  Electromagnetically induced transparency (EIT) is observed in a three-level
system composed of an excited state and two coherent superpositions of the two
ground-state levels. This peculiar ground state basis is composed of the
so-called bright and dark states of the same atomic system in a standard
coherent population trapping configuration. The characteristics of EIT, namely,
width of the transmission window and reduced group velocity of light, in this
unusual basis, are theoretically and experimentally investigated and are shown
to be essentially identical to those of standard EIT in the same system.
