Doppler- and recoil-free laser excitation of Rydberg states via   three-photon transitions
  Three-photon laser excitation of Rydberg states by three different laser beams can be arranged in a star-like geometry that simultaneously eliminates the recoil effect and Doppler broadening. Our analytical and numerical calculations for a particular laser excitation scheme 5S_{1/2}->5P_{3/2}->6S_{1/2}->nP in Rb atoms have shown that compared to the one- and two-photon laser excitation this approach provides much narrower line width and longer coherence time for both cold atom samples and hot vapors, if the intermediate one-photon resonances of the three-photon transition are detuned by more than respective single-photon Doppler widths. This method can be used to improve fidelity of Rydberg quantum gates and precision of spectroscopic measurements in Rydberg atoms. 
I. I. Ryabtsev, I. I. Beterov, D. B. Tretyakov, V. M. Entin, and E. A.   Yakshina
physics.atom-ph cond-mat.quant-gas quant-ph