Two-Atom Rydberg Blockade using Direct 6S to nP Excitation
  We explore a single-photon approach to Rydberg state excitation and Rydberg blockade. Using detailed theoretical models, we show the feasibility of direct excitation, predict the effect of background electric fields, and calculate the required interatomic distance to observe Rydberg blockade. We then measure and control the electric field environment to enable coherent control of Rydberg states. With this coherent control, we demonstrate Rydberg blockade of two atoms separated by 6.6(3) {\mu}m. When compared with the more common two-photon excitation method, this single-photon approach is advantageous because it eliminates channels for decoherence through photon scattering and AC Stark shifts from the intermediate state while moderately increasing Doppler sensitivity. 
A. M. Hankin, Y.-Y. Jau, L. P. Parazzoli, C. W. Chou, D. J. Armstrong,   A. J. Landahl, G. W. Biedermann
physics.atom-ph quant-ph