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arXiv:2307.16469 · Journal of Physics B Atomic Molecular and Optical Physics

Sub-Doppler laser cooling and magnetic trapping of natural-abundance fermionic potassium

Mateusz Bocheński, Mariusz Semczuk

Published Jan 23, 2024• 1 citation

Abstract

Abstract We demonstrate the largest number of 40K atoms that has ever been cooled to deeply sub-Doppler temperatures in a single-chamber apparatus without using an enriched source of potassium. With gray molasses cooling on the D 1-line following a standard D 2-line magneto-optical trap, we obtain 3 × 10 5 atoms at 10(2) µK. We reach densities high enough to measure the temperature via absorption imaging using the time-of-flight method. We magnetically trap a mixture of m F = − 3 / 2 , − 5 / 2 and − 7 / 2 Zeeman states of the F = 7 / 2 hyperfine ground state confining 5 × 10 4 atoms with a lifetime of 0.6 s or ∼ 10 3 atoms with a lifetime of 2.8 s—depending on whether the temperature of the potassium dispensers was chosen to maximize the atom number or the lifetime. The background pressure-limited lifetime of 0.6 s is a reasonable starting point for proof-of-principle experiments with atoms and/or molecules in optical tweezers as well as for sympathetic cooling with another species if transport to a secondary chamber is implemented. Our results show that unenriched potassium can be used to optimize experimental setups containing 40K in the initial stages of their construction, which can effectively extend the lifetime of enriched sources. Moreover, the demonstration of sub-Doppler cooling and magnetic trapping of a relatively small number of potassium atoms might influence experiments with laser-cooled radioactive isotopes of potassium.

DOI: 10.1088/1361-6455/ad2181

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Abstract We demonstrate the largest number of 40K atoms that has ever been cooled to deeply sub-Doppler temperatures in a single-chamber apparatus without using an enriched source of potassium. With gray molasses cooling on the D 1-line following a standard D 2-line magneto-optical trap, we obtain 3 × 10 5 atoms at 10(2) µK. We reach densities high enough to measure the temperature via absorption imaging using the time-of-flight method. We magnetically trap a mixture of m F = − 3 / 2 , − 5 / 2 and − 7 / 2 Zeeman states of the F = 7 / 2 hyperfine ground state confining 5 × 10 4 atoms with a lifetime of 0.6 s or ∼ 10 3 atoms with a lifetime of 2.8 s—depending on whether the temperature of the potassium dispensers was chosen to maximize the atom number or the lifetime. The background pressure-limited lifetime of 0.6 s is a reasonable starting point for proof-of-principle experiments with atoms and/or molecules in optical tweezers as well as for sympathetic cooling with another species if transport to a secondary chamber is implemented. Our results show that unenriched potassium can be used to optimize experimental setups containing 40K in the initial stages of their construction, which can effectively extend the lifetime of enriched sources. Moreover, the demonstration of sub-Doppler cooling and magnetic trapping of a relatively small number of potassium atoms might influence experiments with laser-cooled radioactive isotopes of potassium.

We present a dual isotope magneto-optical trap (MOT), simultaneous sub-Doppler laser cooling, and magnetic trapping of a spin-polarized 39K-40K Bose-Fermi mixture realized in a single-chamber setup with an unenriched potassium dispenser as the source of atoms. We are able to magnetically confine more than 2.2 × 105 fermions (F = 9/2 , m F = 9/2) and 1.4 × 107 bosons (F = 2 , m F = 2) with a lifetime exceeding 1.2 s. For this work, we have developed a versatile laser tailored for sub-Doppler cooling of all naturally occurring potassium isotopes and their mixtures. This laser system incorporates innovative features, such as the capability to select an isotope by activating or deactivating specific acousto-optic modulators that control the light seeding tapered amplifiers. Switching between isotopes takes ∼1 µs without any mechanical adjustment of the components.

Magnetic trapping of an ultracold

^{39}

K-

^{40}

K mixture with a versatile potassium laser system

We present a dual isotope magneto-optical trap (MOT), simultaneous sub-Doppler laser cooling, and magnetic trapping of a spin-polarized

^{39}

K-

^{40}

K Bose-Fermi mixture realized in a single-chamber setup with an unenriched potassium dispenser as the source of atoms. We are able to magnetically confine more than

2.2\times10^5

fermions (

F=9/2\,m_F=9/2

) and

1.4\times10^7

bosons (

F=2\,m_F=2

) with a lifetime exceeding 1.2 s. For this work, we have developed a versatile laser tailored for sub-Doppler cooling of all naturally occurring potassium isotopes and their mixtures. This laser system incorporates innovative features, such as the capability to select an isotope by activating or deactivating specific acousto-optic modulators that control the light seeding tapered amplifiers. Switching between isotopes takes $\sim$1

\mathrmμ

s without any mechanical adjustment of the components. As a final step in characterizing the laser system, we demonstrate sub-Doppler cooling of

^{41}

K.

