Figure 1

Tripod configuration of the atomic levels and transitions: ${\mathrm{\Omega }}_1,{\mathrm{\Omega }}_2$ are the Rabi frequencies of the driving fields, $g$ is the coupling constant, $\widehat{a}$ is the quantized amplitude of the signal field, $\gamma$ is the relaxation rate from the excited state $|4\rangle$, and $T_W,T_R$ are the writing and readout times. Transitions between the three lower levels are dipole forbidden. Initially only the third level is populated.