Spectral Measurements of HF and VHF Radio Frequency Electric Fields Using Rydberg Rabi Matching

Rydberg atoms are useful as calibration-free sensors of electric field amplitude when target radio frequencies are resonant to a Rydberg-Rydberg transition. However, detecting sub-GHz carriers remains challenging. In this work, we generate atomic quasi-energy sidebands using a field in the range of 3 MHz to 300 MHz, the HF and VHF bands, then probe these states using a Rydberg-resonant microwave field to controllably split Rydberg states. By matching the resonant microwave field’s Rabi frequency to the HF/VHF field’s frequency, we convert Townes-Merritt sideband transmission peaks into a secondary line splitting within each Autler-Townes peak that grows non-linearly with HF/VHF field amplitude. This experiment demonstrates reception of meters-long radio waves in a subwavelength centimeter-sized vapor cell and reception of amplitude-modulated (AM) information on MHz-range carriers, useful for long-range reception. We present theory using only two levels and effective polarizabilities, which predicts low field effects well, but is insufficient for the state mixing evident with higher fields approaching 1 V/cm. We compare our sensor to other similar designs, noting that this scheme applies an inherent band-pass filter for the carrier frequency, which is automatically downconverted by the atoms.