Extremely Low Frequency Sensing Using a Rydberg Beam

We perform low frequency electric field sensing using field ionization of Rydberg atoms in a collimated atomic beam. By measuring modulations of an ion signal, we are able to detect a DC Stark shift from external electric fields. Using this approach, we are able to measure electric fields with frequencies as low as 1 Hz, with a spectral noise floor of 0.15 mV/m√Hz. We discuss the fundamental limitations of this approach and prospects for future improvements in sensitivity.