Velocity-selective electromagnetically-induced-transparency measurements of potassium Rydberg states

We demonstrate a velocity selection scheme that mitigates suppression of electromagnetically induced transparency (EIT) by Doppler shifts for coupling wavelengths larger than the probe wavelength. An optical pumping beam counterpropagating with the EIT probe beam transfers atoms between hyperfine states in a velocity-selective fashion. Measurement of the transmitted probe beam synchronous with chopping of the optical pumping beam enables a Doppler-free EIT signal to be detected. Transition frequencies between 5P(1/2) and nS(1/2) states for n = 26, 27, and 28 in K-39 are obtained via EIT spectroscopy in a heated vapor cell with a probe beam stabilized to the 4S(1/2) -> 5P(1/2) transition. Using previous high-resolution measurements of the 4S(1/2) -> nS(1/2) transitions, we make a determination of the absolute frequency of the 4S(1/2) -> 5P(1/2) transition. Our measurement is shifted by 560 MHz from the currently accepted value with a twofold improvement in uncertainty. These measurements will enable novel experiments with Rydberg-dressed ultracold Fermi gases composed of 40 K atoms.