Using a two-step optical excitation scheme, we investigated the symmetric transition spectroscopy in rubidium Rydberg microwave electric-field detection. The study employed two microwave sources to achieve cascade coupling of the $78S_{1/2}\to 78P_{3/2}\to 79S_{1/2}$ transitions, with optical excitation states corresponding to $78S_{1/2}$ and $79S_{1/2}$, respectively. Due to the selection-rule-determined symmetry of the doubly microwave-coupled Rydberg levels, we found they maintain excellent symmetry for microwave field detection, despite the fact that the cascade transitions extend in opposite directions, one toward higher energies and the other toward lower energies. With nearly equal electric dipole moments for adjacent microwave transitions, this symmetric system enables microwave field measurements at wider transition frequencies with comparable sensitivity, also allowing dual-frequency channel-hopping measurements with equal gain, particularly advantageous for a simplified frequency-agile detection system, even at the same optical excitation.

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• Atomic & molecular processes in external fields
• Electromagnetically induced transparency
• Rydberg atoms & molecules
• Electric moment
• Precision measurements