Experimental study of mutual phase locking in Josephson tunnel junctions

Strong mutual phase locking of Josephson oscillations in unshunted tunnel junctions in-coporated into a special common millimeter-wave resonance structure has been observed and studied. The observed locking range was especially wide (up to ±10%) when the both junctions were dc biased at the current steps corresponding to the common resonance modes (frequencies from 25 to 200 GHz). Main features of the junction interaction have been found to be in a qualitative agreement with our earlier theory developed for lumped junctions. For larger junctions (S = 30×30 μm²), quantitative deviations from the theoretical predictions have been observed and identified as resulting from interaction of the (external) common resonance mode with the (internal) Fiske modes of the junctions.