Synchronous Cooper pair tunneling in a 1D-array of Josephson junctions

We have studied 1D-arrays of Josephson junctions with a gate capacitively c oupled to the middle of the array. The SQUID shape of each junction enables tuning of the Josephson coupling energy, El. By adding a signal to the gat e with a radio-frequency f we create a step in the I-V characteristics at t he value I = 2ef: The step is clearly visible as a peak in the differential resistance dI/dV and can be seen in the whole frequency range studied (10- 90 MHz). (C) 2000 Elsevier Science B.V. All rights reserved.