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arXiv:cond-mat/0405013 · Physical Review Letters

Realization of a Magnetically Guided Atomic Beam in the Collisional Regime

Thierry Lahaye, J. M. M. J. Vogels, Kenneth Günter, Zengwei Wang, Jean Dalibard, David Guéry-Odelin

Published Aug 26, 2004• 62 citations
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Abstract

We describe the realization of a magnetically guided beam of cold rubidium atoms, with a flux of $7\ifmmode\times\else\texttimes\fi{}{10}^{9}\text{ }\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}\mathrm{s}/\mathrm{s}$, a temperature of $400\text{ }\ensuremath{\mu}\mathrm{K}$, and a mean velocity of $1\text{ }\mathrm{m}/\mathrm{s}$. The rate of elastic collisions within the beam is sufficient to ensure thermalization. We show that the evaporation induced by a radio-frequency wave leads to appreciable cooling and an increase in the phase space density. We discuss the perspectives to reach the quantum degenerate regime using evaporative cooling.

DOI
10.1103/physrevlett.93.093003

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We describe the realization of a magnetically guided beam of cold rubidium atoms, with a flux of 7\ifmmode×\else\texttimes\fi109 atoms/s7\ifmmode\times\else\texttimes\fi{}{10}^{9}\text{ }\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}\mathrm{s}/\mathrm{s}7\ifmmode×\else\texttimes\fi109 atoms/s, a temperature of 400 \ensuremathμK400\text{ }\ensuremath{\mu}\mathrm{K}400 \ensuremathμK, and a mean velocity of 1 m/s1\text{ }\mathrm{m}/\mathrm{s}1 m/s. The rate of elastic collisions within the beam is sufficient to ensure thermalization. We show that the evaporation induced by a radio-frequency wave leads to appreciable cooling and an increase in the phase space density. We discuss the perspectives to reach the quantum degenerate regime using evaporative cooling.
In this paper, we report our progress towards the realization of a continuous guided atomic beam in the degenerate regime. So far, we have coupled into a magnetic guide a flux of a few 10810^{8}108 atoms/s at 60 cm/s with a propagation in the guide over more than 2 meters. At this stage, the collision rate is not high enough to start an efficient forced evaporative cooling. Here we describe a new approach to reach the collisional regime. It is based on a pulsed feeding of the magnetic guide at a high repetition rate. The overlap of the packets of atoms occurs in the guide and leads to a continuous guided beam. We discuss different ways to increase the collision rate of this beam while keeping the phase space density constant by shaping the external potential.
We report on our recent progress in the manipulation and cooling of a magnetically guided, high-flux beam of 87Rb^{87}\mathrm{Rb}87Rb atoms. Typically, $7\ifmmode\times\else\texttimes\fi{}{10}^{9}\phantom{\rule{0.3em}{0ex}}\text{atoms}\phantom{\rule{0.3em}{0ex}}\text{per}\phantom{\rule{0.3em}{0ex}}\text{second}$ propagate in a magnetic guide providing a transverse gradient of 800G∕cm800\phantom{\rule{0.3em}{0ex}}\mathrm{G}∕\mathrm{cm}800G∕cm, with a temperature $\ensuremath{\sim}550\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$, at an initial velocity of 90cm∕s90\phantom{\rule{0.3em}{0ex}}\mathrm{cm}∕\mathrm{s}90cm∕s. The atoms are subsequently slowed down to $\ensuremath{\sim}60\phantom{\rule{0.3em}{0ex}}\mathrm{cm}∕\mathrm{s}$ using an upward slope. The relatively high collision rate $(5\phantom{\rule{0.3em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1})$ allows us to start forced evaporative cooling of the beam, leading to a reduction of the beam temperature by a factor of 4, and a tenfold increase of the on-axis phase-space density.
We have observed evaporative cooling of magnetically trapped sodium atoms. A novel technique, rf induced evaporation, was used to reduce the temperature by a factor of 12 and increase the phase space density by more than 2 orders of magnitude. The elastic collision cross section of cold sodium atoms in the F=1F\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1F=1, ${m}_{F}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ensuremath{-}1$ hyperfine state was determined to be $6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ cm2{\mathrm{m}}^{2}m2, which implies a positive value of the scattering length.
