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Physical Review A

Near-resonant dark optical lattice with increased occupation

Vladimir Elman, Andreas Hemmerich

Published Oct 31, 2005• 5 citations
Publisher

Abstract

A bichromatic near-resonant dark optical lattice (DOL) with rubidium atoms is demonstrated, which provides confinement in the Lamb-Dicke regime in all spatial dimensions. We apply spatially phase-matched optical potentials for each hyperfine ground state in order to enable improved Sisyphus cooling, undisturbed by optical hyperfine pumping processes. We also explore a method to increase the occupation of the DOL. Initially $2\ifmmode\times\else\texttimes\fi{}{10}^{9}$ rubidium atoms with a temperature of $70\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$ and a density of $5\ifmmode\times\else\texttimes\fi{}{10}^{10}\phantom{\rule{0.3em}{0ex}}\text{atoms}∕{\mathrm{cm}}^{3}$ are prepared in a magneto-optic trap (MOT) and a fraction of $5\ifmmode\times\else\texttimes\fi{}{10}^{7}$ atoms is loaded into a far detuned one-dimensional optical lattice (FOL). Subsequently, the MOT is replaced by the DOL, and the atoms become well localized within the microscopic light-shift potentials at a temperature of $10\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$ with a typical density of $3\ifmmode\times\else\texttimes\fi{}{10}^{11}\phantom{\rule{0.3em}{0ex}}\text{atoms}∕{\mathrm{cm}}^{3}$. We then apply alternating cycles of free evolution in the FOL and cooling and trapping in the DOL, obtaining a fourfold density increase to $1.2\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.3em}{0ex}}\text{atoms}∕{\mathrm{cm}}^{3}$---i.e., 7.5% occupation---while maintaining a temperature of $10\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$. In a final adiabatic cooling step we reduce the well depth to 75 times the single-photon recoil energy, which leads to a temperature of $2.8\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$ and a phase-space density of $1.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$. Despite the increased density, no excess heating or collisional losses are observed.

