High-flux cold lithium-6 and rubidium-87 atoms from compact two-dimensional magneto-optical traps

TL;DR

This work tackles the need for a fast, compact, dual-species Li-6 and Rb-87 atom source suitable for rapid production of ultracold mixtures and molecules. The authors implement in-series 2D MOTs for Li and Rb, augmented by a Li short-distance Zeeman slower, all housed in a compact HV/UHV vacuum system ($55\times65\times70~\mathrm{cm}^3$). They demonstrate a record Li 3D MOT loading rate of $6.6\times10^{9}$ atoms/s at $T_ ext{Li}\approx372^\circ\mathrm{C}$ and a Rubidium flux of $2.3\times10^{9}$ atoms/s at room temperature, with high optical access and decoupled optimization for the two species. These results enable fast, high-flux production of a double-degenerate Li-Rb mixture and pave the way for creating Li-Rb ground-state molecules, while offering a scalable blueprint for other dual-species atom pairs in compact setups.

Abstract

We report a compact setup with in-series two-dimensional magneto-optical traps (2D MOTs) that provides high-flux cold lithium and rubidium atoms. Thanks to the efficient short-distance Zeeman slowing, the maximum 3D MOT loading rate of lithium atoms reaches a record value of $6.6\times 10^{9}$ atoms/s at a moderate lithium-oven temperature of 372 degrees Celsius, which is 44 times higher than that without the Zeeman slowing light. The flux of rubidium is also as high as $2.3\times10^9$ atoms/s with the rubidium oven held at room temperature. Meanwhile, the entire vacuum-chamber system, including an ultra-high-vacuum science cell, is within a small volume of $55\times65\times70~\mathrm{cm}^3$. Our work represents a substantial improvement over traditional bulky and complex dual-species cold-atom setups. It provides a good starting point for the fast production of a double-degenerate lithium-rubidium atomic mixture and large samples of ultracold lithium-rubidium ground-state molecules.

Figures (12)

• Figure 1: Lithium-Rubidium Dual-Species Vacuum Setup. (a): Overview of the vacuum system with laser beams for the Rb 2D MOT (left), the Li 2D MOT (middle), and the dual-species 3D MOT (right) and the short-distance Li Zeeman slower (up). The miniaturized vacuum system has a dimension of $55\times65\times70\ \mathrm{cm}^3$. The vacuum setup sits on a rail system, which allows for 1D translation along the $x$ direction for easy vacuum diagnostics and optics alignment. A gold-coated aluminum mirror (#47-116, Edmund optics) is placed $300\ \mathrm{mm}$ above the center of the octagon chamber to limit the consequences of hot atom flux directly shooting towards the cold viewport and coat the viewport to compromise the transmission. (b): Three-quarter view of the octagon chamber section, where two stages of differential pumping (DP) channels are shown. Four stacks of magnets are attached to the side of the Li octagon chamber in a way that generates a quadruple magnetic field. The center of the magnetic field can be fine-tuned using the translation stages to align the 2D MOT with the center of the DP tube.
• Figure 2: Laser frequency scheme (not to scale). Left: $^6\mathrm{Li}$$D_2$ transition from the $2^2\mathrm{S}_{1/2}$ state to the $2^2\mathrm{P}_{3/2}$ manifold. Red arrows indicate the corresponding laser frequencies, and $\delta^\mathrm{Li}_i$ denote the detunings of the individual beams with respect to the upper manifold. Transition frequencies are taken from gehm2003. Right: $^{87}\mathrm{Rb}$$D_2$ transition from the $5^2\mathrm{S}_{1/2}$ state to the $5^2\mathrm{P}_{3/2}$ state. Red arrows indicate the corresponding laser frequency, and $\delta^\mathrm{Rb}_i$ denote the detunings of the individual beams with respect to the corresponding transition. Transition frequencies are taken from daniela.steck2023.
• Figure 3: Schematic of the lithium and rubidium laser systems. Acousto-optic modulators (AOMs) in double-pass (DP) or single-pass (SP) configurations are used to modulate the laser frequencies. In the Li laser system, a tunable electro-optical modulator (EOM) in the Zeeman-slower beam path generates sidebands at approximately $228\,\mathrm{MHz}$ from the carrier to serve as the repumping beam. A Fabry–Perot cavity monitors the sideband frequency and intensity.
• Figure 4: Simulation of the short-distance Li Zeeman slowing and 2D MOT. (a) Velocity distribution of $10^6$ Li atoms with and without Zeeman slowing. The Zeeman slower reshapes the initial distribution by decelerating atoms in the blue region into the orange peak, bringing them below the 2D MOT capture velocity $v_c$. Inset: velocity deformation caused by Zeeman slowing (figure inspired by lamporesi2013). (b) Numerical simulations of atomic trajectories along the applied magnetic field. The 2D MOT beam region is shown in orange. The light gray area indicates the capture velocity $v_c$ of the 2D MOT.
• Figure 5: Fluorescence measurement of the Li 3D MOT loading process with and without Zeeman-slower enhanced atomic flux. Data were obtained at an oven temperature of $T_\mathrm{Li}=372\,^\circ\mathrm{C}$. Orange solid curve: with Zeeman slower, with a loading rate of $R^\mathrm{Li}_\mathrm{zs}=6.639(25)\times10^9\,\mathrm{atoms/s}$. Gray dashed curve: loading without Zeeman slower, with a loading rate of $R^\mathrm{Li}_\mathrm{no~zs}=1.506(23)\times10^8\,\mathrm{atoms/s}$. The errors given here represent numerical fitting uncertainties. The gain factor of the Li Zeeman slower is $G = R^\mathrm{Li}_\mathrm{zs}/R^\mathrm{Li}_\mathrm{no~zs} \approx 44$. The loading curves are fitted with the MOT loading model in Eq. \ref{['eq:mot_loading']}.