Rydberg states of atoms and molecules are very sensitive to electric fields. This property makes them ideal electric-field sensors but is detrimental to applications of Rydberg states in quantum optics, quantum-information processing, and quantum simulation because of inhomogeneous Stark broadening and the resulting loss of quantum coherence. We demonstrate, with the example of Rydberg states of
39K, the existence of Rydberg-Rydberg transitions with extremely small differential dc Stark shifts, which we call dc-field-magic Rydberg-Rydberg transitions. These transitions hardly exhibit any Stark broadening, even when the electric-field strength varies by as much as $10\phantom{\rule{4pt}{0ex}}\mathrm{V}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ over the experimental volume. We present a systematic study of dc-field-magic Rydberg-Rydberg transitions combining experiment and calculations and classify them in three main types, which should also be encountered in the other alkali-metal atoms, in alkaline-earth-metal atoms, and even in molecules. The observed insensitivity to dc electric fields does not reduce the interactions between Rydberg atoms, even if they are dominantly electric dipole-dipole in nature. Rydberg states coupled by dc-field-magic Rydberg-Rydberg transitions, therefore, have great potential as qubits.
We examine the influence of a static dc electric field on Electromagnetically Induced Transparency (EIT) resonances that involve highly excited Rydberg states. Our focus is on how these resonances are altered when the relative orientation between the laser polarization and the external electric field vectors are varied. We experimentally demonstrate characteristic variations in the amplitude of the Stark-split EIT resonances, which can be explained by the selection rules in various geometries. We also present a simplified semi-analytical model that closely resembles the experimental observations. We use these findings to obtain information about the spatially inhomogeneous electric field, produced by a biased wire, using EIT fluorescence measurements that agrees with the expected angular dependencies. These results suggest that simultaneous analysis of frequency shifts and amplitudes of Rydberg EIT resonances may enable vector electrometry of electrostatic fields, necessary for many quantum sensing applications.