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Review of Scientific Instruments

An enriched K40 source for fermionic atom studies

Brian DeMarco, H. Rohner, D. S. Jin

Published Apr 1, 1999• 42 citations
Publisher

Abstract

We have developed a novel, enriched potassium source for use in trapping and cooling of the fermionic isotope K40. The source is clean, easily controllable, and consistent with high vacuum applications. With this enriched source, we are able to collect 108 K40 atoms in a vapor cell magneto-optical trap.
DOI
10.1063/1.1149695

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We have trapped 40K{}^{40}\mathrm{K}40K 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/cm3{\mathrm{cm}}^{3}cm3. 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 40K{}^{40}\mathrm{K}40K. We discuss the interest of these results in view of possible experiments to study a degenerate Fermi gas.
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.
Laser cooling and trapping of rare, radioactive isotopes has the potential for greatly enhancing experiments in high resolution spectroscopy of exotic species, electron electric-dipole moment searches, β-decay asymmetry, and parity nonconservation. The ability to hold atoms for several seconds at high densities (1011 cm-3) and away from the perturbing effects of the cell walls makes laser cooling and trapping attractive for use in the experiments mentioned above. Because many isotopes of interest for these experiments are difficult to produce, the ability to quickly and efficiently collect these atoms from a dilute vapor is crucial. We have demonstrated a 6% collection efficiency in a special wall-coated cell. We have developed a model that predicts the capture efficiency as a function of trap properties and have developed techniques for measuring the pertinent characteristics.
40 K is one of the most important atomic species for ultra-cold atomic physics. Due to the extremely low concentration (0.012%) of 40 K in natural abundance of potassium, most experiments use 4–10% enriched potassium source, which have greatly suffered from the extremely low annual production and significant price hikes in recent years. Using naturally abundant potassium source, we capture 5.4×10 6 cold 40 K atoms with the help of a high performance of two-dimensional magneto-optical trap (2D + MOT), which is almost three orders of magnitude greater than previous results without the 2D + MOT. The number of the 40 K atoms is sufficient for most ultra-cold 40 K experiments, and our approach provides an ideal alternative for the field.