Two-color magneto-optical trapping of ytterbium atoms

TL;DR

This work addresses the need for fast loading and robust cooling of neutral ytterbium atoms by implementing a two-color MOT that combines the broad $^1S_0\rightarrow{}^1P_1$ and narrow $^1S_0\rightarrow{}^3P_1$ transitions. The authors systematically study loading and loss dynamics, revealing a shielding mechanism where population redistributed to $^3P_1$ suppresses one-body losses, enabling higher steady-state atom numbers and faster capture. The approach yields a rapid, high-number MOT with strong shielding benefits and demonstrates comparable applicability to other alkaline-earth(-like) atoms for scalable quantum experiments. These results provide a practical pathway to prepare large, cold atom samples for quantum simulation, computing, and precision measurements using alkaline-earth-like species.

Abstract

We report laser cooling and trapping of ytterbium atoms in a two-color magneto-optical trap (MOT). Benefited from both the broad singlet transition ($^1\text{S}_0\rightarrow {}^1\text{P}_1$) and the narrow intercombination transition ($^1\text{S}_0\rightarrow {}^3\text{P}_1$) of ytterbium atoms, the two-color MOT enables rapid loading and efficient cooling. We systematically investigate the shielding effect of the intercombination transition by examining the atom loading and loss rates of single-color and two-color MOTs. Our findings are general and can be extended to other alkaline earth(-like) atoms.

Figures (5)

• Figure 1: Experimental setup. Schematic of the experimental apparatus includes (from top left to bottom right) an oven chamber, a Zeeman slower, a magneto-optical trap (MOT) and a glass cell. The two-color MOT consists of three pairs of blue and green beams (indicated by arrows) and a pair of anti-Helmholtz coils. The slower light is shined onto the atomic flux through the bottom right viewport.
• Figure 2: Zeeman slower. (a) The Zeeman slower consists of an inner tube, water-cooled base coils, main coils and water-cooled boost coils. (b) The measured magnetic field (open circles) as a function of position agrees well with the simulation (solid line). Here, the test currents of the main and the boost coils are set to $3.6\,$A and $43\,$A, respectively. Errorbars represent the uncertainty of the measurement. The Zeeman slower is designed for slowing atoms at a temperature of $390\,^\circ$C, with the slower light at $\sim20\,$mW and $756\,$MHz red-detuned from the $^1\text{S}_0\rightarrow {}^1\text{P}_1$ transition.
• Figure 3: Shielding enhancement. (a) Atom loading in the blue MOT ($^1\text{S}_0\rightarrow {}^1\text{P}_1$) is enhanced by two pairs of horizontal green beams ($^1\text{S}_0\rightarrow {}^3\text{P}_1$). After the blue MOT reaches saturation, the green light is turned on from $t_\text{MOT}=1$ to $6\,$s. In contrast to the steady-state MOT number, the initial loading rate is less sensitive to the detuning of the green light. The power per beam is $22.5\,$mW. The dashed lines represent fits to the data using exponential growth and decay curves. (b) The saturated MOT number is probed as a function of the detuning of the green light. (c) The enhancement of the green light is saturated at around 20$\,$mW per beam. Here the detuning is set to $+2\,$MHz.
• Figure 4: MOT loading under different transitions and configurations. (a) The two-color MOT setup and the energy level diagram of $^{174}$Yb atoms. MOT beams indicated by arrows are aligned together and displaced for clarity. The singlet transition $^1\text{S}_0\rightarrow {}^1\text{P}_1$ is not closed. Atoms in the ${}^1\text{P}_1$ state can decay to $^3\text{D}_j (j=1,2)$ states and further to $^3\text{P}_j (j=0,1,2)$ states, before either escaping or returning to the $^1\text{S}_0$ state. (b,c) are MOT loading of the blue MOT and green MOT probed as a function of the slower current $I_{\text{s}}$ and detuning $\delta_{\text{s}}$ from the resonant singlet transition. The power of the Zeeman slower beam is $23\,$mW. (d,e) are the two-color MOT with the power of the slower light of $18\,$mW and $23\,$mW, respectively. The MOT load time is $5\,$s in (b,d,e) and $80\,$s (c).
• Figure 5: MOT loading and holding curves. (a) Atom numbers are measured as a function of loading time for different MOTs. The two-color MOT (cyan squares) benefited from rapid loading of the blue MOT (blue circles) and further cooling of the green MOT (green triangles), has an even higher steady-state number. Errorbars represent the standard deviation over five repetitions. The dashed and solid lines represent fits considering only one-body loss and accounting for two-body loss, respectively. (b) The atom number decreases over time in the MOT after the atomic flux is switched off. Compared with the one-body loss model (dashed lines), the fit that accounts for two-body loss (solid lines) agrees better with the data. The inset presents a complete log-log plot.