We have measured the current-voltage (I-V) characteristics of a loop of four superconducting tunnel junctions at low temperatures (T≈50mK). In each branch of the loop, tunneling occurs through a small capacitance island (C∑≈1fF). In this geometry we can vary the magnetic flux Φ in the loop and the voltage Vg on gate electrodes, which have capacitance Cg to the small islands. We find that both Φ and Vg modulate the I-V curve.
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