Observation of soliton fusion in a Josephson array

The behavior of topological solitons in a parallel array of a Josephson junction is studied experimentally. We observe the fusion of two relativistic solitons of the same polarity into a single soliton. The soliton carries two quanta of magnetic flux and, most strikingly, travels 18% faster than an ordinary soliton under the same driving force. We also find a variety of bunched states composed of solitons of the same polarity, moving with fixed separation.