one has to explore the non-perturbative regime of larger couplings in order to exploit the low-noise, large-nonlinearity properties of EIT and achieve a satisfactory QPG operation.

![image](CPS50.eps)![image](Fshort.eps)

Steady-State QPG Operation {#sec:SSOperation}
==========================

A large amount of work exploring EIT-based nonlinear optical phenomena considers the steady-state of a generic EIT-based system as being the natural state in which to predict and test different phenomena [@Schmidt96; @Ottaviani03; @Matsko03; @Wang01]. We shall see in this Section that it is not possible to achieve a satisfactory QPG performance in such a steady-state regime.

In this Section, we analyze the performance of the QPG at the steady state. To this end, we consider two different parameter regimes: *(i)* the regime of long interaction times, a natural extension of the perturbation analysis of Sec. \[sec:perturbative\], and *(ii)* the regime of short interaction times, corresponding to a non-perturbative regime with strong atom-field coupling.

Long Interaction Time {#sec:SSLong}
---------------------

Naturally extending the perturbative analysis, we solve the master equation (\[eq:mastercoll2\]), and show the results in Fig. \[fig:CPSlong\]. Fig. \[fig:CPSlong\] (*left*) shows the result for the conditional phase shift. Solid line has been calculated from the solution of Eq. (\[eq:mastercoll2\]), as explained in Sec. \[sec:model\]. The dashed line is 