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Journal of Physics B Atomic Molecular and Optical Physics

A high flux of ultra-cold chromium atoms in a magnetic guide

Axel Griesmaier, Anoush Aghajani-Talesh, Markus Falkenau, J. Sebastian, Alexander Greiner, Tilman Pfau

Published Jun 24, 2009• 10 citations
Publisher

Abstract

We report the observation of a very high flux of ultra-cold bosonic chromium atoms in a magnetic guide. The beam is created by operating a magneto-optical trap/moving optical molasses within the magnetic field of the guide. A relative detuning between two pairs of the cooling lasers cools the atoms into a frame moving along the axes of the guide. When the atoms are cooled into a moving frame with a velocity of 6 m s −1 we observe a maximum of the flux of 6 × 10 9 atoms s −1 . For these parameters the transversal temperature of the atoms after a 25 fold increase of the confining magnetic potential is about 1.2 mK. The longitudinal temperature is 400 µK.

DOI
10.1088/0953-4075/42/14/145306

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We report the observation of a very high flux of ultra-cold bosonic chromium atoms in a magnetic guide. The beam is created by operating a magneto-optical trap/moving optical molasses within the magnetic field of the guide. A relative detuning between two pairs of the cooling lasers cools the atoms into a frame moving along the axes of the guide. When the atoms are cooled into a moving frame with a velocity of 6 m s −1 we observe a maximum of the flux of 6 × 10 9 atoms s −1 . For these parameters the transversal temperature of the atoms after a 25 fold increase of the confining magnetic potential is about 1.2 mK. The longitudinal temperature is 400 µK.
We demonstrate experimentally the continuous and pulsed loading of a slow and cold atomic beam into a magnetic guide. The slow beam is produced using a vapor loaded laser trap, which ensures two-dimensional magneto-optical trapping, as well as cooling by a moving molasses along the third direction. It provides a continuous flux larger than 10910^9109 atoms/s with an adjustable mean velocity ranging from 0.3 to 3 m/s, and with longitudinal and transverse temperatures smaller than 100μ100 \mu100μK. Up to 31083 10^83108 atoms/s are injected into the magnetic guide and subsequently guided over a distance of 40 cm.
This thesis examines two complimentary methods for the laser cooling of a magnetically guided ultra-cold atom beam. If combined, these methods could serve as a starting point for high-through put and possibly even continuous production of Bose-Einstein condensates. First, a mechanism is outlined to harvest ultra cold atoms from a magnetically guided atom beam into an optical dipole trap. A continuous loading scheme is described that dissipates the directed kinetic energy of a captured atom via deceleration by a magnetic potential barrier followed by optical pumping to the energetically lowest Zeeman sublevel. The application of this scheme to the transfer of ultra cold chromium atoms from a magnetically guided atom beam into a deep optical dipole trap is investigated via numerical simulations of the loading process. Based on the results of the theoretical studies the feasibility and the efficiency of our loading scheme, including the realisation of a suitable magnetic field configuration, are analysed. Second, experiments were conducted on the transverse laser cooling of a magnetically guided beam of ultra cold chromium atoms. Radial compression by a tapering of the guide is employed to adiabatically heat the beam. Inside the tapered section heat is extracted from the atom beam by a two-dimensional optical molasses perpendicular to it, resulting in a significant increase of atomic phase space density. A magnetic offset field is applied to prevent optical pumping to untrapped states. Our results demonstrate that by a suitable choice of the magnetic offset field, the cooling beam intensity and detuning, atom losses and longitudinal heating can be avoided. Final temperatures below 65 µK have been achieved, corresponding to an increase of phase space density in the guided beam by more than a factor of 30.
We report on the transverse laser cooling of a magnetically guided beam of ultracold chromium atoms. Radial compression by a tapering of the guide is employed to adiabatically heat the beam. Inside the tapered section, heat is extracted from the atom beam by a two-dimensional (2D) optical molasses perpendicular to it, resulting in a significant increase in atomic phase space density. A magnetic offset field is applied to prevent optical pumping to untrapped states. Our results demonstrate that, by a suitable choice of the magnetic offset field, the cooling beam intensity and detuning, atom losses and longitudinal heating can be avoided. Final temperatures below 65 μK have been achieved, corresponding to an increase in phase space density in the guided beam by more than a factor of 30.
By the moving molasses technique we have extracted laser-cooled cesium atoms in a continuous way directly from an optical molasses. The mean launching velocity is precisely tunable from 1 to 12 m/s. The atomic beam has a flux of $1.3\ifmmode\times\else\texttimes\fi{}{10}^{8} \mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}/\mathrm{s}$ at 7 m/s and a longitudinal temperature of $70 \ensuremath{\mu}\mathrm{K},$ which represents the highest flux and lowest velocity spread obtained so far in a continuous beam of cold atoms. These features makes it well suited for atomic fountains. The atomic flux can be slightly increased in a two-dimensional magneto-optical trap operation (+40(+40%).(+40 A simple model accounts for the observed dependence of the flux with the magnetic-field gradient.
We discuss the implementation and characterization of the source of a slow, intense, and collimated beam of rubidium atoms. The cold atomic beam is produced by two-dimensional magneto-optical trapping in directions transverse to the atomic beam axis and unbalanced Doppler cooling in the axial direction. The vacuum design allows use of relatively low laser power and a considerably simplified assembly. The atomic beam has a high flux of about $2\ifmmode\times\else\texttimes\fi{}{10}^{10}\phantom{\rule{0.3em}{0ex}}\mathrm{atoms}∕\mathrm{s}$ at a total cooling laser power of 55mW55\phantom{\rule{0.3em}{0ex}}\mathrm{mW}55mW. It has a narrow longitudinal velocity distribution with mean velocity 15m∕s15\phantom{\rule{0.3em}{0ex}}\mathrm{m}∕\mathrm{s}15m∕s with full width at half maximum 3.5m∕s3.5\phantom{\rule{0.3em}{0ex}}\mathrm{m}∕\mathrm{s}3.5m∕s and has a low divergence of 26mrad26\phantom{\rule{0.3em}{0ex}}\mathrm{mrad}26mrad. The high flux enables ultrafast loading of about 1010{10}^{10}1010 atoms into a three-dimensional (3D) magneto-optical trap within 500 ms. The variation of the atomic beam flux was studied as a function of the rubidium vapor pressure, cooling laser power, transverse cooling laser beam length, detuning of the cooling laser, and relative intensities of the cooling beams along the atomic beam axis. We also discuss a detailed comparison of our measurements of the cold atomic beam with a 3D numerical simulation.