Large Cross-Phase Modulation between Slow Co-propagating Weak Pulses in   Rb
  We propose a scheme to generate double electromagnetically induced transparency and optimal cross-phase modulation for two slow, copropagating pulses with matched group velocities in a single species of atom, namely 87 Rb. A single pump laser is employed and a homogeneous magnetic field is utilized to avoid cancellation effects through the nonlinear Zeeman effect. We suggest a feasible preparational procedure for the atomic initial state to achieve matched group velocities for both signal fields. 
Zeng-Bin Wang, Karl-Peter Marzlin, and Barry C. Sanders