15SIB04 QuADC: Waveform metrology based on spectrally pure Josephson voltages

• Research unit: METAS
• Project number: F-5212.30073

Key words (English)

AC quantum standards, quantum electrical metrology, Josephson Arbitrary Waveform Synthesizer (JAWS), SI traceability, AC voltage, digital measurement, waveform metrology

Short description (English)

This project will develop measurement systems centred on true AC-voltage quantum devices which will both operate at the highest level of accuracy and be simple enough for exploitation outside the national metrology institutes. The term ‘true quantum devices’ refers to the recently achieved breakthrough which provided spectrally pure quantised Josephson AC-voltages exceeding for the first time the usability threshold of 1 V RMS. In this project, innovative use of Josephson junctions is proposed for measurements of arbitrary signals in terms of fundamental constants referenced to the volt in the new SI. The need for this development is clearly driven by development in the application fields: Sensing and measurement are increasingly dependent on fast analogue-to-digital conversion. Recent R&D in precision integrated circuits and measurement equipment has brought about a step change in the sampling rates and accuracies available. Whereas direct traceability of DC electrical metrology to quantum standards is well established, emerging measurement applications in high end equipment are placing new demands on the traceability for dynamic quantities, which cannot be satisfied by the existing approaches. Fluke e.g. has written “One of the barriers to reducing the uncertainty of these multifunction calibrators for AC voltage is the magnitude of uncertainty inherited in the traceability chain.”

The overall objective of this project is to provide for all end-users direct, efficient, and highly accurate traceability of AC-voltages to the SI volt for dynamic measurements in the most relevant range of DC to 1 MHz, up to levels of 1 kV.

This is a joint research project carried out in the framework of the European Metrology Programme for Innovation and Research (EMPIR) (see:http://www.euramet.org/research-innovation/empir/). The EMPIR initiative is co-funded by the European Unions's Horizon 2020 research and innovation programme and the participating states. METAS is one of the project partners in the Project.

Project aims (English)

The specific objectives are:

1. To develop a quantum-based real-time measurement system utilising the Josephson effect representation of the SI volt. Novel methods for biasing Josephson junctions, such as the use of optoelectronic devices, will be exploited to achieve larger voltage levels together with approaches for direct analogue-to-digital conversion in terms of the Josephson constant K_J = 2e/h. Specialised electronic circuits required for interfacing the sensitive and accurate low temperature Josephson devices to room temperature industrial precision waveform instruments will be developed over the range of voltages and frequencies relevant in precision waveform metrology.
2. To develop a robust and user-friendly quantum system as a practical realisation for providing direct traceability of the redefined base unit ‘volt’ to end users, either national measurement laboratories or the next tier of users in the calibration and test sectors. This includes automation techniques, He-free cryogenic systems (4 K) and cost-effective components.
3. To evaluate digital signal processing techniques with respect to their contribution to the measurement uncertainty and to validate measurement methods for AC voltage calibration based on spectrally pure Josephson-AC-voltage references. The target uncertainty of 10 nV/V for frequencies up to 1 kHz and better than 10 μV/V up to 1 MHz will be validated via calibration of commercial instruments against the quantum standard and performed in collaboration with manufactures of precision Instrumentation.
4. To scale quantum waveforms up to 1 kV using voltage dividers or amplifiers. By measuring the divider output directly with a Josephson based digitising system the higher voltage waveform will be linked to the Josephson volt; the aim is to reach uncertainties ranging from 5 μV/V at 1 kV / 50 Hz to 25 μV/V at 120 V / 100 kHz.
5. To engage companies in the project research to facilitate the take up of the technology and measurement infrastructure developed by the project, and to support the development of new, innovative products, thereby enhancing the competitiveness of EU industry.

Abstract (English)

The METAS contribution to this project was to develop a setup that will be used to avoid the loading effect due to the cable and to decrease the difference between the voltage applied to the DUT and the calculated voltage from the pulse driven Josephson junction array (JAWS). This difference is due to the impedance of the cable between the Josephson junction array and the DUT.

The setup proposed is a Load Compensation Bridge (LCB). The primciple of the bridge is to measure the current drawn by the cable capacitance (Vi) and to inject a current through ZS that will compensate this capacitive current and make the output voltage Vout equal to the input voltage Vin (i.e. the Josephson voltage). The signal wire connecting the Josephson voltage to the load ZL is thus an equipotential.

The LCB bridge was moved to PTB in March 2019 to perform the measurement using the PTB JAWS. The JAWS voltage was read by a Fluke 792 thermal transfer standard. The measurements carried out were based on ac-ac differences performed at constant amplitude. The reference frequency was chosen to be 1 kHz, a frequency at which the effect of capacitive loading is known to be smaller than 1 ppm. The rsults demonstarte that with a fully balanced LCB, the quadratic frequency dependence can be fully removed up to a freqauncy of 80 kHz.

In conclusion, the study has shown that the original approach undertaken in this work by using a load compensation bridge is extremely successful. Combining the know-how of METAS and PTB within a strong and fruitful collaboration lead to this successful validation of the LCB in combination with a JAWS system. Using an active guard to compensate the capacitive current drawn by the system wiring, the quadratic frequency dependence present at the output terminal of the JAWS was completely eliminated. Presently the uncertainty of the load compensation bridge is around a few ppm for voltages around 100 mV and frequencies up to 80 kHz.

Transfer and application (English)

The LCB bridge, developed in this project, will be implemented in JAWS systems, allowing a large variety of AC measurement directly with the compensated JAWS.

Publications / Results (English)

F. Overney, Y. Pimsut, S. Bauer, O. Kieler, R. Behr, B. Jeanneret, Meas. Sci. Technol. 2020, 31 (5), 55004. DOI: 10.1088/1361-6501/ab62c7.