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arXiv:quant-ph/0011113 · Physical Review A

Continuous loading of a magnetic trap

J. Stühler, Piet O. Schmidt, S. Hensler, J. Werner, J. Mlynek, Tilman Pfau

Published Aug 20, 2001• 75 citations
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Abstract

We have realized a scheme for continuous loading of a magnetic trap (MT). ${}^{52}\mathrm{Cr}$ 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 ${10}^{8}$ metastable atoms at a rate of ${10}^{8}$ 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.

DOI
10.1103/physreva.64.031405

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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\ensuremathμB),(6 {\ensuremath{\mu}}_{B}),(6\ensuremathμB​), 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\ensuremathμK100 \ensuremath{\mu}\mathrm{K}100\ensuremathμK into the MT. Optical repumping after the loading allows us to realize a MT of ground-state chromium atoms.
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 present a robust continuous optical loading scheme for a Ioffe-Pritchard (IP) type magnetic trap. Atoms are cooled and trapped in a modified magneto-optical trap (MOT) consisting of a conventional 2D-MOT in radial direction and an axial molasses. The radial magnetic field gradient needed for the operation of the 2D-MOT is provided by the IP trap. A small axial curvature and offset field provide magnetic confinement and suppress spin-flip losses in the center of the magnetic trap without altering the performance of the 2D-MOT. Continuous loading of atoms into the IP trap is provided by radiative leakage from the MOT to a metastable level which is magnetically trapped and decoupled from the MOT light. We are able to accumulate 30 times more atoms in the magnetic trap than in the MOT. The absolute number of 2×1082\times 10^82×108~atoms is limited by inelastic collisions. A model based on rate equations shows good agreement with our data. Our scheme can also be applied to other atoms with similar level structure like alkaline earth metals.
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 realization of a magneto-optical trap (MOT) for europium atoms using a narrow-line cooling transition with a natural linewidth of 97 kHz. Europium is a strongly dipolar species with interesting properties, such as the presence of hyperfine structure and the absence of electronic orbital angular momentum. Our starting point of the MOT is continuous capturing and cooling of optically pumped metastable europium atoms. We have employed simultaneous MOT for the metastable and ground-state atoms. The trapped metastable atoms are successively pumped back to the ground state and then continuously loaded to the narrow-line MOT, where up to $4.7\ifmmode\times\else\texttimes\fi{}{10}^{7}$ atoms are captured. A spin-polarized sample at a temperature of $6\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{K}$ and with a peak number density of $2.2\ifmmode\times\else\texttimes\fi{}{10}^{11}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ is obtained through the compression process, resulting in a phase space density of $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$.
Metastable calcium atoms, produced in a magneto-optic trap (MOT) operating within the singlet system, are continuously loaded into a magnetic trap formed by the magnetic quadrupole field of the MOT. At MOT temperatures of 3 mK and 240 ms loading time, we observe $1.1\ifmmode\times\else\texttimes\fi{}{10}^{8}$ magnetically trapped 3P2{}^{3}{P}_{2}3P2​ atoms at densities of $2.4\ifmmode\times\else\texttimes\fi{}{10}^{8}{\mathrm{cm}}^{\ensuremath{-}3}$ and temperatures of 0.61 mK. In a modified scheme, we first load a MOT for metastable atoms at a temperature of 0.18 mK and subsequently release these atoms into the magnetic trap. In this case, 240 ms of loading yields $2.4\ifmmode\times\else\texttimes\fi{}{10}^{8}$ trapped 3P2{}^{3}{P}_{2}3P2​ atoms at a peak density of $8.7\ifmmode\times\else\texttimes\fi{}{10}^{10}{\mathrm{cm}}^{\ensuremath{-}3}$ and a temperature of 0.13 mK. The temperature decrease observed in the magnetic trap for both loading schemes can be explained only in part by trap size effects.