Rabi frequency $\Omega_1$, couples to the transition ${\left|1\right\rangle} \leftrightarrow {\left|2\right\rangle}$, while a second classical pump field, with frequency $\omega_4$ and Rabi frequency $\Omega_4$, couples to the transition ${\left|4\right\rangle} \leftrightarrow {\left|5\right\rangle}$ (see Fig. \[fig:5levelscheme\]). We consider a cylindrical, quasi-1D, atomic medium with the two classical pump beams propagating along its axis, collinear with the two quantum fields in order to avoid Doppler broadening. When the probe field is on two-photon resonance with the pump field with Rabi frequency $\Omega_1$, and the trigger field is on two-photon resonance with the pump field with Rabi frequency $\Omega_4$, the system exhibits EIT for probe and trigger simultaneously. This simultaneous EIT condition is achieved when $$\label{eq:twophotres}
\delta_1=\delta_2, \;\;\;\;\;\;\;\;\;\; \delta_3=\delta_4,$$ where the detunings $\delta_j$ are defined by

\[eq:det2\] $$\begin{aligned}
E_{2}-E_{1}&=&\hbar\omega_{1}+\hbar\delta_{1},\\
E_{2}-E_{3}&=&\hbar\omega_{p}+\hbar\delta_{2}, \\
E_{4}-E_{3}&=&\hbar\omega_{t}+\hbar\delta_{3}, \\
E_{4}-E_{5}&=&\hbar\omega_{4}+\hbar\delta_{4}.\end{aligned}$$

A nonzero CPS occurs whenever a nonlinear cross-phase modulation (XPM) between probe and trigger is present. This cross-Kerr interaction takes place if the two-photon resonance condition is violated. For small frequency mismatch $\epsilon_{12}=\delta_1-\delta_2$ and $\epsilon_{34}=\delta_3-\delta_4$ (both chosen to be within the EIT window), absorption remains negligible and the cross-Kerr interaction between probe and trigger photons may be strong. The consequent CPS may become large, of the order of $\pi$, if the 