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arXiv:physics/0106028 · Physical Review A

Ferromagnetic atom guide within situloading

Mukund Vengalattore, Wilbert Rooijakkers, Mara Goff Prentiss

Published Nov 11, 2002• 27 citations
arXivHTMLPDF

Abstract

Magneto-optic and magnetostatic trapping is realized near a surface using current carrying coils wrapped around magnetizable cores, with up to ${10}^{9}$ cesium atoms. Ramping up the current while maintaining optical dissipation leads to tightly confined atom clouds with an aspect ratio of 1:1000. We study the three-dimensional character of the magnetic potential and characterize the atom number and density as functions of the applied current. The field gradient in the transverse dimension has been varied from $<10\mathrm{G}/\mathrm{c}\mathrm{m}$ to $>1\mathrm{k}\mathrm{G}/\mathrm{c}\mathrm{m}.$

DOI
10.1103/physreva.66.053403

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Quantum Electronics and Laser Science Conference•Nov 13, 2002•0 citations
Magneto-optic and magnetostatic trapping is realized near a surface using current carrying coils wrapped around magnetizable cores, with up to 109{10}^{9}109 cesium atoms. Ramping up the current while maintaining optical dissipation leads to tightly confined atom clouds with an aspect ratio of 1:1000. We study the three-dimensional character of the magnetic potential and characterize the atom number and density as functions of the applied current. The field gradient in the transverse dimension has been varied from <10G/cm<10\mathrm{G}/\mathrm{c}\mathrm{m}<10G/cm to >1kG/cm.>1\mathrm{k}\mathrm{G}/\mathrm{c}\mathrm{m}.>1kG/cm.
We report measurements of the stability of magneto-optical traps (MOTs) for neutral atoms in the limit of tight confinement of a single atom. For quadrupole magnetic field gradients at the trap center greater than $\ensuremath{\sim}1\mathrm{kG}/\mathrm{cm}$, we find that stochastic diffusion of atoms out of the trapping volume becomes the dominant particle loss mechanism, ultimately limiting the MOT size to greater than $\ensuremath{\sim}5\ensuremath{\mu}\mathrm{m}$. We measured and modeled the diffusive loss rate as a function of laser power, detuning, and field gradient for trapped cesium atoms. In addition, for as few as two atoms, the collisional loss rates become very high for tightly confined traps, allowing the direct observation of isolated two-body atomic collisions in a MOT.
The use of a cryogenic apparatus has several advantages for the magneto-optical and magnetostatic trapping of atoms. We have measured the lifetimes of magneto-optical and magnetostatic traps for cesium to be greatly increased in a cryogenic vacuum environment relative to a room-temperature vacuum environment, due to reduced background pressure and reduced mass and kinetic energy of background gas atoms. Another advantage of a cryogenic apparatus is that superconducting magnetic field coils can easily be used to generate very high magnetic field gradients for stronger confinement of the trapped atoms. We report the first measurements of the stability of MOT's at high magnetic field gradients and laser intensities, and compare the results to a model based on evaporation of atoms from the MOT using a Fokker-Planck equation approach. This model is more consistent with the data than an alternative model based on ejection of atoms by magnetic dipole forces. We also measure the dynamical parameters for magneto-optically trapped atoms at both high and low magnetic field gradients and with trapping laser intensities above saturation and compare the data to a simple physical model based on polarization gradient cooling. Consistent with measurements by other groups, the measured friction is consistent with the polarization gradient model, but the measured spring constant is drastically reduced and more consistent with the predictions of the Doppler theory.
Summary form only given. The magneto-optical trap (MOT) has become the workhorse for many, if not most atom-optics experiments. The traditional MOT uses a pair of coils in anti-Helmholtz configuration to generate a quadrupole field with a typical gradient in the order of 10 Gauss/cm and a typical MOT diameter of 1 mm. In this paper we describe the formation of a surface MOT in a skewed magnetic field, produced by a magnetic laminate.We have imaged the fluorescence of the trapped cloud onto a CCD camera, and made cross sections of the trapped atom cloud.
We present an investigation of the cesium magneto-optical trap, with particular regard to the best combination of atomic density and temperature that can be produced. Conditions in the trap depend on four independent parameters: the detuning and intensity of the light, the gradient of the magnetic field, and the number of atoms trapped. We have varied all these parameters and measured the temperature and density distribution of the trapped cloud. Both the nonlinear variation with position of the restoring force and the reabsorption of photons scattered in the cloud limit the maximum density, and we present an empirical model that takes this into account. This in turn limits the density in phase space \ensuremath{\rho} (defined as the number of atoms in a box with sides of one thermal de Broglie wavelength). We have observed a maximum \ensuremath{\rho}=(1.5\ifmmode\pm\else\textpm\fi{}0.5)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$, with a spatial density of about 2\ifmmode\times\else\texttimes\fi{}1011{10}^{11}1011 atoms/cm3{\mathrm{cm}}^{3}cm3.
We investigate the behavior of a cloud of atoms in a magneto-optical trap, which—after collection—is compressed when the field gradients of the trap magnetic field are increased. We measure sizes and shapes of the atom cloud as a function of laser detuning, magnetic field gradient, and number of trapped atoms. A transient density increase of more than an order of magnitude has been achieved. Moreover, reproducible Gaussian density distributions are observed at large detunings and intermediate magnetic-field gradients, permitting an accurate determination of density.
Summary form only given. The integration of atom optics components on a small surface may lead to many new experiments exploring the boundaries of quantum mechanics. Coherent transport of atoms is a key ingredient, and for this reason many groups are developing magnetic waveguides for cold atoms. So far the confinement in these guides is rather poor, resulting in the population of many transverse modes. Typically 10 Gauss over 100 /spl mu/m gives a mode spacing of 40 kHz for Cs atoms. We have developed a new magnetic waveguide, based on ferromagnetic material. At saturation the surface-field is 1 Tesla, which can be applied over a 10 /spl mu/m distance corresponding to a mode spacing of 18 MHz. We use a sandwich of mu-metal and copper sheets to generate a magnetic field. An electrical current of up to 5 A is running through the copper, encircling the mu-metal sheet. This induces an upward or downward field in the mu-metal, similar to a solenoid with a ferromagnetic core. Thin kapton sheets are used to provide electrical isolation.