Figure 1.

Schematic representation of a coherently pumped degenerate three-level atom in a cascade configuration. The transitions from |a⟩ → |b⟩ and from |b⟩ → |c⟩ at frequency ω_a are taken to be resonant with the cavity, whereas the transition from |a⟩ → |c⟩ is dipole forbidden. However, the transition from |c⟩ → |a⟩ is induced by pumping the atoms externally with a resonant radiation of frequency 2ω_a. Moreover, biased noise fluctuations enter the cavity via one of the coupler mirrors.