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arXiv:2511.16220 · Physical Review Research

Diffuse laser cooling based on the 6 P 3 / 2 excited state of rubidium atoms via 420 nm blue light

Jia Zhang, Xun Gao, Zheng Xiao, Xiaolei Guan, Ruihang Chen, 韩梦媛 Han Mengyuan, Tiantian Shi, Jingbiao Chen

Published Aug 18, 2026• 0 citations

Abstract

To date, the laser cooling of rubidium atoms has inevitably relied on 780 nm cooling light corresponding to the first excited state 5 P 3 / 2 . Here, we demonstrate diffuse laser cooling of Rb 87 atoms using the 420 nm 5 S 1 / 2 → 6 P 3 / 2 transition without a preceding 780 nm cooling stage. A high-power 420 nm laser is used as the cooling light to produce cold atoms in a 1-m-long diffuse-cooling cell. Absorption spectroscopy yields a cold-atom density of approximately 1.4 × 10 11 m − 3 . We compare the cooling performance of the 420 nm and conventional 780 nm diffuse-cooling schemes and verify the feasibility of using the high-excited-state transition as an independent cooling channel in this large-volume diffuse-cooling system. This approach provides an alternative cooling/pumping pathway for continuous cold-atom active optical clocks, where compatibility with the clock-lasing process and reduced cooling-light-induced perturbations are important. It is also expected to open up research directions and application prospects in frontier fields such as Rydberg atoms, quantum information, and so on.

DOI: 10.1103/4jtk-471b

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To date, the laser cooling of rubidium atoms has inevitably relied on 780 nm cooling light corresponding to the first excited state 5 P 3 / 2 . Here, we demonstrate diffuse laser cooling of Rb 87 atoms using the 420 nm 5 S 1 / 2 → 6 P 3 / 2 transition without a preceding 780 nm cooling stage. A high-power 420 nm laser is used as the cooling light to produce cold atoms in a 1-m-long diffuse-cooling cell. Absorption spectroscopy yields a cold-atom density of approximately 1.4 × 10 11 m − 3 . We compare the cooling performance of the 420 nm and conventional 780 nm diffuse-cooling schemes and verify the feasibility of using the high-excited-state transition as an independent cooling channel in this large-volume diffuse-cooling system. This approach provides an alternative cooling/pumping pathway for continuous cold-atom active optical clocks, where compatibility with the clock-lasing process and reduced cooling-light-induced perturbations are important. It is also expected to open up research directions and application prospects in frontier fields such as Rydberg atoms, quantum information, and so on.

Using modulation transfer spectroscopy, we achieve the frequency stabilization of a high-power 3-W blue laser at the wavelength of 420 nm to the

\mathrm{Rb}

5{S}_{1/2}

--

6{P}_{3/2}

transition. The in-loop frequency stability of this laser is $1.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}/\sqrt{\ensuremath{\tau}}$, reaching $1.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ at 100 s and $7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}$ at 1000 s. An external power feedback loop is established using an acousto-optic modulator, employing the zeroth-order diffracted light for power stabilization, achieving an in-loop power stability of $1.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ at 1 s. Moreover, the continuous mode-hop free interval of this high-power laser can simultaneously cover the

5{S}_{1/2}

--

6{P}_{3/2}

transitions of

{}^{85}\text{Rb}

and

{}^{87}\text{Rb}

, with successful locking achieved for both isotopes, providing a comprehensive analysis of the

\mathrm{Rb}

atomic transitions in the blue spectral region. As an application, this 3-W 420-nm laser with excellent power and frequency stabilities is used as a repumping source for diffuse laser cooling of

{}^{87}\text{Rb}

atoms, realizing a one-meter-long cold-atom cloud. This paves the way for using blue-light cooling to realize a cold-

\mathrm{Rb}

-atom active optical clock. In addition, the ultrahigh-stability 420-nm laser also finds applications in various fields such as the Rydberg atom experiment, Bose-Einstein condensation, ultranarrow-bandwidth Faraday atomic filter, and so on.

The cavity-pulling suppression advantage of active optical clocks (AOCs) helps to realize atomic clocks with superior performance. However, realization of continuous-wave cold-atom AOCs is a highly challenging task. In this work, we proposed an AOC scheme with diffuse-laser-cooled atoms. Combining the advantages of diffuse laser cooling to easily prepare cold-atom cloud with large volume on the scale of meters and the four-level structure to greatly reduce the light shift, it is expected to realize the continuous output of AOC superradiant signals. We experimentally realized a 100-cm-long cold87Rb atom cloud as the gain medium. The 1367 nm AOC superradiant signal with a power of 0.3

\mu \mathrm{W}

has been achieved without resonant cavity feedback. In the future, we will continue to advance the experiment and also conduct research on 420 nm blue light cooling.

In this paper we describe an experiment on laser cooling of

^{87}\mathrm{Rb}

atoms directly from a vapor background in diffuse light. Diffuse light is produced in a ceramic integrating sphere by multiple scattering of two laser beams injected through multimode fibers. A probe beam, whose propagation direction is either horizontal or vertical, is used to detect cold atoms. We measured the absorption spectra of the cold atoms by scanning the frequency of the probe beam, and observed both the absorption signal and the time of flight signal after we switched off the cooling light, from which we estimated the temperature and the number of cold atoms. This method is clearly attractive for building a compact cold atom clock.

We have constructed a simplified setup for the demonstration of laser cooling and trapping of rubidium atoms. An external-cavity diode laser was swept across the F = 3 → F' = 2, 3, 4 resonances of the D 2 -line of 85 Rb at 780 nm to resolve the saturated absorption features. Subsequently, the transition F = 3 → F' = 4 was used to cool rubidium atoms in a roomtemperature vapor cell. A cold cloud of trapped atoms with a diameter of approximately 1 mm was produced. The lifetime of the trap was measured to be 1 second.