Direct Comparison of Josephson Waveforms Using an AC Quantum Voltmeter

A new method for calibrating ac voltage waveforms based on measuring the difference relative to a synchronously synthesized waveform supplied from a programmable Josephson array is presented. In order to test the accuracy of this approach, the differences between two Josephson waveforms have been measured at the 1.2-V level for frequencies up to 5 kHz. The uncertainties achieved are at least a factor of ten lower than those from the direct sampling method. Below 400 Hz, this new method gives an uncertainty of 5 10^-8 (k = 1), while at 1 kHz, the uncertainty is about 2 10⁷ (k = 1)