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A Quantum Voltmeter for Industry

To measure ac voltages up to audio frequencies, sampling methods are commonly used. Here, an unknown voltage is rapidly measured (sampled). The gain and internal voltage reference of the sampling meter limit the overall uncertainty. This problem can be solved by comparing the unknown voltage with a Josephson voltage, which is accurate to 0.1 nV/V.

PTB has developed a method, to implement this idea. Ac voltages are synthesized with programmable Josephson arrays. A chip with 69632 Josephson Junctions is cooled down to 4.2 K and it is irradiated at 70 GHz. These junctions are divided in a binary format (1, 2, 4, 8, 16, …). By driving these junction segments with fast switching current sources, we are able to switch individual segments on and off, thus alternating between quantized voltages and superconducting short circuit at a fast switching time of 100 ns. In this way, a synthesized waveform of quantum precision could be compared with the unknown waveform under test.

The Quantum Voltmeter system is incorporated with precise synchronization of the voltage under test, the current sources and the sampling voltmeter. Thus, the differential voltage can be achieved at a sufficiently low voltage for maximum sensitivity.


Commercially available calibrator is being calibrated by an AC-Quantum Voltmeter  

A commercially available calibrator (middle) is being calibrated by an AC-Quantum Voltmeter in ac voltage mode.

The federal ministry of economics sponsored a technology transfer project (MNPQ project) between PTB and two industry counterparts to establish a Josephson measurement system for dc and ac voltage – an AC Quantum Voltmeter – for industrial calibration laboratories. The system uses PTB manufactured Josephson arrays with peak voltages of ±10 V and is able to synthesize quantized ac voltage with frequencies up to 2 kHz.

The AC Quantum Voltmeter is also able to calibrate commercially available dc references and voltmeters, hence, it comes with the capabilities of a DC Quantum Voltmeter. A DC comparison between a classic Josephson DC system and the AC Quantum Voltmeter showed no discrepancy within the uncertainty on 0.1 nV/V in 15 minutes.

The AC Quantum Voltmeter has been optimized for practical use at an accredited calibration laboratory of one of the project participants, esz AG. With some input from the operatorend users, Supracon AG automated the system and increased the user friendliness. During a comparison with a PTB prototype, an uncertainty of a few µV/V could be reached within one minute. This makes the AC Quantum Voltmeter 20 times more accurate than common calibrators and 60 times faster than measurements using thermal converter.

The development followed a modular concept, which grants further research towards a "Quantum Calibrator" covering not only a larger voltage range, but resistance and current standards calibration as well.


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