A permanent-magnet Zeeman slower and magneto-optical trap for calcium atoms for ultracold Rydberg physics

TL;DR

This work reports a compact experimental platform for producing ultracold $^{40}$Ca and performing high-Rydberg excitations via a resonant three-photon scheme. A passive Halbach-based permanent-magnet Zeeman slower decelerates an atomic beam from the oven to ~ $35~\mathrm{m\,s^{-1}}$, enabling capture in a $423$ nm MOT, which reaches $T\approx 1.0(3)$ mK and $N\approx 10^{6}$ atoms. Ground-state Ca atoms in the MOT are excited to Rydberg states using a three-photon sequence and detected with a pulsed-field ionization system housed in a segmented electrode stack that also allows Stark switching and stray-field compensation. The demonstrated velocity control, MOT performance, and Rydberg spectroscopy validate the setup as a robust platform for ultracold Ca Rydberg physics with potential extensions to repumping and higher-$\ell$ state manipulation.$

Abstract

We report the construction and characterization of an experimental setup for producing a cold gas of $^{40}$Ca atoms and excite them to high Rydberg states with a resonant three-photon-excitation scheme. The apparatus comprises four stages, each designed in-house. An oven heated to $\sim 500^\circ$C generates an atomic beam that is collimated by a capillary stack. The beam is sent into a passive, permanent-magnet-based Zeeman slower that reduces the atomic velocity to $30$ m/s. The slow atoms are captured in a magneto-optical trap (MOT) and cooled to $1.0(3)$ mK with a trapping time of $16(2)$ ms. Ground-state atoms in the cold gas are excited to high Rydberg states via resonant excitation through the intermediate $4s4p\, ^1P_1$ and $4s4d\, ^1D_2$ states. The MOT is operated at the center of an electrode stack, which serves to apply continuous and pulsed electric fields and field-ionize the Rydberg atoms for detection. We benchmark our MOT against previous implementations and find its performance consistent with state-of-the-art results in terms of temperature and trapping lifetime. Finally, we demonstrate Rydberg spectroscopy of calcium, confirming the system's suitability for ultracold Rydberg physics experiments.

Figures (16)

• Figure 1: Relevant energy levels and transitions of $^{40}$Ca. Transition wavelengths were obtained from Ref. AtomicSpectraDatabase2009. Einstein $A$ coefficients were obtained from Ref. adamczykTwophotonCoolingCalcium2025wilpersAbsoluteFrequencyMeasurement2007.
• Figure 2: Sectional view of the experimental setup, where (1) is the Ca oven, (2) is the permanent-magnet-based Zeeman slower, and (3) is the magneto-optical trap.
• Figure 3: Sectional view of the Ca oven. (1) is the cartridge where the Ca granules are loaded, (2) are the capillary tubes, and (3) is the tantalum wire.
• Figure 4: Optical path schematic. HWP: half-wave plate. QWP: quarter-wave plate. PBS: polarizing beam splitter. PCX: plano-convex lens. PCC: plano-concave lens. AOM: acousto-optic modulator. M: mirror. I: iris. The optical paths $1-4$ correspond to the slowing beam, frequency stabilization, beam diagnosis, and MOT beam, respectively.
• Figure 5: Zeeman slower 3D model. The red and blue segments represent the north and south poles of the magnets, respectively. The black elements are the $3$D-printed holding structure.