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arXiv:cond-mat/0212147

Injection of a cold atomic beam into a magnetic guide

Christian F. Roos, P. Cren, Thierry Lahaye, Jean Dalibard, David Guéry-Odelin

Published Dec 6, 2002• 5 citations
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Abstract

We report the continuous or pulsed loading of a slow and cold atomic beam into a magnetic guide. In order to optimize the transfer into the guide, we have studied two coupling schemes. The first one is based on an auxiliary two-dimensional MOT that confines the atoms transversally before they enter the guide. However, for low atomic velocities, the atoms are strongly heated in the longitudinal direction. This limitation does not occur in the second coupling scheme which relies on magnetic confinement in the transfer zone. For this purpose, we have constructed a miniature magnetic guide located between the atomic source and the long magnetic guide. The latter scheme allows to inject atoms with velocities down to 70 cm/s into the guide.

DOI
10.48550/arxiv.cond-mat/0212147

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•Jan 1, 2002•0 citations
We report the continuous or pulsed loading of a slow and cold atomic beam into a magnetic guide. In order to optimize the transfer into the guide, we have studied two coupling schemes. The first one is based on an auxiliary two-dimensional MOT that confines the atoms transversally before they enter the guide. However, for low atomic velocities, the atoms are strongly heated in the longitudinal direction. This limitation does not occur in the second coupling scheme which relies on magnetic confinement in the transfer zone. For this purpose, we have constructed a miniature magnetic guide located between the atomic source and the long magnetic guide. The latter scheme allows to inject atoms with velocities down to 70 cm/s into the guide.
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.
We demonstrate the continuous injection and propagation of a cold atomic beam in a high-gradient (up to 2.7kG∕cm2.7\phantom{\rule{0.3em}{0ex}}\mathrm{kG}∕\mathrm{cm}2.7kG∕cm) magnetic guide of 1.7m1.7\phantom{\rule{0.3em}{0ex}}\mathrm{m}1.7m length. Continuous injection is accomplished using a side-loading scheme that involves a sequence of two modified magneto-optic traps. Methods are developed to measure the atomic-flow temperatures and the flux under steady-state conditions. In the high-gradient portion of the guide, the guided atomic beam has a transverse temperature of $420\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}\ifmmode\pm\else\textpm\fi{}40\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$, a longitudinal temperature of 1mK1\phantom{\rule{0.3em}{0ex}}\mathrm{mK}1mK, and an average velocity of order 1m∕s1\phantom{\rule{0.3em}{0ex}}\mathrm{m}∕\mathrm{s}1m∕s. Using a radio-frequency current of a fixed frequency $\ensuremath{\nu}$ coupled directly into the guide wires, atoms exceeding a transverse energy of $h\ensuremath{\nu}$ can be continuously and selectively removed from the atomic beam.
We present an intense source of 87 Rb atoms that has been set up to produce a continuous, slow and cold beam in a magnetic guide. It consists of a two-dimensional magneto-optical trap whose cooling laser power is provided by a master-oscillator tapered-amplifier system. This trap produces an atomic beam with a flux of over 10 10 atoms/s and a mean velocity of 40 m/s. The beam is recaptured by a second trap whose purpose consists in reducing the beam's velocity, in further cooling the atoms and injecting them into the magnetic guide. The article focusses on the first stage of the process described above.
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.
Summary form only given. Small guides can produce very strong confining fields but the transfer of cold atoms from a magneto optical trap (MOT) into such guides present a problem. This is because MOTs require a fixed trapping volume of /spl sim/ 1 cm/sup 3/ and cannot be miniaturized in the same way as atom guides. We demonstrate the loading of atoms from a cycled MOT into a tapered magnetic atom guide that overlaps with the MOT. This integrated loading strategy involves the formation of a MOT directly inside the atom guide.
We have constructed a tapered magnetic atom guide that is integrated with a magneto-optical trap. We have observed atom guiding over distances of up to 20 cm. This integrated strategy is useful for loading atoms into wire guides. We have shown that tapered sections in atom guides. i.e. sections with a variable steepness of the transverse guiding potential, can reflect atoms propagating in the guide. The reflection arises from the adiabatic conservation of the action of the transverse motion; the latter can be used as a control parameter for the atom reflectivity. A method of producing a slow, dark atomic beam of atoms is demonstrated. Atoms that are extracted from a magneto-optical trap using a push beam are quickly pumped into a dark state, which does not interact with the push laser beam. This method is capable of producing atoms with a velocity of order 1 m/s. The slow atoms can be coupled into atom guides. In related research, we observe tunneling resonances of atoms in a 1D optical lattice through the measurement of the magnetization. These resonances manifest themselves as modulations in the magnetization curves. The resonances are shown to be related to the coherence properties of the atoms, and exist only under conditions where the coherence time exceeds the tunneling period of the atoms.