The intensity of bright solitons propagating
in linearly coupled identical fibers can be distributed either in
stable symmetric state if the coupling is strong enough or in a
stable asymmetric state if the coupling is small enough. In the
first case, if the initial state is not the equilibrium state,
intensity may switch periodically from fiber to fiber, while in
second case the a-symmetrical state remains forever, with most of
its energy in either fiber. Classically the latter situation
a state of propagation with two exactly reciprocal realizations.
the quantum case, such a situation does not exist as an eigenstate
because of the possibility of quantum tunneling between the two
fibers. Such a tunneling is a purely quantum phenomenon
without counterpart in the classical theory seeing the solitons
objects made of classical polarization and electromagnetic fields.
When estimating the rate of tunneling one realizes that quantum
effects may be often relevant in the dynamics of optical
because the order of magnitude of the action is not much greater
than Planck's constant.
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