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Physical Review Letters

Reconstruction of a Cold Atom Cloud by Magnetic Focusing

C. V. Saba, P. A. Barton, M. G. Boshier, Ifan G. Hughes, P. Rosenbusch, B. E. Sauer, Edward A. Hinds

Published Jan 18, 1999• 83 citations
Publisher

Abstract

We have imaged an $18\ensuremath{\mu}\mathrm{K}$ cloud of ${}^{85}\mathrm{Rb}$ atoms bouncing freely on a horizontal, concave magnetic atom mirror (radius of curvature $R$). A high-quality image is observed even after 14 reflections. Initially compact, the cloud is alternately collimated (odd bounces) and brought back to a focus (even bounces) when dropped from a height $R/4$. This multiple reconstruction of a thermally expanding cloud hinges on the microscopic reversal of the atomic motion. We discuss the factors limiting the resolution of the mirror and describe the method of construction.

DOI
10.1103/physrevlett.82.468

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We have imaged an 18\ensuremathμK18\ensuremath{\mu}\mathrm{K}18\ensuremathμK cloud of 85Rb{}^{85}\mathrm{Rb}85Rb atoms bouncing freely on a horizontal, concave magnetic atom mirror (radius of curvature RRR). A high-quality image is observed even after 14 reflections. Initially compact, the cloud is alternately collimated (odd bounces) and brought back to a focus (even bounces) when dropped from a height R/4R/4R/4. This multiple reconstruction of a thermally expanding cloud hinges on the microscopic reversal of the atomic motion. We discuss the factors limiting the resolution of the mirror and describe the method of construction.
Multiple bounces of cold rubidium atoms have been observed for times up to one second in a trap formed by gravity and a 2 cm-diameter spherical mirror made from a sinusoidally magnetized floppy disk. We have studied the dynamics of the atoms bouncing in this trap from several different heights up to 40.5 mm and we conclude that the atoms are reflected specularly and with reflectivity 1.01(3). Slight roughness of the mirror is caused by harmonics in the magnetization of the surface and by discontinuities at the boundaries between recorded tracks. As the next step in this atom optics program we propose using a magnetic mirror to create a 2D atomic gas. We discuss how cold atoms can be loaded into the ground state of a static magnetic potential well that exists above the surface of the mirror as a consequence of the intermediate-field Zeeman effect.
In a recent experiment we studied cold rubidium atoms bouncing on a magnetic mirror made from a flexible computer disk with sinusoidal magnetization. The motion was well described by a model in which the mirror was a perfect specular reflector, but complete agreement with the data required the reflecting surface to be slightly corrugated. Here we explore the physical origins of the corrugation both theoretically and experimentally. First, we develop a theory relating the reflecting force on the atoms to the magnetization of the mirror, taking into account the finite thickness of the magnetic film. We find that if the signal on the floppy disk is not harmonic the atoms appear to have been reflected from a corrugated surface, as observed in our recent experiment. Next, we describe magnetic force microscope measurements which allow us to determine the distortion on the disk and hence to quantify its effect on the reflected atoms. We show that recording nonlinearity is indeed a major cause of the mirror roughness. We also consider other sources of roughness and identify an important effect associated with the boundaries between recorded tracks. Agreement between our experiment and theory suggests that we have identified the limiting factors in a real atom-optical element made from a floppy disk. At present the angular resolution of the mirror is approximately 35 mrad for atoms dropped from a height of 4 cm. We discuss how this can be improved to reach a level of 5 mrad or better.
We have demonstrated that a suitably magnetized surface can be used to retroreflect cold atoms for applications in atom optics. This has some advantages relative to evanescent wave mirrors because no light is involved. Multiple bounces of cold rubidium atoms have been observed for times up to 1 s in a trap formed by gravity and a 2 cm diameter spherical mirror made from a flexible computer disk (`floppy disk'). We have studied the dynamics of the atoms bouncing in this trap from several different heights up to 40.5 mm and we conclude that the atoms are reflected specularly and with reflectivity 1.01(3). The performance of this mirror is limited at present by collisions with the background gas and by unwanted harmonics in the magnetization of the surface. This is the first in a series of papers concerning the use of magnetized surfaces in atom optics.
Cryogenically cooled microelectromagnet mirrors were used to reflect a cloud of free-falling laser-cooled 85Rb{}^{85}\mathrm{Rb}85Rb atoms at normal incidence. The mirrors consisted of microfabricated current-carrying Au wires in a periodic serpentine pattern on a sapphire substrate. The fluorescence from the atomic cloud was imaged after it had bounced off a mirror. The transverse width of the cloud reached a local minimum at an optimal current corresponding to minimum mirror roughness. A distinct increase in roughness was found for mirror configurations with an even versus odd number of lines. These observations confirm theoretical predictions.
We have demonstrated that cold atoms can be retroflected from a ferromagnetic surface by the Stern-Gerlach effect. When the surface is magnetized periodically, the reflectivity for suitably polarized atoms is $(94\ifmmode\pm\else\textpm\fi{}8)%$ and the reflection is specular. A demagnetized surface is also highly reflecting but the reflections are diffuse. These magnetic processes are of interest for atom optics because they permit the manipulation of cold atoms without the use of laser beams. In our experiments, Rb atoms released from a magneto-optic trap (MOT) fall under gravity until they are reflected, after which they are recaptured in the MOT. Multiple bounces of these atoms have been studied for times up to 600 ms.