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Quantum voltmeter for alternating voltages

The quantum voltmeter for alternating voltages conceived at PTB achieved already in the test phase an uncertainty of 5 · 10–8 during the measurement of a 400 Hz signal, a value ten times lower than previously. The outstanding performance of superconducting quantum standards used so far for dc voltage calibrations has thus been extended for the measurement of alternating voltages.

Part of a Josephson-series-array. From the left, the microwave striplines appear; from below the control leads for the individual segments.

In the low frequency range, alternating voltages are measured using sampling methods during which the time-varying voltage is measured repeatedly (“sampled”) in rapid succession. The amplification factor and the internal voltage reference of the sampling voltmeter limit, however, the attainable uncertainty. This can in practice be completely avoided if the sampled voltage is directly compared with the voltage of a Josephson quantum standard, known at 1 V to better than 0.1 nV.

This idea is realised in a method developed and patented at PTB. For this purpose, alternating voltages are synthesised with programmable Josephson-series-arrays. A chip cooled down to the temperature of liquid helium contains 8192 superconducting tunnel elements – so-called Josephson junctions – supplied with a microwave frequency of 70 GHz. They are distributed over segments with 1, 2, 4, 8, 16 … junctions. Switchable current sources control the individual segments such that they pro-duce quantised partial voltages which add up to the total voltage. A transition between quantised voltages requires less than 100 ns. Therefore, the slowly changing voltage to be measured can be compensated. If the two time-varying voltages and a sampling voltmeter are synchronized, the differences between the two alternating voltages can be measured with high resolution.

It is nowadays possible – with new programmable Josephson circuits, as yet only produced at PTB – to synthesise alternating voltages with amplitudes of even 10 V, making possible a range of additional applications. In particular, however, the attainable relative measurement uncertainty should, due to the greater signal-to-noise ratio, decrease by an additional order of magnitude.

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