Magneto-optical trapping of holmium atoms

We demonstrate sub-Doppler laser cooling and magneto-optical trapping of the rare-earth element holmium. Atoms are loaded from an atomic beam source and captured in six-beam ${\sigma }_{+}-{\sigma }_{-}$ molasses using a strong $J=15/2\leftrightarrow J=17/2$ cycling transition at $\lambda =410.5$ nm. Due to the small difference in hyperfine splittings and Landé $g$ factors in the lower and upper levels of the cooling transition the MOT is self-repumped without additional repump light, and deep sub-Doppler cooling is achieved with the magnetic trap turned on. We measure the leakage out of the cycling transition to metastable states and find a branching ratio $<$${10}^{-5}$, which is adequate for state-resolved measurements on hyperfine encoded qubits.

Figure 1

Energy levels of Ho. The upper levels of the cycling and cooling transitions are labeled 1–7. The diagram on the right shows the cooling, repumper, and slowing light used on the 410.5-nm transition to level $|e\rangle$.

Figure 2

Vacuum and laser cooling setup. The length of the apparatus from end to end is about 1 m. The slowing beam enters the vacuum chamber through a vertical window and is reflected from a Cu mirror to propagate towards the atomic source.

Figure 3

Number of trapped atoms (circles) and peak density (squares) as a function of cooling-light detuning with the slower light turned on and the repump light turned off. The magnetic-field gradient was 0.3 T/m. The insets show an averaged fluorescence image of the trapped atoms from 100 exposures at $-36$ MHz detuning and population decay curves with exponential fits at $-15$ MHz (fast decay) and $-45$ MHz (slow decay).

Figure 4

MOT population as a function of repumper detuning relative to the $F_g=11\to F_e=12$ transition and repumper power, with the slowing beam off. The population is normalized to the value with no repumper. The data stop at 620 MHz, which was the limit of the acousto-optic modulator used for frequency shifting.

Figure 5

MOT temperature with quadrupole magnetic field on vs detuning at saturation parameter $I/I_s=1.1$. The temperatures were extracted from fits to time-of-flight data for free expansion times up to 15 ms. The Doppler temperature is 780 $\mu \mathrm{K}$.