Analyzing the optical pumping on the $5s4d\,{}^1D_2-5s8p\,{}^1P_1$ transition in a magneto-optical trap of Sr atoms

TL;DR

This study investigates optical pumping in a strontium MOT, demonstrating significant atom number enhancement and identifying decay pathways and loss mechanisms, which inform improved cooling and imaging techniques for quantum applications.

Contribution

It provides the first experimental demonstration of the effectiveness of 448 nm pumping light in suppressing atom escape in Sr MOTs and quantifies relevant decay rates.

Findings

Atom number increased by a factor of 12 with repumping.

Decay pathways bypassing the $5s4d\,{}^1D_2$ state account for 8% of atom loss.

448 nm pumping light suppresses atom escape in small-diameter traps.

Abstract

We explore the efficacy of optical pumping on the $5s4d\,{}^1D_2 - 5s8p\,{}^1P_1$ ($448\,\mathrm{nm}$) transition in a magneto-optical trap (MOT) of Sr atoms. The number of trapped atoms is enhanced by a factor of $12.0(6)$ relative to the case without repumping light, which is six times as large as that obtained using the pumping transition $5s4d\,{}^1D_2 - 5s6p\,{}^1P_1$ ($717\,\mathrm{nm}$). This enhancement is limited by decay pathways that bypass the $5s4d\,{}^1D_2$ state, namely $5s5p\,{}^1P_1 \to 5s4d\,{}^3D_1 \to 5s5p\,{}^3P_0$ and $5s5p\,{}^1P_1 \to 5s4d\,{}^3D_2 \to 5s5p\,{}^3P_2$, which account for 8% of the total loss of the trapped atoms. We determine the decay rates for the $5s5p\,{}^1P_1 \to 5s4d\,{}^3D_1$ and $5s5p\,{}^1P_1 \to 5s4d\,{}^3D_2$ transitions to be $66(6)\,\mathrm{s^{-1}}$ and $2.4(2)\times10^2\,\mathrm{s^{-1}}$, respectively. Furthermore, we experimentally demonstrate for the first time that, when the trap beam diameter is small, escape of atoms in the $5s4d\,{}^1D_2$ state, which has a relatively long lifetime of $400\,\mathrm{\mu s}$, becomes a dominant loss mechanism, and that the $448\,\mathrm{nm}$ pumping light effectively suppresses this escape. Our findings will contribute to improved laser cooling and fluorescence imaging in cold strontium atom platforms, such as quantum computers based on optical tweezer arrays.