The atom laser has long been a holy grail within atom physics and with the creation of an atom laser we hope to bring a similar revolution in to the field of atom optics. With the creation of the Bose-Einstein Condensate (BEC) in 1995 the path to an atom laser was initiated. An atom laser is continues source of BEC. In a Bose condensate all the atoms occupy the same quantum state and can be described by the same wave function and phase. With an atom laser the De Broglie wavelength of atoms can be much smaller than the wavelength of light. Due to the ultimate control over the atoms the atom laser is very interesting for atom optics, lithography, metrology, etching and deposition of atoms on a surface. All previous atom lasers have been created from atoms coupled out from an existing Bose-Einstein Condensate. There are different approaches but common to them all is that the duration of the output of the atom laser is limited by the size of the initial BEC and they all have a low flux. This leaves the quest to build a continuous high flux atom laser. An alternative approach to a continuous BEC beam is to channel a continuous ultra cold atomic beam into a magnetic guide and then cool this beam down to degeneracy. Cooling down a continuous beam of atoms faces three large problems: The collision rate has to be large enough for effective rethermalization, since evaporative cooling in 2D is not as effective as in 3D and a large thermal conductivity due to atoms with a high angular momentum causes heating downstream in the guide. We have built a 4 meter magnetic guide that is placed on a downward slope with a magnetic barrier in the end. In the guide we load packets of ultra cold rubidium atoms with a frequency rate large enough for the packets to merge together to form a continuous atomic beam. The atomic beam is supersonic and when the beam reaches the end barrier it will return and collide with itself. The collisions lowers the velocity of the beam into subsonic velocities and a shock wave is created between the two velocity regions. In order to conserve number of particle, momentum and enthalpy the density of the atomic beam passing through the shock wave must increase. We have build such a shock wave in an atomic beam and observed the density increase due to this. As an extra feature having a subsonic beam on a downward slope adds an extra density increase due to gravitational compression. Loading ultra cold atoms into a 3D trap from the dense subsonic beam overcomes the problem with 2D cooling and thermal conductivity. This was done and evaporative cooling was applied creating an unprecedented large number rubidium BEC.
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.
The continuous injection and propagation of a cold atomic beam in a 1.7 m long high-gradient (up to 2.7 kG/cm) magnetic guide is demonstrated. Continuous injection is accomplished by side-loading a magnetic guide with atoms from a sequence of two modified magneto-optical traps. In the high-gradient portion of the guide, the guided atomic beam has a transverse temperature of 420 muK +/- 40 muK, a longitudinal temperature of 1 mK, and an average velocity of order 1 m/s. Essential for the characterization of this steady-state atomic beam, imaging methods compatible with the high field-gradients associated with this magnetic guide have been investigated and developed. By utilizing a probe laser tuned to an open transition, the fluorescence yield per atom is largely fixed throughout the trap volume, independent of the trapping field. This enables a reliable conversion of fluorescence images into atomic-density profiles. Using a fixed radio-frequency (nurf) current coupled directly onto the guide wires, continuous and selective removal of atoms with a transverse energy exceeding hnurf is demonstrated. This removal scheme is the essence of an rf filter, where the number of modes occupied by the atomic flow is reduced. Through sufficient rf-filtering, a near-single-mode guided atomic beam should be achievable, enabling basic atom-interferometric experiments. The extension of this fixed-frequency removal of atoms for continuous position-resolved evaporative cooling in a long magnetic guide is outlined. Employing an innovative gridless technique for performing Direct Simulation of rarefied gas dynamics via Monte Carlo (DSMC), the evaporative cooling process in a guided atomic beam has been simulated. Both fixed-frequency as well as variable-frequency evaporative cooling layouts have been explored and the results of these simulations are presented. The implementation of this gridless technique is presented and its benefits are briefly outlined. In parallel with the development of a continuous-wave atom laser, tools suited to manipulate cold atomic beams in atom guides have been explored. An atomic guide with a tunable aspect ratio, using rf-dressed state potentials, is presented.