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arXiv:2208.05160 · Physical Review A

Degenerate Raman sideband cooling of K40 atoms

Elad Zohar, Yanay Florshaim, Oded Zilberman, Amir Stern, Yoav Sagi

Published Dec 23, 2022• 8 citations
arXivHTMLPDF

Abstract

We report on the implementation of degenerate Raman sideband cooling of $^{40}\mathrm{K}$ 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 ${\mathrm{D}}_{2}$ atomic transition, while the optical pumping is operated near the ${\mathrm{D}}_{1}$ 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.

DOI
10.1103/physreva.106.063111

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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 $1.4\ifmmode\times\else\texttimes\fi{}{10}^{8}$ 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 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.
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Summary form only given. Laser cooled atoms play a critical role in modern designs of frequency standards such as atomic fountains. Recently, an elegant and efficient method of two-dimensional cooling to the vibrational ground state of a far-off resonance optical lattice has been demonstrated. This method is a variant of Raman sideband cooling based on the Raman transitions between the vibrational manifolds of adjacent Zeeman substates. A static magnetic field is used to tune the Zeeman levels so that Raman resonance occurs on the red sideband and results in cooling. Two circularly polarized fields are used to recycle the atoms for repetitive Raman cooling. Unfortunately, a direct application of this scheme to frequency standards meets with several difficulties. Stimulated by these concepts and results, we propose a new variant of transverse sideband cooling, where these problems are avoided, while still maintaining most of the attractive features. The field configuration used for the optical lattice consists of three linearly polarized beams having equal amplitudes and propagating in the xy plane with angles of 2/spl pi//3 between each other. The polarization vectors of these beams are tilted through a small angle with respect to the z-axis. All the beams have the same frequency far-detuned to the red of the D/sub 2/ resonance line. Raman transitions between vibrational levels of adjacent magnetic substates are induced by the small in-plane component of the field. Two other important requirements for Raman sideband cooling are a uniform energy shift of the magnetic sublevels and optical pumping.
Sub-Doppler laser cooling of 39^{39}39K via the 4S$\to$5P transition
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We demonstrate laser cooling of 85Rb atoms in a two-dimensional optical lattice. We follow the two-step degenerate Raman sideband cooling scheme, Kerman el al., Phys. Rev. Lett. 84, 439 (2000), where a fast cooling of atoms to an auxiliary state followed by a slow cooling to a dark state. This method has the advantage of independent control of the heating rate and cooling rate from the optical pumping beam. We operate the lattice at Lamb-Dicke parameter eta=0.45 and show the cooling of spin-polarized 85Rb atoms to the recoil temperature in both dimension within 2.4 ms with 90% of initial atom numbers.
We demonstrate the laser cooling of 85Rb^{85}\mathrm{Rb}85Rb atoms in a two-dimensional optical lattice. We follow the two-step degenerate Raman sideband cooling scheme [Kerman et al., Phys. Rev. Lett. 84, 439 (2000)], where a fast cooling of atoms to an auxiliary state is followed by a slow cooling to a dark state. This method has the advantage of independent control of the heating rate and cooling rate from the optical pumping beam. We operate the lattice at a Lamb-Dicke parameter $\ensuremath{\eta}=0.45$ and show the cooling of spin-polarized 85Rb^{85}\mathrm{Rb}85Rb atoms to the recoil temperature in both dimensions within 2.4 ms with the aid of adiabatic cooling.