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Australian Journal of Physics

Manipulating Beams of Ultra-cold Atoms with a Static Magnetic Field

W J Rowlands, DC Lau, G. I. Opat, A. I. Sidorov, R. J. McLean, Peter Hannaford

Published Apr 1, 1996• 9 citations
Publisher

Abstract

We report preliminary results on the deflection of a beam of ultra-cold atoms by a static magnetic field. Caesium atoms trapped in a magneto-optical trap (MOT) are cooled using optical molasses, and then fall freely under gravity to form a beam of ultra-cold atoms. The atoms pass through a static inhomogeneous magnetic field produced by a single current-carrying wire, and are deflected by a force ?(µB) dependent on the magnetic substate of the atom. The population of atoms in various magnetic substates can be altered by using resonant laser radiation to optically pump the atoms.

DOI
10.1071/ph960577

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We report preliminary results on the deflection of a beam of ultra-cold atoms by a static magnetic field. Caesium atoms trapped in a magneto-optical trap (MOT) are cooled using optical molasses, and then fall freely under gravity to form a beam of ultra-cold atoms. The atoms pass through a static inhomogeneous magnetic field produced by a single current-carrying wire, and are deflected by a force ?(µB) dependent on the magnetic substate of the atom. The population of atoms in various magnetic substates can be altered by using resonant laser radiation to optically pump the atoms.
We propose a novel method for guiding cold, neutral atoms using static magnetic fields. A theoretical study of the magnetic field produced by a tube consisting of two identical, interwound solenoids carrying equal but opposite currents is presented. This field is almost zero throughout the centre of the tube, but it increases with exponential rapidity as one approaches the walls formed by the current carrying wires. Hence, cold atoms passing through the tube may be reflected by magnetic mirror effects near the walls. Applying this technique to a free-falling cloud of magneto-optically cooled caesium atoms we hope to construct atomic guides to facilitate the manipulation of cold atomic beams.
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.
An experimental investigation of the effects of magnetic field on cooling and trapping of neutral atoms is presented. Applied to optical molasses, a static magnetic field often produces spectacular effects. It can either induce a sub-Doppler cooling effect or shift a sub-Doppler-cooled atomic sample away from its zero-velocity position. Some of the effects have potential applications in the manipulation of cesium atoms (deflecting an atomic beam or launching an optical molasses into an atomic fountain) and can probably be used in the development of a future cesium atomic frequency standard. The observed experimental effects are interpreted by simple physical models and calculations.>
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.
The advent of the laser opened the way to the development of powerful methods for producing and manipulating cold atomic gases. To set the stage we describe a typical experiment, which is shown schematically in Fig. 4.1. A beam of sodium atoms emerges from an oven at a temperature of about 600 K, corresponding to a speed of about 800 m s −1 , and is then passed through a so-called Zeeman slower, in which the velocity of the atoms is reduced to about 30 m s −1 , corresponding to a temperature of about 1 K. In the Zeeman slower, a laser beam propagates in the direction opposite that of the atomic beam, and the radiation force produced by absorption of photons retards the atoms. Due to the Doppler effect, the frequency of the atomic transition in the laboratory frame is not generally constant, since the atomic velocity varies. However, by applying an inhomogeneous magnetic field designed so that the Doppler and Zeeman effects cancel, the frequency of the transition in the rest frame of the atom may be held fixed. On emerging from the Zeeman slower the atoms are slow enough to be captured by a magneto-optical trap (MOT), where they are further cooled by interactions with laser light to temperatures of order 100 μK. Another way of compensating for the changing Doppler shift is to increase the laser frequency in time, which is referred to as ‘chirping’.