Large Cross-phase Modulation Based on Double EIT in a Four-level Tripod   Atomic System
  We report the experimental observations on the simultaneous EIT effects for probe and trigger fields (double EIT) as well as the large cross-phase modulation (XPM) between the two fields in a four-level tripod EIT system of the D1 line of 87Rb atoms. The XPM coefficients (larger than 2*10-5cm2/W) and the accompanying transmissions (higher than 60%) are measured at slightly detuning of the probe field from the exact EIT resonance condition. The presented system can be applied in the recently proposed quantum information processing with weak cross-Kerr nonlinearities. 
Shujing Li, Xudong Yang, Xuemin Cao, Chunhong Zhang, Changde Xie, Hai   Wang