Diffuse Laser Cooling Based on the $6\mathrm{P}_{3/2}$ Excited State of Rubidium Atoms via 420 nm Blue Light

TL;DR

This paper reports the first direct demonstration of diffuse laser cooling of $^{87}\mathrm{Rb}$ using 420 nm light addressing the $5\mathrm{S}_{1/2}\rightarrow 6\mathrm{P}_{3/2}$ transition, achieving a meter-scale cold-atom cloud with $N\approx 4.4\times10^{7}$. By constructing a high-power 420 nm source and employing an isotropic diffuse light field in a 1 m cell, the authors show that blue-light cooling can directly prepare atoms in the high excited state, enabling potential gain for continuous cold-atom active optical clocks. Theoretical analysis highlights the narrow linewidth and corresponding Doppler limit, while experiments reveal detuning- and power-dependent optimization with 420 nm outperforming 780 nm in certain regimes. This approach provides a compact route for simultaneous cooling and pumping, with implications for Rydberg experiments, quantum information, and precision timekeeping.

Abstract

To date, the laser cooling of rubidium atoms has inevitably relied on 780 nm cooling light corresponding to the first excited state $5\mathrm{P}_{3/2}$. Surprisingly, we demonstrate laser cooling directly utilizing 420 nm blue light for active optical clock, which corresponds to the high excited state $6\mathrm{P}_{3/2}$ of Rb atom. Experimentally, we successfully apply the 420 nm diffuse laser cooling technique to prepare a cold $^{87}\mathrm{Rb}$ atomic cloud with a length of up to one meter, and measure the cold-atom absorption spectroscopy. The cold atom number is approximately $4.4\times10^{7}$. We systematically compare the cooling effects of 420 nm and 780 nm diffuse laser cooling, and verify the feasibility of blue light cooling using high excited state. This work directly employs blue light to cool and manipulate ground-state Rb atoms to the 6P excited state, providing a new and efficient approach for the cold-atom active optical clock. It is also expected to open up research directions and application prospects in frontier fields such as Rydberg atoms, ultracold quantum gases Bose-Einstein condensation, quantum information, and so on.

Figures (7)

• Figure 1: (a) Diagram of 420 nm blue light diffuse laser cooling, which produced a cold rubidium atom cloud up to one meter in length. (b) Relevant energy levels involved in 420 nm blue light cooling. The cooling light has a certain red detuning relative to the 420 nm $\mathrm{5S_{1/2}}(F=2)\rightarrow\mathrm{6P_{3/2}}(F^{\prime}=3)$ transition. The repumping light corresponds to the 780 nm $\mathrm{5S_{1/2}}(F=1)\rightarrow\mathrm{5P_{3/2}}(F^{\prime}=2)$ transition and the probe light corresponds to the 780 nm $\mathrm{5S_{1/2}}(F=2)\rightarrow\mathrm{5P_{3/2}}$ transition. Simultaneously, the 420 nm laser cooling light also functions as a pumping light, creating a population inversion between the $\mathrm{6S_{1/2}}$ and $\mathrm{5P_{3/2}}$, $\mathrm{5P_{1/2}}$ energy levels, which facilitates the realization of lasing at 1367 nm and 1323 nm for the cold-atom active optical clock.
• Figure 2: In the calculation, the saturation coefficient $s_{0}$ is set to 1, and the one-dimensional optical molasses model is used to calculate the variation of the scattering force with the velocity of $^{87}\mathrm{Rb}$ atoms under different detuning $\delta$ conditions. (a) 780 nm laser as cooling light; (b) 420 nm laser as cooling light. When both the 780 nm cooling light and the 420 nm cooling light are at saturation intensity, with the same detuning, the velocity of the atomic group acted on by the 780 nm cooling light is greater than that of the 420 nm cooling light.
• Figure 3: Experimental system. (a) The 420 nm laser undergoes frequency shifting before frequency locking, resulting in a 420 nm cooling light with a specific red detuning relative to the $\mathrm{5S_{1/2}}(F=2)\rightarrow\mathrm{6P_{3/2}}(F^{\prime}=3)$ transition. The 780 nm repumping light is locked to the $\mathrm{5S_{1/2}}(F=1)\rightarrow\mathrm{5P_{3/2}}(F^{\prime}=2)$ transition, while the probe light corresponds to the 780 nm $\mathrm{5S_{1/2}}(F=2)\rightarrow\mathrm{5P_{3/2}}$ transition. (b) Relevant energy levels. (c) Schematic illustration of the principle of diffuse reflection cooling.
• Figure 4: (a) Photograph of the vacuum atomic cell. Under the cooling effect of the diffuse light field, a cold $^{87}\mathrm{Rb}$ atomic cloud with a length of up to one meter is formed. (b) Absorption spectroscopy of cold $^{87}\mathrm{Rb}$ atoms for the 780 nm probe light (blue curve). The red curve serves as the reference rubidium saturated absorption spectroscopy. Obvious cold atomic absorption can be observed at the position corresponding to the $^{87}\mathrm{Rb}$$\mathrm{5S_{1/2}}(F=2)\rightarrow\mathrm{5P_{3/2}}(F^{\prime}=3)$ transition.
• Figure 5: When a 780 nm laser is used as the cooling light, the absorption spectroscopy of cold $^{87}\mathrm{Rb}$ atoms for the 780 nm probe light is shown as the blue spectral line. The red spectral line is the saturated absorption spectroscopy of rubidium atoms used as a reference. (a) It contains all spectral lines of the 780 nm transitions of $^{85}\mathrm{Rb}$ and $^{87}\mathrm{Rb}$, with $^{87}\mathrm{Rb}$ atoms cooled to the $\mathrm{5S_{1/2}}(F=2)$ state. (b) For the expanded spectral lines, obvious cold atom absorption can be observed at the position corresponding to the $^{87}\mathrm{Rb}$$\mathrm{5S_{1/2}}(F=2)\rightarrow\mathrm{5P_{3/2}}(F^{\prime}=1,2,3)$.