Simple, narrow, and robust atomic frequency reference at 993 nm exploiting the rubidium (Rb) 5S to 6S transition using one-color two-photon excitation.

We experimentally demonstrate a one-color two-photon transition from the 5S ground state to the 6S excited state in rubidium (Rb) vapor using a continuous wave laser at 993 nm. The Rb vapor contains both isotopes (Rb and Rb) in their natural abundances. The electric dipole-allowed transitions are characterized by varying the power and polarization of the excitation laser. Since the optical setup is relatively simple, and the energies of the allowed levels are impervious to stray magnetic fields, this is an attractive choice for a frequency reference at 993 nm, with possible applications in precision measurements and quantum information processing.