Effects of quantum interference on coherent population trapping states of a four-level atom interacting with coherent fields

We find that a four-level atom-two-driving field coupled system has four coherent population trapping states. We show that quantum interference between different spontaneous emission pathways from the upper to the lower levels can lead to coherent population trapping states stable against the radiative decay of the upper levels. By adjusting the detuning of the driving fields, we can control the population trapped in the upper levels via quantum interference. We also find that quantum interference can much largely delay the relaxation of the atom to the steady state. We also notice that the quantum interference results in the dependence of the atomic steady state on the initial condition although the interaction of the atom with the vacuum modes is considered.