Three-photon electromagnetically induced transparency using Rydberg   states
  We demonstrate electromagnetically induced transparency (EIT) in a four-level cascade system where the upper level is a Rydberg state. The observed spectral features are sub-Doppler and can be enhanced due to the compensation of Doppler shifts with AC Stark shifts. A theoretical description of the system is developed which agrees well with the experimental results and an expression for the optimum parameters is derived. 
Christopher Carr, Monsit Tanasittikosol, Armen Sargsyan, David   Sarkisyan, Charles S. Adams and Kevin J. Weatherill
physics.atom-ph