MUTUAL PHASE LOCKING IN JOSEPHSON JUNCTION ARRAYS

Arrays of interacting Josephson junctions have been studied experimentally. The Josephson element in these arrays consisted of indium microbridges having sub-micron dimensions. An external resistive inductor connected across the array provided long range interaction between the junctions. The results of these experiments have been analyzed in light of the theory developed recently for such systems by Likharev, Kuzmin and Ovsyannikov.; Detailed experimental results on the simplest such array of two junctions are presented. The interaction leads to frequency pulling and then to phase locking (and voltage locking) as the unperturbed frequencies of the two junctions are brought closer. The effects of the interaction on the current voltage characteristic of the junctions and the behavior of the radiation linewidth and power in the neighborhood of voltage locking have been studied. These experiments clearly show the differing effects of the junction and coupling loop resistance noise on the locking stability and radiation linewidth for the series and parallel biasing configurations. All these observations are in accordance with the calculations of Likharev, et al.; In addition, measurements have also been made on two microbridge arrays coupled by a superconducting loop. In this case, the phase difference between the junction oscillations was dependent on the flux through the loop, and both the radiated power and the linewidth varied periodically with this flux. An additional normal shunt was found to considerably decrease the variation of the phase difference with the flux.; We have also studied larger linear arrays containing up to 99 junctions. A principal requirement for obtaining coherence in these arrays is small dispersion in the junction voltages ((TURN) 10%). Because with current fabrication techniques, the junction parameters vary by about 20%, a biasing scheme using superconducting loops was used to ensure the uniformity of the junction voltages. The external resistive inductor connected across the array provided the long range interaction between the junctions. The effectiveness of this biasing and coupling scheme has been demonstrated. The observed increase in the power ((alpha) N('2)) and the decrease in the linewidth (('1)/N) are in agreement with the theory.