  Electromagnetically induced transparency (EIT) and EIT-like effects have been
investigated in a wide variety of coupled resonant systems. Here, a
classification of the phase characteristics of the EIT-like spectral responses
is presented. Newly identified phase responses reveal unexplored operation
regimes of EIT-like systems. Taking advantage of the new phase regimes, one can
obtain group delay, dispersion and nonlinearity properties greatly enhanced by
almost one order of magnitude, compared to the traditionally constructed
EIT-like devices, which breaks the fundamental limitation (e.g.,
delay-bandwidth product) intrinsic to atomic EIT and EIT-like effects. Optical
devices and electrical circuits are analyzed as examples showing the
universality of our finding. We show that cavity-QED-based quantum phase gates
can be greatly improved to achieve a phase shift of {\pi}. The new phase
characteristics are also believed to be useful to build novel doubly resonant
devices in quantum information based on cavity QED, optomechanics, and
metamaterials.
