Fast and high-yield loading of a $D_2$ magneto-optical trap of potassium from a cryogenic buffer-gas beam

We demonstrate the direct loading of a ${\mathrm{D}}_2$ magneto-optical trap (MOT) of potassium-39 atoms from a cryogenic buffer gas beam source. We load ${10}^8$ atoms in a 10 ms pulse, with no degradation in performance up to a 10 Hz repetition rate. Observed densities reach $\approx {10}^{11}$ atoms/${\mathrm{cm}}^3$ in a single pulse, achieved with a modest Zeeman slower but no sub-Doppler cooling or transverse compression. This system produces an ideal starting point for ultracold atom experiments where high experimental repetition rates are desirable and initial high densities are critical. Extension to other atomic species (e.g., refractory metals) that present technical challenges to high-yield production using oven-based sources is straightforward.

Figure 1

Experimental schematic and relevant laser frequencies. (a) Potassium energy level diagram depicting the ${\mathrm{D}}_2$ transition of ${}^{39}\mathrm{K}$ employed in this paper. Both the cycling and the repumping transitions were equally detuned from resonance by $\delta =-14$ MHz. Energy shifts in parentheses are given relative to the idealized case of vanishing hyperfine constants. (b) Schematic of the experiment depicting the CBGB source, the Zeeman slower, and the MOT (not to scale). The in-vacuum shutter is placed on the room-temperature plate of the beam source vacuum chamber.

Figure 2

Forward velocity and MOT fluorescence. (a) Potassium beam forward velocity measured at the MOT location with the Zeeman slower off. Peak velocity from a Gaussian fit to the data is 182 m/s, with negligible population at and below the MOT capture velocity of 67 m/s. Velocities beyond 250 m/s are not shown due to spurious signal in the velocity profile arising from the ground-state hyperfine structure. (b) PMT trace of an unperturbed atomic beam (i.e., with Zeeman slower and MOT magnetic fields off) arriving at the MOT location is shown in red and the MOT trace (with Zeeman slower and MOT magnetic fields on) is shown in black. The MOT trace is fit (dashed green line) to the theoretical profile given in Eq. (3). We have fully loaded the MOT by 12 ms after ablation and measure a MOT lifetime of 116.4(4) ms.

Figure 3

MOT absorption images and temperature. (a) Optical density from absorption images of the MOT after 910 $\mu \mathrm{s}$ time of flight and averaged over 36 shots. (b) Fit of absorption image to a two-dimensional Gaussian function. (c) Time-of-flight trace of the $1\sigma$ radius obtained from the Gaussian fits in each direction (horizontal in yellow circles, vertical in green squares), with best-fit curves shown for the model ${\sigma }^2\left(t\right)={\sigma }_0^2+k_{B}T{t}^2/m$.

Figure 4

MOT fluorescence as a function of repetition rate. For clarity, PMT traces are shown offset in time only for 2 (black, solid), 6 (dashed, blue), and 10 (dot-dashed, green) Hz repetition rates. Peak fluorescence signals (red diamonds), proportional to peak MOT densities, are indicated for 1, 2, 4, 6, 8, and 10 Hz. Fluctuations of $\approx 10\%$ in peak signal are typical of ablation yields in CBGB sources. At the fastest repetition rates, due to the short experimental cycle time (at minimum, 100 ms), the tail of the MOT decay curve is not recorded. Transient spikes near the MOT turn-on time are due to scattered light arising from switching lasers; signal before these transients is set to 0 for readability.