We present the experimental realization and characterization of a Ramsey interferometer and electrometer based on optically trapped ultracold potassium atoms, where one state is continuously coupled by an off-resonant laser field to a highly-excited Rydberg state.
Atomic clocks based on trapped ensembles of neutral atoms or single ions have enabled the most precise measurements ever made. Besides their importance for defining future time and frequency standards, they also hold great promise for searches for physics beyond the standard model, exploring the physics of complex quantum systems, and for realizing sensors capable of operating at the fundamental quantum limit. By design however, atomic clocks typically involve the coherent evolution of atoms that interact very weakly, either with one another or with external fields, seemingly precluding many possible applications. Here we demonstrate an alkali atomic clock involving two magnetically insensitive hyperfine ground states, where one state is continuously coupled to a Rydberg state by an off-resonant laser field. This Rydberg-dressing approach provides the means to combine the outstanding coherence properties of atomic clocks with greatly enhanced sensitivity to external fields or controllable interparticle interactions mediated by the Rydberg state admixture. As a proof-of-principle, we use the Rydberg-enhanced interferometer to demonstrate an application for measuring small, static electric fields with high sensitivity. The basic essence of our scheme can be applied to atomic and molecular systems in metrological applications and studies of new phases of matter.