A magneto-optical trap of silver and potassium atoms

Abstract

We demonstrate a dual magneto-optical trap of $^{109}$Ag and $^{39}$K. For silver, a decreasing-field Zeeman slower loads a MOT of $1.5{\times}10^8$ atoms at a temperature of 0.74(5) mK, with laser cooling occurring primarily on the $D_2$ line of $4d^{10}5s\; {}^2S_{1/2}\rightarrow 5p\; {}^2P_{3/2}$ at 328 nm. We create a novel Ag "dark spot MOT," where shelving the atoms in a dark state enhances the captured atom number by a factor of two and the lifetime by a factor of four. For potassium, we obtain $2{\times}10^8$ trapped atoms, and further cooling on the $D_1$ transition via grey molasses results in a cloud of $1.2{\times} 10^8$ atoms at 7(1) $μ$K. We observe evidence of photoionization loss of the K MOT in the presence of Ag laser-cooling light, with implications for optimal dual species loading strategies. Our results on Ag point to simple and general laser cooling strategies for other coinage metals (Au, Cu). Furthermore, this work lays the foundation for the production of alkali-coinage metal degenerate quantum mixtures and highly polar molecules.

Figures (4)

• Figure 1: (a) Illustration of vacuum apparatus, including the Ag effusion oven, Zeeman slower, Ag atom shutter, K 2D MOT, main chamber, magnetic field coils for the MOT, and the heated Ag Zeeman viewport. The inset shows a cross-section of the main chamber with high numerical aperture viewports and another set of magnetic coils for eventual control over Feshbach resonances. (b) Energy level diagrams for $^{39}$K and $^{109}$Ag, with lasers responsible for the MOT and grey molasses indicated as red and blue arrows, respectively. Note Ag’s inverted hyperfine structure.
• Figure 2: Ag slowing and magneto-optical trapping. (a) Magnetic field profile of the Ag Zeeman slower, comprising the simulated profile of 9 segments of the Zeeman solenoid (orange), 2 segments of a "bridge" solenoid (dark red), and the MOT gradient (green). The combined Zeeman field is shown in blue, and the total field is shown in red, emulating the ideal profile (black). The measured fields are shown as circles. (b) Top-down view of beams used for trapping Ag, including the slowing beam (ZS, blue), MOT cooling (pink), and two repump beams with imaged dark spots (yellow, R1 and R2). (c) Fluorescence images of the bright Ag MOT with two unmasked repumping beams, a sub-optimal dark MOT with only one repumper with a dark spot mask, and an optimized dark MOT with two repumpers with intersecting dark spots. (d) Ag atom number captured in the bright (orange circles) and dark MOT (blue squares) as a function of axial magnetic field gradient. Error bars are standard errors of the mean.
• Figure 3: MOT loading rate and lifetimes, showing atom number during loading for (a) Ag bright and dark MOTs in orange circles and blue squares, respectively and (b) K MOT. (c) Lifetime of the Ag MOT measured after turning off the slow beam via the atom shutter, with fitted one-body loss rates of $\alpha{=}1.5(4) {\times}10^{-1}$ /s and $3.6(5) {\times}10^{-2}$ /s and two-body loss rates of $\beta{=}7(1){\times}10^{-11} \: \rm{cm^3/s}$ and $3.5(6) {\times} 10^{-13}\: \rm{cm^3/s}$ for the bright and dark MOTs, respectively (see text). (d) Lifetime of K after turning off the 2D MOT source, both with (green) and without (purple) UV light, leading to one-body rates of $\alpha{=}1.8(1)$ /s and $\alpha{=}4.0(4)\times10^{-2}$ /s, respectively.
• Figure 4: Steady-state Ag population for the bright MOT (orange circles) and dark MOT (blue squares) as a function of (a) cooling intensity, (b) detuning of the cooling beam $\Delta_{\rm C}$, (c) repump intensity, and (d) repump detuning $\Delta_{\rm R}$. As our limited UV power is split between our Zeeman slower and cooling beams, we scan the cooling intensity in (a) with the Zeeman beam at 1/5 of the intensity that optimizes the MOT. For (b-d), the non-scanned parameters were held constant and set to optimize atom number. The MOT gradient was set to 46 G/cm for all measurements.