laser cooled atoms in a magneto-optical trap (MOT) in an ultra-high vacuum (UHV) chamber (pressure ∼\sim∼ 1.5 ×\times× 10−1010^{-10}10−10 Torr) were trapped in a quadrupole magnetic trap and evaporatively cooled before transferring them to the rf-dressed potential. The experimentally observed hollow shell type atom trapping geometry has been explained by theoretical modelling of the trapping potential.