In the intermediate magnetic field regime, the photon-atom interaction is strongly modified, which can be used to open forbidden transition channels. Using the master equation, we have modeled the cesium $D_2$ optical pumping under different magnetic fields, light intensities, and interaction times. Experimentally, a vacuum cesium atomic beam was employed, and the results show good agreement with the theoretical calculations. Transitions that were originally accessible only via the $|F=3⟩\to |F^{\prime }=3⟩$ and $|F=3⟩\to |F^{\prime }=4⟩$ channels can now be pumped through $|F=3⟩\to |F^{\prime }=2⟩$ and $|F=3⟩\to |F^{\prime }=5⟩$ as well. Based on this effect, we have realized a magneto-optically pumped cesium-beam atomic clock achieving a short-term stability of $1.9\times {10}^{-12}{\tau }^{-1/2}$. This is approximately twice as good as that obtained without optical pumping, using monochromatic light resonant with the $|F=3⟩\to |F^{\prime }=2⟩$ transition employed for both optical pumping and detection. Moreover, this effect is generally applicable to a wide range of systems involving laser cooling and quantum state preparation.

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• Atomic, optical & lattice clocks
• Light-matter interaction
• Magneto-optical spectra
• Time & frequency standards
• Atomic & molecular beams
• Precision measurements
• Pump-probe spectroscopy
• Quantum master equation