Comparative study of the transient evolution of Hanle EIT/EIA resonances

The temporal evolutions of coherent resonances corresponding to electromagnetically induced transparency (EIT) and absorption (EIA) were observed in a Hanle absorption experiment carried on the $D_{2}$ lines of $% ^{87}$Rb vapor by suddenly turning the magnetic field on or off. The main features of the experimental observations are well reproduced by a theoretical model based on Bloch equation where the atomic level degeneracy has been fully accounted for. Similar (opposite phase) evolutions were observed at low optical field intensities for Hanle/EIT or Hanle/EIA resonances. Unlike the Hanle/EIA\ transients which are increasingly shorter for driving field intensities approaching saturation, the $B\neq 0$ transient of the Hanle/EIT signal at large driving field intensities present a long decay time approaching the atomic transit time. Such counterintuitive behavior is interpreted as a consequence of the Zeno effect.