We have observed for the first time a single sine-Gordon soliton trapped in an annular Josephson junction. This system offers a unique possibility to study undisturbed soliton motion. In the context of perturbation theory, the soliton may be viewed as a relativistic particle moving under a uniform force with damping. Accordingly our experimental results are displayed directly in a force-versus-momentum plane, where they may be described by a theoretically derived universal curve.
Physical Review Letters, 1985, Vol 55, Issue 19, p. 2059-2062