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arXiv:quant-ph/0603028 · Physical Review A

Simultaneous magneto-optical trapping of bosonic and fermionic chromium atoms

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

Published May 24, 2006• 53 citations
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Abstract

We report on magneto-optical trapping of fermionic $^{53}\mathrm{Cr}$ 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 $P$ 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 $^{53}\mathrm{Cr}$ atoms in this magnetic trap. We also report on the simultaneous magneto-optical trapping of bosonic $^{52}\mathrm{Cr}$ and fermionic $^{53}\mathrm{Cr}$ atoms. Finally, we characterize the light-assisted collision losses in this Bose-Fermi cold mixture.

DOI
10.1103/physreva.73.053406

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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×\else\texttimes\fi106s\ensuremath−13\ifmmode\times\else\texttimes\fi{}{10}^{6}\phantom{\rule{0.3em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}3\ifmmode×\else\texttimes\fi106s\ensuremath−1. We present systematic characterization of the magneto-optical trap (MOT). We obtain up to 5\ifmmode×\else\texttimes\fi1055\ifmmode\times\else\texttimes\fi{}{10}^{5}5\ifmmode×\else\texttimes\fi105 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.
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 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 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 realize a combined trap for bosonic chromium (52Cr) and rubidium (87Rb) atoms. Initial experiments focus on exploring a suitable loading scheme for the combined trap and on studies of new trap loss mechanisms originating from simultaneous trapping of two species. By comparing the trap loss from the 87Rb magneto-optical trap (MOT) in the absence and presence of magnetically trapped ground state 52Cr atoms we determine the scattering cross-section of σ inel,RbCr = 5.0 ± 4.0 × 10−18 m2 for light-induced inelastic collisions between the two species. Studying the trap loss from the Rb magneto-optical trap induced by the Cr cooling-laser light, the photoionization cross-section of the excited 5P3/1 state at an ionizing wavelength of 426 nm is measured to be σ P = 1.1 ± 0.3 × 10−21 m2.
We simultaneously confine fermionic metastable 3He^{3}\mathrm{H}\mathrm{e}3He atoms and bosonic metastable 4He^{4}\mathrm{H}\mathrm{e}4He atoms in a magneto-optical trap. The trapped clouds, containing up to $1.5\ifmmode\times\else\texttimes\fi{}{10}^{8}$ atoms of each isotope, are characterized by measuring ions and metastable helium atoms escaping from the trap. Optical pumping of 3He^{3}\mathrm{H}\mathrm{e}3He atoms to a nontrapped hyperfine state is investigated and it is shown that large atom numbers can be confined without additional repumping lasers. Unique possibilities for quantum degeneracy experiments with mixtures of spin-polarized metastable 3He^{3}\mathrm{H}\mathrm{e}3He and 4He^{4}\mathrm{H}\mathrm{e}4He atoms are indicated.