Interlocked pair-tunneling phenomenon in a series array of small S-S´-S Josephson junctions

A novel macroscopic quantum phenomenon in a series array of small S-S´-S Josephson junctions was found by examination of high-frequency responses and mixing characteristics at 100-115 GHz. The experiments reveal that the M multiphoton process is exclusively enhanced in the M stage array driven by a DC voltage bias source and that pair tunneling at each stage occurs simultaneously. Each Josephson element is a quasiplanar-type Josephson weak-link made of niobium films with small capacitance and bridge length shorter than 3.49 ξ, where the ideal Josephson effect can be expected. Current is a natural common variable in a series array. It is therefore reasonable that fluctuation of the Josephson phase φ_k is effectively reduced in the series array, and that φ_k is locked on a common value, where k means the kth junction (k=1,2,3. . . N)