Dark transition states quantum interference

The effect of quantum interference on spontaneous emission in atomic and molecular systems is the generation of superposition states that can be manipulated, to reduce the interaction with the environment, by adjusting the polarizations of the transition dipole moments, or the amplitudes and phases of the external driving fields. With a suitable choice of parameters, the superposition states can decay with controlled and significantly reduced rates. This modification can lead to subnatural linewidths in the fluorescence and absorption spectra [5,10]. Furthermore, as will be shown in this review, the superposition states can even be decoupled from the environment and the population can be trapped in these states without decaying to the lower levels. These states, known as dark or trapped states, were predicted in many configurations of multilevel systems [11], as well as in multiatom systems [12],... 

Thus, in terms of the quantum dressed states, the gain features predicted by Menon and Agarwal [48] actually appear on completely inverted transitions whose dipole moments are canceled by quantum interference. Therefore, the gain features can be regarded as the amplification on dark transitions [51]. 

IY In their case the atom prefers to stay in the transition with the larger decay rate (strong transition) and there is a small probability of finding the system in the other (weak) transition. The extended dark periods, predicted for the V-type atom with almost parallel dipole moments, appear simultaneously on both transitions independent of the decay rates. This indicates that in the presence of quantum interference the atomic states 1) and 3) are not the preferred radiative states of the atom. 

---