  We investigate the generation and transfer of entangled states between two
coupled microtoroidal cavities considering two different types of couplings,
namely i) via a bridge qubit and ii) via evanescent fields. The cavities
support two counter-propagating whispering-gallery modes (WGMs) that may also
interact with each other. We firstly show that it is possible to transfer, with
high fidelity, a maximally entangled state between the two modes of the first
cavity (cavity 1) to the two modes of the second cavity (cavity 2),
independently of the type of coupling. Interesting differences, though, arise
concerning the generation of entangled states from initial product states; if
the cavities are coupled via a bridge qubit, we show that it is possible to
generate a 4-partite entangled state involving all four cavity modes. On the
other hand, contrarily to what happens in the qubit coupling case, it is
possible to generate bipartite maximally entangled states between modes of
different cavities from initial separable states for cavities coupled by
evanescent waves. Besides, we show that different entangled states between the
propagating and counter-propagating modes of distinct cavities may be generated
by tuning the interaction between modes belonging to the same cavity
(intra-cavity couplings). Again, this is possible only for the couplings via
evanescent waves. For the completion of our work, we discuss the effects of
losses on the dynamics of the system.