DOI
10.1103/physreva.72.043410

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This thesis explores ensembles of ultracold atoms, trapped and cooled in dark optical lattices. In this type of optical lattice, atoms are trapped in quantum states which are decoupled from the trapping light field. Therefore, elastic scattering - the main process limiting the density of atoms in optical lattices - is significantly reduced. This entails the chance to overcome the density limit existing in conventional optical lattices. We explore a novel scheme for density enhancement which allows for a four-fold density increase over the initial density of the magneto-optical trap (MOT). We load atoms from the MOT into a modified optical lattice consisting of a three-dimensional dark optical lattice and a far-detuned optical dipole trap. Atoms are now cooled down in the potentials of dark optical lattice at typical densities of a MOT. The enhancement procedure is based on a time sequence composed of cooling and trapping cycles inside the dark optical lattice and cycles of free evolution in an array of microscopic potentials formed by the dipole trap. Im Rahmen dieser Arbeit werden kalte Atome in einem dunklen optischen Gitter untersucht. In diesem Typ von optischem Gitter sind die Atome in Zustanden gefangen, die nur schwach mit dem Lichtfeld wechselwirken. Dadurch ist die elastische Komponente der Photonenstreuung, die in konventionellen optischen Gittern fur die Begrenzung der atomaren Dichte verantwortlich ist, erheblich reduziert. Deswegen wird es moglich in dunklen optischen Gittern die Dichtebegrenzung konventioneller optischer Gitter zu ubertreffen. Wir untersuchen eine neue Methode der Dichteerhohung, mit der wir einen vierfachen Anstieg der atomaren Dichte gegenuber der Anfangsdichte in der MOT erreichen. Die Atome werden aus einer magneto-optischen Falle in eine Uberlagerung von zwei optischen Fallen geladen. Diese ist durch ein drei-dimensionales dunkles optisches Gitter und eine fernverstimmte optische Stehwellen-Dipolfalle gebildet. Das Verfahren der Dichteerhohung basiert auf abwechselnden Zyklen der Kuhlung und Lokalisierung in dem dunklen optischen Gitter und Zyklen der freier Expansion in den Mikropotentialen der optischen Stehwellen-Dipolfalle.
We form a 1D optical lattices for Cs atoms using light tuned a few thousand linewidths below the 6S1/2(F equals 4) yields 6P3/2(F' equals 5) transition at 852 nm. In this far-off- resonance lattice the time scale for damping of motional coherences and kinetic energy can be orders of magnitude longer than the vibrational oscillation period for atoms trapped in the lattice potential wells. Atoms are loaded directly into deeply bound states, by adiabatic transfer from a superimposed, near-resonance optical lattice. This yields a mean vibrational excitation n approximately equals 0.3, and localization (Delta) z approximately (lambda) /20 deep in the Lamb-Dicke regime. Light scattering subsequently heats the atoms, but the initial rate is only of order 10-3 vibrational quanta per oscillation period. Low vibrational excitation, localization in the Lamb-Dicke regime and low heating rates make these atoms good candidates for resolved- sideband Raman cooling, and for the generation and study of non-classical states of center-of-mass motion. We propose a scheme for resolved-sideband Raman cooling and quantum state preparation; the scheme employs Raman coupling between magnetic sublevels induced by the lattice light field itself.
We demonstrate subrecoil Sisyphus cooling using the long-lived ^{3}P_{0} clock state in alkaline-earth-like ytterbium. A 1388-nm optical standing wave nearly resonant with the ^{3}P_{0}→^{3}D_{1} transition creates a spatially periodic light shift of the ^{3}P_{0} clock state. Following excitation on the ultranarrow clock transition, we observe Sisyphus cooling in this potential, as the light shift is correlated with excitation to ^{3}D_{1} and subsequent spontaneous decay to the ^{1}S_{0} ground state. We observe that cooling enhances the loading efficiency of atoms into a 759-nm magic-wavelength one-dimensional (1D) optical lattice, as compared to standard Doppler cooling on the ^{1}S_{0}→^{3}P_{1} transition. Sisyphus cooling yields temperatures below 200 nK in the weakly confined, transverse dimensions of the 1D optical lattice. These lower temperatures improve optical lattice clocks by facilitating the use of shallow lattices with reduced light shifts while retaining large atom numbers to reduce the quantum projection noise. This Sisyphus cooling can be pulsed or continuous and is applicable to a range of quantum metrology applications.
We report a way to overcome heating due to the reabsorption of scattered light when relatively many atoms are laser cooled at high density. A sample of $8\ifmmode\times\else\texttimes\fi{}{10}^{7}\mathrm{Cs}$ atoms, with $6\ifmmode\times\else\texttimes\fi{}{10}^{11}{\mathrm{atoms}\mathrm{}\mathrm{cm}}^{\mathrm{\ensuremath{-}}3},$ is loaded into a deep far-detuned one-dimensional optical lattice, which acts as a large volume three-dimensional (3D) trap. Polarization gradient cooling with a separate set of cooling beams is found to work well in this trap, populating the trap's vibrational ground state with more than half the atoms. After cooling, the lattice is shut off adiabatically, leaving a still dense sample at 1.2 \ensuremath{\mu}K. We propose and demonstrate a generalization to a 3D lattice, which will allow many more atoms to be loaded into conservative light traps, and can greatly increase the initial phase-space density for loading magnetic traps.
A cooling process for optical dipole traps, rf-induced Sisyphus cooling in a circularly polarized far-off-resonance trap, is demonstrated. Cooling rates of up to $\ensuremath{\approx}300 \ensuremath{\mu}\mathrm{K}/\mathrm{s}$ are observed without significant atom loss. The final temperature achieved was (a somewhat disappointing) $17 \ensuremath{\mu}\mathrm{K}.$ Experimental studies and theoretical analysis of this cooling process were carried out, which explain that the temperature limit is due to an equilibrium between the small fraction of total thermal energy that is removed per Sisyphus cycle and the multiple photon recoils that are required to complete each cycle. Light-induced collisional loss made it difficult to operate under conditions where a larger fraction of the energy would be removed. A method for storing atoms in an optical trap with only a single confined spin state is also demonstrated.
A complete laser cooling setup was built, with focus on threedimensional near-resonant optical lattices for cesium. These consist of regularly ordered micropotentials, created by the interference of four laser beams. One key feature of optical lattices is an inherent ”Sisyphus cooling” process. It efficiently extracts kinetic energy from the atoms, leading to equilibrium temperatures of a few µK. The corresponding kinetic energy is lower than the depth of the potential wells, so that atoms can be trapped.We performed detailed studies of the cooling processes in optical lattices by using the time-of-flight and absorption-imaging techniques. We investigated the dependence of the equilibrium temperature on the optical lattice parameters, such as detuning, optical potential and lattice geometry. The presence of neighbouring transitions in the cesium hyperfine level structure was used to break symmetries in order to identify, which role “red” and “blue” transitions play in the cooling. We also examined the limits for the cooling process in optical lattices, and the possible difference in steady-state velocity distributions for different directions. Moreover, in collaboration with ´Ecole Normale Sup´erieure in Paris, numerical simulations were performed in order to get more insight in the cooling dynamics of optical lattices.Optical lattices can keep atoms almost perfectly isolated from the environment and have therefore been suggested as a platform for a host of possible experiments aimed at coherent quantum manipulations, such as spin-squeezing and the implementation of quantum logic-gates. We developed a novel way to trap two different cesium ground states in two distinct, interpenetrating optical lattices, and to change the distance between sites of one lattice relative to sites of the other lattice. This is a first step towards the implementation of quantum simulation schemes in optical lattices.