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arXiv:1612.02309 · Physical Review A

Degenerate Raman sideband cooling of K39

Michael Gröbner, Philipp Weinmann, Emil Kirilov, Hanns‐Christoph Nägerl

Published Mar 15, 2017• 24 citations
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Abstract

We report on a realization of sub-Doppler laser cooling of $^{39}\mathrm{K}$ atoms using degenerate three-dimensional Raman sideband cooling. We take advantage of the well-resolved excited hyperfine states on the ${D}_{1}$ optical transition to produce spin-polarized samples with $1.4\ifmmode\times\else\texttimes\fi{}{10}^{8}$ atoms at temperatures of 1.8 $\ensuremath{\mu}\mathrm{K}$. The phase-space densities are $\ensuremath{\ge}{10}^{\ensuremath{-}4}$, which significantly improves the initial conditions for a subsequent evaporative cooling step. The presented cooling technique using the ${D}_{1}$ line can be adapted to other atomic species and is applicable to high-resolution imaging schemes in far off-resonant optical lattices.

DOI
10.1103/physreva.95.033412

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We report on a realization of sub-Doppler laser cooling of 39K^{39}\mathrm{K}39K atoms using degenerate three-dimensional Raman sideband cooling. We take advantage of the well-resolved excited hyperfine states on the D1{D}_{1}D1​ optical transition to produce spin-polarized samples with 1.4\ifmmode×\else\texttimes\fi1081.4\ifmmode\times\else\texttimes\fi{}{10}^{8}1.4\ifmmode×\else\texttimes\fi108 atoms at temperatures of 1.8 \ensuremathμK\ensuremath{\mu}\mathrm{K}\ensuremathμK. The phase-space densities are \ensuremath≥10\ensuremath−4\ensuremath{\ge}{10}^{\ensuremath{-}4}\ensuremath≥10\ensuremath−4, which significantly improves the initial conditions for a subsequent evaporative cooling step. The presented cooling technique using the D1{D}_{1}D1​ line can be adapted to other atomic species and is applicable to high-resolution imaging schemes in far off-resonant optical lattices.
Sub-Doppler laser cooling of 39^{39}39K via the 4S$\to$5P transition
We demonstrate sub-Doppler laser cooling of ^{39} 39 K using degenerate Raman sideband cooling via the 4S _{1/2} \rightarrow 1 / 2 → 5P _{1/2} 1 / 2 transition at 404.8 nm. By using an optical lattice in combination with a magnetic field and optical pumping beams, we obtain a spin-polarized sample of up to 5.6 \times 10^{7} 5.6 × 10 7 atoms cooled down to a sub-Doppler temperature of 4 \upmu μ K, reaching a peak density of 3.9 \times 10^{9} 3.9 × 10 9 atoms/cm ^{3} 3 , a phase-space density greater than 10^{-5} 10 − 5 , and an average vibrational level of \langle \nu \rangle=0.6 ⟨ ν ⟩ = 0.6 in the lattice. This work opens up the possibility of implementing a single-site imaging scheme in a far-detuned optical lattice utilizing shorter wavelength transitions in alkali atoms, thus allowing improved spatial resolution.
We report on the implementation of degenerate Raman sideband cooling of 40K^{40}\mathrm{K}40K atoms. The scheme incorporates a three-dimensional (3D) optical lattice, which confines the atoms and drives the Raman transitions. The optical cooling cycle is closed by two optical pumping beams. The wavelength of the laser beams forming the lattice is close to the D2{\mathrm{D}}_{2}D2​ atomic transition, while the optical pumping is operated near the D1{\mathrm{D}}_{1}D1​ transition. With this cooling method, we achieve a temperature of $\ensuremath{\sim}1\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{K}$ of a cloud with $\ensuremath{\sim}{10}^{7}$ atoms. This corresponds to a phase space density of $\ensuremath{\ge}{10}^{\ensuremath{-}3}$. Moreover, the fermionic ensemble is spin polarized to conditions which are favorable for subsequent evaporative cooling. We study the dependence of the cooling scheme on several parameters, including the applied magnetic field and the detuning, duration, and intensity profile of the optical pumping beams. Adding this optical cooling stage to current Fermi gas experiments can improve the final conditions and increase the data rate.
We develop a simple and practical scheme to apply sideband cooling to a cloud of rubidium atoms. A sample containing 4 × 10 70 87 Rb is trapped in a far red detuned optical lattice. Through optimizing the relevant parameters, i.e., laser detuning, magnetic field, polarization, and duration time, a temperature around 1.5 μK and phase space density close to 1/500 are achieved. Compared with polarization gradient cooling, the temperature decreases by around one order of magnitude. This technique could be used in high precision measurement such as atomic clocks and atom interferometer. It could also serve as a precooling means before evaporation cooling in a dipole trap, and may be a promising method of achieving quantum degeneracy with purely optical means.
We trap 107{10}^{7}107 cesium atoms in a far red detuned 1D optical lattice. With degenerate Raman sideband cooling we achieve a vibrational ground state population of 80% for the steep trapping direction. Collisional coupling enables us to cool the spin-polarized gas in 3D without loss of atoms to a peak phase space density of 1/1801/1801/180 at a mean temperature of $2.8\ensuremath{\mu}\mathrm{K}$ and a density of $1.4\ifmmode\times\else\texttimes\fi{}{10}^{13}{\mathrm{cm}}^{\ensuremath{-}3}$.
We laser cool $5\ifmmode\times\else\texttimes\fi{}{10}^{7}$ Cs atoms to a spin-polarized phase space density of 1/30, the highest ever obtained by laser cooling. It is achieved by compression and polarization gradient cooling in a 3D far-off-resonant optical lattice, followed by 3D Raman sideband cooling optimized at a density of $1.5\ifmmode\times\else\texttimes\fi{}{10}^{12}\mathrm{atoms}/{\mathrm{cm}}^{3}$, and adiabatic release. In the lattice, 2323%23 of the sites are occupied, 9595%95 of the atoms are in the lowest energy magnetic sublevel, and 3737%37 are in the lowest 3D vibrational state.
We demonstrate a simple, general purpose method to cool neutral atoms. A sample containing $3\ifmmode\times\else\texttimes\fi{}{10}^{8}$ cesium atoms prepared in a magneto-optical trap is cooled and simultaneously spin polarized in 10 ms at a density of $1.1\ifmmode\times\else\texttimes\fi{}{10}^{11}\mathrm{cm}{}^{\ensuremath{-}3}$ to a phase space density $n{\ensuremath{\lambda}}_{\mathrm{dB}}^{3}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1/500$, which is almost 3 orders of magnitude higher than attainable in free space with optical molasses. The technique is based on 3D degenerate Raman sideband cooling in optical lattices and remains efficient even at densities where the mean lattice site occupation is close to unity.