Realization of a Rydberg-dressed atomic clock.

We present the experimental realization and characterization of an atomic clock 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. We show that the observed Ramsey interference signals can be used to precisely measure the Rydberg atom-light coupling strength as well as the population and coherence decay rates of the Rydberg-dressed states with sub-kilohertz accuracy and for Rydberg state fractions as small as one part in 10^6. This provides the means to combine the outstanding coherence properties of atomic clocks with controllable coupling to strongly interacting states, thus expanding the number of systems suitable for metrological applications and many-body physics studies.