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arXiv:0807.3672 · Physical Review A

Light-shift tomography in an optical-dipole trap for neutral atoms

Jean-Philippe Brantut, Jean‐François Clément, Martin Robert-De-Saint-Vincent, Gael P. Varoquaux, Robert A. Nyman, Alain Aspect, Thomas Bourdel, Philippe Bouyer

Published Sep 25, 2008• 37 citations
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Abstract

We report on light-shift tomography of a cloud of $^{87}\mathrm{Rb}$ atoms in a far-detuned optical-dipole trap at $1565\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. Our method is based on standard absorption imaging, but takes advantage of the strong light shift of the excited state of the imaging transition, which is due to a quasiresonance of the trapping laser with a higher excited level. We use this method to (i) map the equipotentials of a crossed optical-dipole trap and (ii) study the thermalization of an atomic cloud by following the evolution of the potential energy of atoms during the free-evaporation process.

DOI
10.1103/physreva.78.031401

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•Nov 23, 2016•0 citations
We report on light-shift tomography of a cloud of 87Rb^{87}\mathrm{Rb}87Rb atoms in a far-detuned optical-dipole trap at 1565nm1565\phantom{\rule{0.3em}{0ex}}\mathrm{nm}1565nm. Our method is based on standard absorption imaging, but takes advantage of the strong light shift of the excited state of the imaging transition, which is due to a quasiresonance of the trapping laser with a higher excited level. We use this method to (i) map the equipotentials of a crossed optical-dipole trap and (ii) study the thermalization of an atomic cloud by following the evolution of the potential energy of atoms during the free-evaporation process.
This project reports on loading 87-Rb in a red-detuned optical dipole trap formed by three 1560 nm crossed beams. The work describes the implementations made to the system for proper trap operation and the designed experimental sequence. The characteristics of the dipole potential are calculated to understand and optimize the trap loading process. The light-shift tomography technique is employed to map the trapping potential and to ensure the alignment of the trap beams. As a result, the experiment yielded 2 × 10^5 atoms in the optical dipole trap.
Abstract In this work, the effect of AC-Stark shift of atomic energy levels in an optical dipole trap (ODT) on in situ absorption imaging of the trapped atoms has been investigated. The Stark shift of various energy levels of 87 Rb atoms relevant for trapping in the optical dipole trap (ODT) has been calculated. The spatially varying AC-Stark shift of energy levels due to ODT beam results in position dependent absorption cross-section of the atom at the absorption probe laser wavelength, which significantly modifies the measured optical density (OD) of the trapped atom cloud during the in situ absorption imaging. We have estimated the number of atoms trapped in the ODT taking AC-Stark shift into account. The estimated numbers are found to be higher at higher ODT beam power, making a qualitative agreement with larger trap depth at higher ODT beam power. This work can be particularly useful in estimating the in situ loss rate of atoms from the trap during the evaporative cooling of the trapped atom cloud.
Microscopically controlled neutral atoms in optical tweezers and lattices have led to exciting advances in the study of quantum information and quantum many-body systems. The light shifts of atomic levels from the trapping potential in these systems can result in detrimental effects such as fluctuating dipole force heating, inhomogeneous detunings, and inhibition of laser cooling, which limits the atomic species that can be manipulated. In particular, these light shifts can be large enough to prevent loading into optical tweezers directly from a magneto-optical trap. We implement a general solution to these limitations by loading, as well as cooling and imaging the atoms with temporally alternating beams, and present an analysis of the role of heating and required cooling for single atom tweezer loading. Because this technique does not depend on any specific spectral properties, it should enable the optical tweezer platform to be extended to nearly any atomic or molecular species that can be laser cooled and optically trapped.
We present the experimental production and characterization of a dense cold atomic cloud of 88Sr^{88}\text{Sr}88Sr atoms, optimized for the future studies of light transport in highly dense regimes. Using narrow-line molasses on the 689 nm transition, combined with a far off-resonant optical dipole trap, we achieve spatial densities as high as 7.9×1013 atoms/cm37.9 \times 10^{13} \, \text{atoms/cm}^37.9×1013atoms/cm3 and optical depths up to 64. This approach stands out from previous methods by integrating narrow-line molasses with an optical dipole trap, enabling high-density samples without relying on evaporative cooling. Unlike traditional absorption imaging, which becomes inaccurate in such dense regimes, we demonstrate that phase-contrast imaging (PCI) can reliably reconstruct the in-situ density profile even for highly spatially and optically dense samples. The use of a spatial light modulator instead of a fixed phase plate in the PCI setup provides enhanced flexibility and control of imaging parameters, making this imaging technique robust against imaging artifacts and adaptable to varying experimental conditions. Moreover, we derive theoretical conditions for reliable PCI operation in dense regimes and validate these experimentally, showing excellent agreement with time-of-flight measurements even at the highest densities. Our results establish a robust method for producing and characterizing dense atomic clouds.
The mapping of the potential landscape with high spatial resolution is crucial for quantum technologies based on ultracold atoms. However, the imaging of optical dipole traps is challenging because purely optical methods, commonly used to profile laser beams in free space, are not applicable in a vacuum. In this work, we demonstrate precise in situ imaging of optical dipole traps by probing a hyperfine transition with Ramsey interferometry. Thereby, we obtain an absolute map of the potential landscape with micrometer resolution and shot-noise-limited spectral precision. The idea of the technique is to control the polarization ellipticity of the trap laser beam to induce a differential light shift proportional to the trap potential. By studying the response to polarization ellipticity, we uncover a small but significant nonlinearity in addition to a dominant linear behavior, which is explained by the geometric distribution of the atomic ensemble. Our technique for imaging of optical traps can find wide application in quantum technologies based on ultracold atoms, as it applies to multiple atomic species and is not limited to a particular wavelength or trap geometry.
We have developed a technique to fully characterize an arbitrary potential energy of an atom trap.The characterization allows one to measure the symmetry of the trap, a determining factor in many trapped atom applications.Created through a magneto-optical trap and evaporative cooling, a cold atom cloud is first loaded into our weakly-confined time-orbiting potential (TOP) trap.The whole atom cloud is then optically pumped into a dark state, an energy level that does not interact with the probe laser light.A selected part of the atom cloud is reactivated by a repump light which optically pump the atoms back into a state that can be probed.The repump light is shaped into a light sheet 168 µm thick.The reactivated region interacts with the probe laser light to create a fluorescence image.Since the light sheet is much thinner than the atom cloud, which is roughly 2 mm wide, the fluorescence image obtained is a cross-section of the atom cloud.A movable light sheet allows us to generate crosssection images of the cloud at different positions.A composite image of all the crosssection images shows the complete potential profile of our atom trap.This is similar to tomographic imaging used in medical imaging.We have verified the technique with two other methods: direct oscillation measurement at varying amplitude and numerical simulation of the atom trap.The technique is able to measure the potential up to the fourth-order terms in spatial coordinates.A complete characterization of the atom trap's symmetry will allow us to develop an atomic Foucault pendulum, a novel application for trapped atoms.5.3 Acceptable level of anharmonicity . . . . . . .