Magnetically tuned single-Zeeman-sublevel Rydberg EIT of a ⁸⁷ Rb 38F state for resonant L-band microwave electric field sensing

We report the Rydberg electromagnetically induced transparency (EIT) spectra of the ⁸⁷Rb 5<i>S</i>_1/2 → 5<i>P</i>_3/2 → 5<i>D</i>_5/2 → 38<i>F</i>_7/2 transition in a vapor cell with a DC magnetic field and demonstrate its use for tunable, resonant L-band microwave electric field detection via the Autler-Townes (AT) splitting. Compared to conventional two-photon Rydberg excitation, three-photon EIT exhibits a markedly richer and more intricate spectral structure resulting from extra transition pathways under a DC magnetic field. Under copropagating linearly-polarized optical fields and a static magnetic field, we resolve as many as eight distinct EIT peaks and extract their linear Zeeman-shift coefficients. The measured Zeeman-shifts are all in agreement with the predictions of a simple analytical equation. More interestingly, we demonstrate a single-Zeeman-sublevel Rydberg excitation using three-photon EIT with circular light fields and a quantization axis. We subsequently exploit single-Zeeman-sublevel Rydberg-EIT at the low principal quantum number <i>n</i> = 38 to perform a tunable, resonant EIT-AT measurement of 1.5 GHz (L-band) microwave electric fields. The observed characteristics confirm that L-band microwave measurement via three-photon excited low-<i>n</i>F Rydberg states is advantageous by avoiding the autoionization and high laser power demands of two-photon excited high-<i>n</i> Rydberg states.