The main goal of this research is to cool potassium-41 atoms to ultracold temperatures (<10 μK) in order to study the spin dynamics of atom to atom interactions. However, our current goal is to finish constructing the apparatus that will allow us to confine and cool these atoms in three dimensions. This apparatus will cool and confine the potassium atoms in 3 steps. First, the atoms will be cooled and trapped in our 3D magneto-optical trap. This step of the experiment will cool the atoms near the Doppler-cooling limit, which for potassium atoms is 145 μK. Second, atoms will be loaded into a FORT (far off resonance trap) with an expected depth of 25 μK. In order to cool the atoms below the Doppler-cooling limit, we will use sub-Doppler cooling techniques. What is particularly unique about our experiment is that we plan to use an octupole magnetic field to achieve sub-Doppler cooling. In typical laser cooling experiments sub-Doppler temperatures are achieved when the quadrupole magnetic field is off, which leaves the atoms untrapped. Third, once the atoms have been loaded into the FORT one last cooling mechanism will be implemented. This mechanism is referred to as evaporative cooling and will cool the atoms below 10 μK.

We have trapped

{}^{40}\mathrm{K}

in a vapor-cell magneto-optical trap starting from a natural abundance sample. This rare, weakly radioactive atom is a fermion. The number of trapped atoms is approximately 8000, with a density of $\ensuremath{\sim}{10}^{8}$ atoms/

{\mathrm{cm}}^{3}

. Contrary to what is observed for the other isotopes of potassium, our data indicate that temperatures lower than the Doppler-cooling limit can be reached for

{}^{40}\mathrm{K}

. We discuss the interest of these results in view of possible experiments to study a degenerate Fermi gas.

Simultaneous sub-Doppler laser cooling and optical trapping of bosonic

^{39}

K-

^{133}

Cs and

^{41}

K-

^{133}

Cs mixtures

We report simultaneous sub-Doppler cooling and optical dipole trapping of

^{39}

K-Cs and

^{41}

K-Cs mixtures. Both mixtures are cooled to temperatures

\sim10

^{41}

K, achieving performance comparable to that obtained with each species individually. To the best of our knowledge, this constitutes the first realization of a laser-cooled and optically trapped

^{41}

K-Cs mixture. For the

^{39}

K-Cs mixture, we additionally implement parallel spin-resolved Feshbach spectroscopy enabled by Stern-Gerlach separation. For potassium isotopes, we implement sub-Doppler cooling with

D_1

-line gray molasses, while for cesium the entire cooling sequence is implemented using

D_2

transitions. We spin-polarize the atoms and confine Bose-Bose mixtures in a 1064 nm optical dipole trap. Using the resulting

^{39}

K-Cs samples, we observe 14 heteronuclear Feshbach loss features. Five agree with resonances reported previously, while nine have, to the best of our knowledge, not been observed experimentally before, including

p

-wave features and resonances in additional spin channels. The shorter lifetime of the

^{41}

K-Cs mixture in the optical dipole trap currently hinders systematic Feshbach spectroscopy, which we therefore do not pursue in this work. To characterize this limitation quantitatively, we study the decay dynamics of

^{39}

K-Cs and

^{41}

K-Cs mixtures under comparable temperature and density conditions, revealing a substantially stronger nonexponential loss in the

^{41}

K-Cs mixture. The demonstrated preparation of ultracold

^{41}

K-Cs mixtures provides a starting point for Feshbach-resonance and photoassociation spectroscopy of this previously unexplored isotopologue. Such measurements are essential for identifying suitable pathways for magnetoassociation and coherent optical transfer, and ultimately for the production of ultracold ground-state

^{41}

KCs molecules.

We investigate the sub-Doppler laser cooling of bosonic potassium isotopes, whose small hyperfine splitting has so far prevented cooling below the Doppler temperature. We find instead that the combination of a dark optical molasses scheme that naturally arises in this kind of system and an adiabatic ramping of the laser parameters allows us to reach sub-Doppler temperatures for small laser detunings. We demonstrate temperatures as low as $25\ifmmode\pm\else\textpm\fi{}3$ $\ensuremath{\mu}$K and $47\ifmmode\pm\else\textpm\fi{}5$ $\ensuremath{\mu}$K in high-density samples of the two isotopes

{}^{39}

K and

{}^{41}

K, respectively. Our findings should find application to other atomic systems.