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IEEE Transactions on Instrumentation and Measurement

Effects of a static magnetic field on sub-Doppler molasses: manipulation of cesium atoms

Jia Yu, Marie-Claude Gagné, Constance Valentin, Saïda Guellati-Khélifa, J. Djemaa, Ruixia Yuan, P. Pillet

Published Apr 1, 1993• 2 citations
Publisher

Abstract

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.>

DOI
10.1109/19.278531

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We present a simple review of the basic physical processes allowing one to control, with laser beams, the velocity and the position of neutral atoms. The control of the velocity corresponds to a cooling of atoms, that is, a reduction of the atomic velocity spread around a given value. The control of the position means a trapping of atoms in real space. The best present performances will be given, in terms of the lowest temperatures and the highest densities. The corresponding highest quantum degeneracy will also be estimated. It is imposed by fundamental limits, which will be described briefly. We also give the general trends in this field of research and outline the new directions which look promising for observing quantum statistical effects in laser cooled atomic samples, but which are for the moment restricted by unsolved problems. Introducing the Simple Schemes The radiative forces acting on atoms in a light field can be split into two parts, a reactive one and a dissipative one. The dissipative force (radiation pressure), which basically involves scattering processes, is velocity dependent. We will see that this dependence leads to the Doppler cooling scheme and to the concept of optical molasses, and we will give the corresponding minimal achievable temperature. The dissipative force can be made position dependent, through a gradient of the magnetic field, so that the atoms are also trapped in the so-called magneto-optical trap.

Cambridge University Press eBooks•Apr 6, 1995•6 citations
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.>
Sub-Doppler cooling has been achieved for the two closed hyperfine transitions: F = 4 → F ' = 5 and F = 3 → F ' = 2, of the cesium D 2 line. One-dimension optical molasses is experimentally investigated and compared for both j → j + 1 and j → j - 1 transitions, on different laser polarization configurations, in the presence, and absence of a static magnetic field. One remarkable point is that cooling is obtained with blue laser detuning for the j → j - 1 transition. An applied static magnetic field can also shift the sub-Doppler-cooled molasses peaks to nonzero velocities without any extra heating, for the polarization gradient configurations.
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 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 present a simple review of the basic physical processes allowing one to control, with laser beams, the velocity and the position of neutral atoms. The control of the velocity corresponds to a cooling of atoms, that is, a reduction of the atomic velocity spread around a given value. The control of the position means a trapping of atoms in real space. The best present performances will be given, in terms of the lowest temperatures and the highest densities. The corresponding highest quantum degeneracy will also be estimated. It is imposed by fundamental limits, which will be described briefly. We also give the general trends in this field of research and outline the new directions which look promising for observing quantum statistical effects in laser cooled atomic samples, but which are for the moment restricted by unsolved problems. Introducing the Simple Schemes The radiative forces acting on atoms in a light field can be split into two parts, a reactive one and a dissipative one. The dissipative force (radiation pressure), which basically involves scattering processes, is velocity dependent. We will see that this dependence leads to the Doppler cooling scheme and to the concept of optical molasses, and we will give the corresponding minimal achievable temperature. The dissipative force can be made position dependent, through a gradient of the magnetic field, so that the atoms are also trapped in the so-called magneto-optical trap.