Vortex interference in Josephson arrays in the insulating phase

A ring made from a Josephson array in the insulating phase is considered. T he centre of the ring contains a time-dependent charge Q(t). Interference o f vortices winding around the ring is studied. In the case Q(t) = It + Q(0) sin(omegat), the time-averaged intensity shows interference only if Phi I- 0 = N omega (integral Bloch steps). In the case Q(t) = Q, sin(omega (1)t) Q(0) sin(wt), the time-averaged intensity of vortices shows interference o nly for rational values of the ratio omega (1)/omega (fractional Bloch step s). The use of this non-linear device as frequency converter at THz frequen cies is discussed. (C) 2001 Elsevier Science B.V. All rights reserved.