Ratchet-like dynamics of fluxons in annular Josephson junctions driven   by bi-harmonic microwave fields
  Experimental observation of the unidirectional motion of a topological soliton driven by a bi-harmonic ac force of zero mean is reported. The observation is made by measuring the current-voltage characteristics for a fluxon trapped in an annular Josephson junction that was placed into a microwave field. The measured dependence of the fluxon mean velocity (rectified voltage) at zero dc bias versus the phase shift between the first and second harmonic of the driving force is in qualitative agreement with theoretical expectations. 
A. V. Ustinov, C. Coqui, A. Kemp, Y. Zolotaryuk, M. Salerno