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The Rochester Conferences on Coherence and Quantum Optics and the Quantum Information and Measurement meeting

Technique for In-Situ Measurement of Magnetic Sublevel Populations in a Magneto-Optical Trap Based on Free-Expansion Imaging

Gehrig Carlse, Alexander Pouliot, Thomas Vacheresse, Adam C. Carew, Hermina C. Beica, Anantharaman Kumarakrishnan

Published Jan 1, 2019• 1 citation
Publisher

Abstract

We track the centroid position of a laser-cooled atomic cloud falling in a magnetic field gradient and show that the effective acceleration can be used to probe magnetic sublevel populations in the atomic sample.
DOI
10.1364/cqo.2019.w6a.10

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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.
We describe a simple time-of-flight technique for measuring the magnetic moment of an optically pumped magneto-optical trap. The technique relies on free-expansion imaging of a cold atom cloud in a small magnetic field gradient without the need to detect spatial separation between magnetic sublevels. We find that the effective acceleration of the cloud can be used to characterize extreme state optical pumping. In the general case, we show that the integrated displacement of the falling cloud can be accurately modeled using rate equation simulations of magnetic sublevel populations, and knowledge of local magnetic fields, field gradients, and light intensities. The agreement between the model and the data allows the reconstruction of magnetic moments and suggests that this technique may be suitable for the measurement of population distributions over a range of optical pumping conditions.
We demonstrate a technique for imaging magnetic fields using velocity-selective two-photon resonances in a cold atom cloud. Freely expanding (85)Rb atoms released from a magneto-optical trap are exposed to a brief (approximately 1 ms), off-resonant, retro-reflected laser pulse in a lin-perp-lin configuration. Two-photon resonance between magnetic sublevels occurs only for atoms in narrow velocity classes dependent on the magnetic field strength. The momentum of resonant atoms is altered by the pulse, and this two-photon momentum change is easily visible after further ballistic expansion. When the momentum pulse is applied to an atom cloud with finite size, magnetic field variations across the sample result in position-dependent features in images of the expanded cloud. We demonstrate the technique by imaging magnetic field variations over approximately 5 mm with approximately 250 microm spatial resolution.