  Magnetic flux quanta, of value h/2e, in long Josephson junctions behave as
(quasi) solitons. Fluxon dynamical states are well described by a perturbed
sine-Gordon equation model, with boundary conditions determined by the junction
geometry and by externally applied magnetic fields, and they give rise to
readily measurable physical phenomena, such as step structure in
current-voltage characteristics and microwave radiation emission. Devices based
on fluxon propagation offer potentially interesting applications as oscillators
and amplifiers - as well as digital applica- tions, described elsewhere in this
volume - in high performance integrated superconductive circuits.
