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

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 3×105 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 mF=−3/2,−5/2 and −7/2 Zeeman states of the F=7/2 hyperfine ground state confining 5×104 atoms with a lifetime of 0.6 s or ∼103 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.