  We investigate a two-way cascaded cavity QED system consisting of
microtoroidal resonators coupled through an optical fiber. Each microtoroidal
cavity supports two counter-propagating whispering-gallery modes coupled to
single atoms through their evanescent fields. We focus on a pair of
atom-microtoroid systems and compute the spectrum of spontaneous emission into
the fiber with one atom initially excited. Explicit results are presented for
strong-coupling and bad-cavity regimes, where the latter allows the effective
atom-atom interaction to be controlled through the atom-cavity coupling and
detuning: the atoms exhibit either collective spontaneous emission with no
dipole-dipole interaction or a (coherent) dipole-dipole interaction and
independent (single-atom) emission. This capacity for switching the character
of the interaction is a feature of bi-directional coupling and connects our
two-way cascaded system to work on one-dimensional waveguides. Building upon
our bad-cavity results, we generalize to many atom-microtoroid systems coupled
through an optical fiber.
