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arXiv:quant-ph/0503045

Experimental Characterization of 1-D Velocity Selection

Stefan H. Myrskog, J. K. Fox, A. M. Jofre, L. R. Segal, Mishra, S. R., Adam Michael Steinberg

Published Mar 3, 2005• 0 citations
arXivHTMLPDF

Abstract

We demonstrate a 1-D velocity selection technique which relies on combining magnetic and optical potentials. We have selected atom clouds with temperatures as low as 2.9% of the initial temperature, with an efficiency of 1%. The efficiency (percentage of atoms selected) of the technique can vary as slowly as the square root of the final temperature. In addition to selecting the coldest atoms from a cloud, this technique imparts a sharp cut-off in the velocity distribution. The cold selected atoms are confined in a small well, spatially separated from higher energy atoms. Such a non-thermal distribution may be useful for atom optics experiments, such as studies of atom tunneling.

DOI
10.48550/arxiv.quant-ph/0503045

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We demonstrate a 1-D velocity selection technique which relies on combining magnetic and optical potentials. We have selected atom clouds with temperatures as low as 2.9% of the initial temperature, with an efficiency of 1%. The efficiency (percentage of atoms selected) of the technique can vary as slowly as the square root of the final temperature. In addition to selecting the coldest atoms from a cloud, this technique imparts a sharp cut-off in the velocity distribution. The cold selected atoms are confined in a small well, spatially separated from higher energy atoms. Such a non-thermal distribution may be useful for atom optics experiments, such as studies of atom tunneling.
We discuss a velocity selection technique for obtaining cold atoms, in which all atoms below a certain energy are spatially selected from the surrounding atom cloud. Velocity selection can in some cases be more efficient than other cooling techniques for the preparation of ultracold atom clouds in one dimension. With quantum mechanical and classical simulations and theory we present a scheme using a dipole force barrier to select the coldest atoms from a magnetically trapped atom cloud. The dipole and magnetic potentials create a local minimum which traps the coldest atoms. A unique advantage of this technique is the sharp cut-off in the velocity distribution of the sample of selected atoms. Such a non-thermal distribution should prove useful for a variety of experiments, including proposed studies of atomic tunneling and scattering from quantum potentials. We show that when the rms size of the atom cloud is smaller than the local minimum in which the selected atoms are trapped, the velocity selection technique can be more efficient in 1-D than some common techniques such as evaporative cooling. For example, one simulation shows nearly 6% of the atoms retained at a temperature 100 times lower than the starting condition.
We demonstrate techniques for imaging both magnetic fields and magnetic sublevel distributions using velocity-selective Raman transitions in cold 87Rb and 85Rb ensembles. When an expanding atom cloud is exposed to counterpropagating laser beams, resonant atom trajectories undergoing Raman transitions are perturbed and result in distinctive features in images of the expanded cloud. If the counterpropagating laser fields couple magnetic sublevels in different hyperfine manifolds, the images provide a spectrograph of sublevel occupation. The techniques are simple to implement and provide a useful, rapid diagnostic tool for cold atom experiments.
We have studied velocity-selective resonances in the presence of a uniform magnetic field and shown how they can be used for rapid, single-shot assessment of the ground state magnetic sublevel spectrum in a cold atomic vapor. Cold atoms are released from a magneto-optical trap in the presence of a small bias magnetic field ( approximately 300 mG) and exposed to a laser field comprised of two phase-locked counterpropagating beams connecting the two ground state hyperfine manifolds. An image of the expanded cloud shows the velocity-selected resonances as distinct features, each corresponding to specific magnetic sublevel, in a direct, intuitive manner. We demonstrate the technique with both 87Rb and 85Rb, and show the utility of the technique by optically pumping into particular magnetic sublevels. The results are shown to agree with a theoretical model, and are compared to traditional Raman spectroscopy.
We experimentally demonstrated the transport of cold atoms in a magnetic quadrupole trap in the nonadiabatic regime on the order of the trap period. The trap potential velocity was controlled by varying the currents of the two pairs of anti-Helmholtz coils. We efficiently transported cold atoms with two velocity regimes. For example, with a potential velocity of 18 (28 cm/s), the final velocity of a cold atom cloud is 0.6±2.7 (58.6±1.5 cm/s). The trap motion with proper velocity could expel energetic atoms from the trap, resulting in a decline in temperature of high residual velocity atoms.
Summary form only given. Atoms without ground state splitting, e.g. alkaline earths, are of special interest for atom interferometry and frequency standards due to their small sensitivity to external fields and for BEC and the study of cold collisions due to the simplified theoretical interpretation of their interaction. We demonstrate Maxwell's-demon cooling, a novel method which allows to further cool alkaline earth atoms in one dimension. It consists of three consecutive steps which are repeated several times. First, from a cloud of cooled atoms released from a standard magneto-optical trap a small part of the velocity distribution is selected by excitation on a narrow transition (intercombination transition). Second, these atoms are optically pumped to another long lived state. Third, the ground state velocity distribution is restored by turning on the optical molasses on the strong transition used for the magneto-optical trapping. In addition to Maxwell's thought experiment, where a demon separates the cold atoms from the hot ones, in the novel scheme the hot atoms are furthermore cooled again to the original 3 mK. By repeating these steps slow atoms are accumulated in the excited state. The velocity distribution of the atoms is measured after the cold atoms are decayed to the ground state by recording the Doppler broadening of the intercombination transition.