Precision Comparison of Sine Waveforms With Pulse-Driven Josephson Arrays

The equivalence of two sinusoidal waveforms synthesized by pulse-driven Josephson arrays using two Josephson arbitrary waveform synthesizers (JAWS) has been investigated by a direct, high-precision comparison. The arrays are based on double-stacked SNS-type Josephson junctions (superconductor-normal conductor-superconductor) with Nb
x
Si
_1-
x
as barrier material. Two arrays, consisting of 4000 junctions each, are arranged on one silicon chip and are connected in opposite voltage direction directly at the chip at 4.2 K. Careful phase tuning between the two JAWS systems allowed the minimization of the residual ac voltage, which was characterized by a fast digitizer and a lock-in amplifier. For an output voltage of 8.8 mV, a difference voltage of 0.14 nV with 0.25 nV uncertainty was measured, i.e., (1.6 ±2.9) × 10
^-8
in relative units. This result demonstrates the excellent agreement of both voltages and therefore the quantization of the JAWS itself.