  Electromagnetically induced transparency (EIT) is a signature of quantum
interference in an atomic three-level system. By driving the dressed
cavity-qubit states of a two-dimensional circuit QED system, we generate a set
of polariton states in the nesting regime. The lowest three energy levels are
utilized to form the $\Lambda$-type system. EIT is observed and verified by
Akaike's information criterion based testing. Negative group velocities up to
$-0.52\pm0.09$ km/s are obtained based on the dispersion relation in the EIT
transmission spectrum.
