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Europhysics Letters (EPL)

Observation of individual neutral atoms in magnetic and magneto-optical traps

D. Haubrich, Harald Schadwinkel, Frank Strauch, B. Ueberholz, R. Wynands, Dieter Meschede

Published Jun 20, 1996• 76 citations
Publisher

Abstract

We have identified and photographed individual cesium atoms in a magneto-optical trap with steep magnetic gradients. By switching off the trapping light fields, single atoms were released to a bound state of the magnetic potential. A storage time of 38 s was measured for purely magnetic trapping, whereas a storage time of 147 s was observed in the corresponding magneto-optical trap.

DOI
10.1209/epl/i1996-00512-5

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We have identified and photographed individual cesium atoms in a magneto-optical trap with steep magnetic gradients. By switching off the trapping light fields, single atoms were released to a bound state of the magnetic potential. A storage time of 38 s was measured for purely magnetic trapping, whereas a storage time of 147 s was observed in the corresponding magneto-optical trap.
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.
A variety of experiments are underway in the Quantum Optics Group at Caltech which investigate the quantum nature of atom-field interactions at the level of individual atoms and quanta. A recent technical advance in support of this research is the observation of cooling and trapping of single neutral cesium atoms in magneto-optical trap. Discrete steps are recorded in the fluorescence signal from the trap and are associated with the arrival and departure of individual trapped atoms. Such a spatially localized sample of a single atom with small kinetic energy is an enabling advance for diverse studies in quantum optics, including the possibility of spectroscopy with squeezed and other forms of nonclassical light and cavity quantum electrodynamics with strong coupling of an atom to the field of an optical cavity.
We report on the observation of a new laser trap for neutral atoms. It uses three orthogonal pairs of counterpropagating laser beams having linear polarizations at 45° and a quadrupole magnetic field. 10 8 cesium atoms were thus confined in a 0.15 mm 3 volume at a temperature of 60 μK, a factor of 2 below the Doppler cooling limit. We interpret this trapping as being due to the new magneto-optical force recently observed by Grimm et al. and which is essentially a dipole force rectified by the magnetic field. This trap opens new possibilities for increasing the phase-space density of laser-cooled atoms.
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.