Quantum beat spectroscopy of Rydberg hyperfine structure

Measurements of the hyperfine interval $\nu_{hfs}$ for highly excited Rydberg levels are challenging since the interval scales inversely with principal quantum number $\propto n^{-3}$. In our experiment a sample of ultra-cold Rb atoms is prepared in a state-insensitive lattice to preserve ground-Rydberg atomic coherence. A two-photon 5s-ns transition is used to excite a manifold of nuclear Zeeman states in the Paschen-Back regime with energy splittings $\approx \nu_{hfs}/4$. Time-dependent atomic interference between the nuclear Zeeman states modulates the polarization of the coherently retrieved light field on a $\sim10\mu s$ timescale. The effect of nuclear state dependent light shifts on the observed beat frequency is investigated, and $\nu_{hfs}$ is measured for principal quantum numbers between 30 and 50.