Figure 2

Four-level QD with the two upper levels |c〉 and |b〉 which are near one another, and two lower levels |a〉 and |d〉. Here, ω is the probe field frequency while ω_ca and ω_ba are the transition frequencies.Γ_c and Γ_b are the decay rates from the exited states to the |a〉 state. The decay rate from |a〉 to |d〉 is given as Γ_a, whereas Δ_c = ω_ca − ω and Δ _b = ω − ω_ba are the detunings of the atomic transition energies.δ _k is the detuning of the probe field, which has a central frequency at the middle point of the two upper levels